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The Twisted Documentation The Twisted Development Team March 6, 2010 Contents 1 Introduction 7 1.1 TheVisionForTwisted ............................. ............. 7 1.2 High-Level Overview of Twisted . . . . . . . . . . . . . . . . . . . . . ................ 7 1.3 OverviewofTwistedInternet . . . . . . . . . . . . . . . . . . . . . . . ................ 8 2 Tutorial 9 2.1 WritingServers ................................... ........... 9 2.1.1 Overview ...................................... ....... 9 2.1.2 Protocols ..................................... ........ 9 2.1.3 Factories ..................................... ........ 11 2.2 WritingClients ................................... ........... 13 2.2.1 Overview ...................................... ....... 13 2.2.2 Protocol...................................... ........ 13 2.2.3 Simple,single-useclients . . . . . . . . . . . . . . . . . . . . . . .............. 13 2.2.4 ClientFactory................................. .......... 14 2.2.5 A Higher-Level Example: ircLogBot . . . . . . . . . . . . . . . . .............. 15 2.2.6 FurtherReading ................................ ......... 18 2.3 Setting up the TwistedQuotes application . . . . . . . . . . . . .................... 18 2.3.1 Goal.......................................... ...... 18 2.3.2 Setting up the TwistedQuotes project directory . . . . . .................... 18 2.4 DesigningTwistedApplications . . . . . . . . . . . . . . . . . . . . ................. 20 2.4.1 Goals ......................................... ...... 20 2.4.2 Example of a modular design: TwistedQuotes . . . . . . . . . ................. 21 2.5 Twisted from Scratch, or The Evolution of Finger . . . . . . . ..................... 23 2.5.1 Introduction.................................. .......... 23 2.5.2 Contents ...................................... ....... 23 2.6 The Evolution of Finger: building a simple finger service ........................ 24 2.6.1 Introduction.................................. .......... 24 2.6.2 RefuseConnections ............................. .......... 24 2.6.3 DoNothing..................................... ....... 24 2.6.4 DropConnections ............................... ......... 25 2.6.5 Read Username, Drop Connections . . . . . . . . . . . . . . . . . . ............. 25 2.6.6 Read Username, Output Error, Drop Connections . . . . . . .................. 26 2.6.7 OutputFromEmptyFactory . . . . . . . . . . . . . . . . . . . . . . . . ........... 26 2.6.8 Output from Non-empty Factory . . . . . . . . . . . . . . . . . . . . ............. 27 2.6.9 UseDeferreds.................................. ......... 27 2.6.10 Run’finger’Locally . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... 28 2.6.11 ReadStatusfromtheWeb . . . . . . . . . . . . . . . . . . . . . . . . . ........... 29 2.6.12 UseApplication ............................... .......... 30 2.6.13 twistd....................................... ........ 30 2.7 The Evolution of Finger: adding features to the finger service...................... 31 2.7.1 Introduction.................................. .......... 31 2.7.2 SettingMessageByLocalUsers . . . . . . . . . . . . . . . . . . . . ............. 31 2.7.3 Use Services to Make Dependencies Sane . . . . . . . . . . . . . ............... 32 2.7.4 ReadStatusFile ................................ ......... 34 1 CONTENTS 2 2.7.5 AnnounceonWeb,Too.. .... ..... ..... ..... ..... ... ......... 35 2.7.6 AnnounceonIRC,Too............................. ......... 37 2.7.7 AddXML-RPCSupport ............................. ........ 39 2.8 The Evolution of Finger: cleaning up the finger code . . . . . ..................... 41 2.8.1 Introduction.................................. .......... 41 2.8.2 WriteReadableCode.............................. ......... 41 2.9 The Evolution of Finger: moving to a component based architecture .................. 43 2.9.1 Introduction.................................. .......... 43 2.9.2 WriteMaintainableCode. . . . . . . . . . . . . . . . . . . . . . . . . . ........... 43 2.9.3 Advantages of Latest Version . . . . . . . . . . . . . . . . . . . . . ............. 48 2.9.4 Aspect-Oriented Programming . . . . . . . . . . . . . . . . . . . . .............. 53 2.10 The Evolution of Finger: pluggable backends . . . . . . . . . ..................... 53 2.10.1 Introduction................................. ........... 53 2.10.2 AnotherBack-end .............................. .......... 53 2.10.3 Yet Another Back-end: Doing the Standard Thing . . . . . ................... 58 2.11 The Evolution of Finger: a web frontend . . . . . . . . . . . . . . ................... 64 2.11.1 Introduction................................. ........... 64 2.12 The Evolution of Finger: Twisted client support using Perspective Broker . . . . . . . . . . . . . . . 68 2.12.1 Introduction................................. ........... 68 2.12.2 UsePerspectiveBroker. . . . . . . . . . . . . . . . . . . . . . . . . ............. 68 2.13 The Evolution of Finger: using a single factory for multipleprotocols ................. 74 2.13.1 Introduction................................. ........... 74 2.13.2 SupportHTTPS................................. ......... 74 2.14 The Evolution of Finger: a Twisted finger client . . . . . . . ...................... 80 2.14.1 Introduction................................. ........... 80 2.14.2 FingerProxy .................................. ......... 80 2.15 The Evolution of Finger: making a finger library . . . . . . . ..................... 82 2.15.1 Introduction................................. ........... 82 2.15.2 Organization ................................. .......... 82 2.15.3 EasyConfiguration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... 83 2.16 The Evolution of Finger: configuration and packaging of thefingerservice. . . . . . . . . . . . . . . 85 2.16.1 Introduction................................. ........... 85 2.16.2 Plugins ...................................... ........ 85 2.16.3 OSIntegration ................................ .......... 91 3 Low-Level Twisted 93 3.1 ReactorOverview................................. ............ 93 3.1.1 ReactorBasics ................................. ......... 93 3.1.2 Usingthereactorobject . . . . . . . . . . . . . . . . . . . . . . . . . ............ 93 3.2 UDPNetworking ................................... .......... 94 3.2.1 Overview ...................................... ....... 94 3.2.2 DatagramProtocol .............................. .......... 94 3.2.3 ConnectedUDP .................................. ....... 94 3.2.4 MulticastUDP .................................. ........ 95 3.2.5 Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... 96 3.3 UsingProcesses.................................. ............ 96 3.3.1 Overview ...................................... ....... 96 3.3.2 RunningAnotherProcess . . . . . . . . . . . . . . . . . . . . . . . . . ........... 97 3.3.3 WritingaProcessProtocol . . . . . . . . . . . . . . . . . . . . . . . . ............ 97 3.3.4 Things that can happen to your ProcessProtocol . . . . . . ................... 98 3.3.5 Things you can do from your ProcessProtocol . . . . . . . . . ................. 99 3.3.6 VerboseExample................................ ......... 99 3.3.7 DoingittheEasyWay ............................. ......... 100 3.3.8 MappingFileDescriptors . . . . . . . . . . . . . . . . . . . . . . . . ............ 101 3.4 DeferredReference ............................... ............. 104 3.4.1 Deferreds..................................... ........ 104 3.4.2 Callbacks..................................... ........ 104 CONTENTS 3 3.4.3 Errbacks ...................................... ....... 108 3.4.4 Handling either synchronous or asynchronous results ...................... 109 3.4.5 DeferredList .................................. ......... 110 3.4.6 ClassOverview................................. ......... 112 3.4.7 Seealso....................................... ....... 113 3.5 GeneratingDeferreds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............. 113 3.5.1 Classoverview ................................. ......... 113 3.5.2 What Deferreds don’t do: make your code asynchronous . . .................. 113 3.5.3 Advanced Processing Chain Control . . . . . . . . . . . . . . . . ............... 114 3.5.4 Returning Deferreds from synchronous functions . . . . .................... 115 3.5.5 Integrating blocking code with Twisted . . . . . . . . . . . . ................. 115 3.5.6 Possiblesourcesoferror . . . . . . . . . . . . . . . . . . . . . . . . ............. 116 3.6 Deferredsarebeautiful!(ATutorial) . . . . . . . . . . . . . . . .................... 117 3.6.1 Introduction.................................. .......... 117 3.6.2 Asimpleexample ................................ ........ 117 3.6.3 Errbacks ...................................... ....... 118 3.6.4 addBoth: the deferred version of finally . . . . . . . . . . . . ................. 126 3.6.5 addCallbacks: decision making based on previous successorfailure . . . . . . . . . . . . . . 128 3.6.6 Hints, tips, common mistakes, and miscellaney . . . . . . ................... 134 3.6.7 Conclusion .................................... ........ 137 3.7 Schedulingtasksforthefuture . . . . . . . . . . . . . . . . . . . . . ................. 137 3.8 UsingThreadsinTwisted. . . . . . . . . . . . . . . . . . . . . . . . . . . .............. 139 3.8.1 Running code in a thread-safe manner . . . . . . . . . . . . . . . ............... 139 3.8.2 Runningcodeinthreads . . . . . . . . . . . . . . . . . . . . . . . . . . ........... 139 3.8.3 UtilityMethods................................ .......... 139 3.8.4 ManagingtheThreadPool . . . . . . . . . . . . . . . . . . . . . . . . . ........... 140 3.9 Choosing a Reactor and GUI Toolkit Integration . . . . . . . . ..................... 140 3.9.1 Overview ...................................... ....... 140 3.9.2 ReactorFunctionality. . . . . . . . . . . . . . . . . . . . . . . . . . ............. 141 3.9.3 GeneralPurposeReactors . . . . . . . . . . . . . . . . . . . . . . . . ............ 142 3.9.4 Platform-SpecificReactors . . . . . . . . . . . . . . . . . . . . . . .............. 142 3.9.5 GUIIntegrationReactors. . . . . . . . . . . . . . . . . . . . . . . . ............. 143 3.9.6 Non-Reactor GUI Integration . . . . . . . . . . . . . . . . . . . . . ............. 143 4 High-Level Twisted 145 4.1 TheBasics....................................... .......... 145 4.1.1 Application................................... ......... 145 4.1.2 twistd........................................ ....... 145 4.1.3 tap2deb....................................... ....... 145 4.1.4 tap2rpm....................................... ....... 146 4.2 TheTwistedPluginSystem. . . . . . . . . . . . . . . . . . . . . . . . . . .............. 146 4.2.1 WritingExtensiblePrograms. . . . . . . . . . . . . . . . . . . . . . ............. 146 4.2.2 ExtendinganExistingProgram . . . . . . . . . . . . . . . . . . . . ............. 147 4.2.3 AlternatePluginPackages . . . . . . . . . . . . . . . . . . . . . . . ............. 147 4.2.4 PluginCaching ................................. ......... 148 4.2.5 FurtherReading ................................ ......... 148 4.3 WritingatwistdPlugin ............................. ............. 148 4.3.1 Goals ......................................... . . . . . . 148 4.3.2 Anoteon.tapfiles ............................... ......... 149 4.3.3 Alternativestotwistdplugins . . . . . . . . . . . . . . . . . . . ............... 149 4.3.4 Creatingtheplugin ............................. ........... 149 4.3.5 UsingcredwithyourTAP . . . . . . . . . . . . . . . . . . . . . . . . . . .......... 150 4.3.6 Conclusion .................................... ........ 151 4.4 Components: Interfaces and Adapters . . . . . . . . . . . . . . . . .................. 151 4.4.1 Interfaces and Components in Twisted code . . . . . . . . . . ................. 154 4.5 Cred: Pluggable Authentication . . . . . . . . . . . . . . . . . . . . ................. 157 4.5.1 Goals ......................................... . . . . . . 157 CONTENTS 4 4.5.2 Credobjects ................................... ........ 158 4.5.3 Responsibilities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............ 160 4.5.4 Credplugins ................................... ........ 162 4.5.5 Conclusion .................................... ........ 163 4.6 Using the Twisted Application Framework . . . . . . . . . . . . . ................... 163 4.6.1 Introduction.................................. .......... 163 4.6.2 Overview ...................................... ....... 164 4.6.3 Usingapplication.............................. ........... 164 5 Utilities 168 5.1 Parsing command-lines with usage.Options . . . . . . . . . . . .................... 168 5.1.1 Introduction.................................. .......... 168 5.1.2 BooleanOptions ................................ ......... 168 5.1.3 Parameters .................................... ........ 169 5.1.4 OptionSubcommands ............................. ......... 170 5.1.5 GenericCodeForOptions . . . . . . . . . . . . . . . . . . . . . . . . . ........... 171 5.1.6 ParsingArguments .............................. .......... 171 5.1.7 PostProcessing................................ .......... 172 5.1.8 Typeenforcement............................... .......... 172 5.2 Loggingwithtwisted.python.log . . . . . . . . . . . . . . . . . . . .................. 173 5.2.1 Basicusage.................................... ........ 173 5.2.2 Writinglogobservers. . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... 174 5.2.3 Customizingtwistdlogging . . . . . . . . . . . . . . . . . . . . . . ............. 174 5.3 DirDBM: Directory-based Storage . . . . . . . . . . . . . . . . . . . ................. 175 5.3.1 dirdbm.DirDBM ................................. ........ 175 5.3.2 dirdbm.Shelf .................................. ......... 175 5.4 Using telnet to manipulate a twisted server . . . . . . . . . . . ..................... 175 5.5 WritingtestsforTwistedcodeusingTrial . . . . . . . . . . . . . ................... 176 5.5.1 Trialbasics ................................... ......... 176 5.5.2 Trialdirectories.............................. ............ 176 5.5.3 Twisted-specific quirks: reactor, Deferreds, callLater ...................... 177 6 Twisted RDBMS support 179 6.1 twisted.enterprise.adbapi: Twisted RDBMS support . . . ........................ 179 6.1.1 Abstract...................................... ........ 179 6.1.2 Whatyoushouldalreadyknow. . . . . . . . . . . . . . . . . . . . . . . ........... 179 6.1.3 QuickOverview ................................. ........ 179 6.1.4 HowdoIuseadbapi?.............................. ......... 180 6.1.5 Examples of various database adapters . . . . . . . . . . . . . ................. 181 6.1.6 Andthat’sit! .................................. ......... 181 6.2 TwistedEnterpriseRowObjects . . . . . . . . . . . . . . . . . . . . . ................ 181 6.2.1 ClassDefinitions ............................... .......... 181 6.2.2 Initialization ................................ ........... 182 6.2.3 CreatingRowObjects . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... 182 6.2.4 Relationships Between Tables . . . . . . . . . . . . . . . . . . . . .............. 183 6.2.5 DuplicateRowObjects. . . . . . . . . . . . . . . . . . . . . . . . . . . ........... 183 6.2.6 UpdatingRowObjects . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... 183 6.2.7 DeletingRowObjects . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... 183 7 Perspective Broker 184 7.1 OverviewofTwistedSpread . . . . . . . . . . . . . . . . . . . . . . . . . .............. 184 7.1.1 Rationale ..................................... ........ 184 7.2 Introduction to Perspective Broker . . . . . . . . . . . . . . . . . ................... 184 7.2.1 Introduction.................................. .......... 184 7.2.2 ObjectRoadmap ................................. ........ 185 7.2.3 Things you can Call Remotely . . . . . . . . . . . . . . . . . . . . . . ............ 185 7.2.4 Things you can Copy Remotely . . . . . . . . . . . . . . . . . . . . . . ........... 186 CONTENTS 5 7.3 UsingPerspectiveBroker . . . . . . . . . . . . . . . . . . . . . . . . . . ............... 187 7.3.1 BasicExample .................................. ........ 187 7.3.2 CompleteExample ............................... ......... 189 7.3.3 References can come back to you . . . . . . . . . . . . . . . . . . . . ............ 190 7.3.4 References to client-side objects . . . . . . . . . . . . . . . . ................. 192 7.3.5 RaisingRemoteExceptions . . . . . . . . . . . . . . . . . . . . . . . ............ 193 7.3.6 Try/Except blocks and Failure.trap . . . . . . . . . . . . . . . ................ 195 7.4 PB Copyable: Passing Complex Types . . . . . . . . . . . . . . . . . . ................ 198 7.4.1 Overview ...................................... ....... 198 7.4.2 Motivation .................................... ........ 198 7.4.3 PassingObjects................................ .......... 198 7.4.4 pb.Copyable ................................... ........ 200 7.4.5 pb.Cacheable .................................. ......... 205 7.5 Authentication with Perspective Broker . . . . . . . . . . . . . .................... 210 7.5.1 Overview ...................................... ....... 210 7.5.2 Compartmentalizing Services . . . . . . . . . . . . . . . . . . . . .............. 210 7.5.3 AvatarsandPerspectives . . . . . . . . . . . . . . . . . . . . . . . . ............. 214 7.5.4 PerspectiveExamples . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... 215 7.5.5 UsingAvatars.................................. ......... 222 8 Manual Pages 227 8.1 MANHOLE.1....................................... ........ 227 8.1.1 NAME .......................................... . . . . 227 8.1.2 SYNOPSIS...................................... . . . . . . 227 8.1.3 DESCRIPTION ................................... . . . . . . 227 8.1.4 AUTHOR........................................ . . . . . 227 8.1.5 REPORTINGBUGS ................................. . . . . . . 227 8.1.6 COPYRIGHT..................................... . . . . . . 227 8.2 TAP2DEB.1....................................... ......... 228 8.2.1 NAME .......................................... . . . . 228 8.2.2 SYNOPSIS...................................... . . . . . . 228 8.2.3 DESCRIPTION ................................... . . . . . . 228 8.2.4 AUTHOR........................................ . . . . . 228 8.2.5 REPORTINGBUGS ................................. . . . . . . 228 8.2.6 COPYRIGHT..................................... . . . . . . 228 8.2.7 SEEALSO....................................... . . . . . 228 8.3 TAP2RPM.1....................................... ......... 229 8.3.1 NAME .......................................... . . . . 229 8.3.2 SYNOPSIS...................................... . . . . . . 229 8.3.3 DESCRIPTION ................................... . . . . . . 229 8.3.4 AUTHOR........................................ . . . . . 229 8.3.5 REPORTINGBUGS ................................. . . . . . . 229 8.3.6 COPYRIGHT..................................... . . . . . . 229 8.3.7 SEEALSO....................................... . . . . . 229 8.4 TAPCONVERT.1 .................................... ......... 230 8.4.1 NAME .......................................... . . . . 230 8.4.2 SYNOPSIS...................................... . . . . . . 230 8.4.3 DESCRIPTION ................................... . . . . . . 230 8.4.4 AUTHOR........................................ . . . . . 230 8.4.5 REPORTINGBUGS ................................. . . . . . . 230 8.4.6 COPYRIGHT..................................... . . . . . . 230 8.5 TRIAL.1......................................... ......... 231 8.5.1 NAME .......................................... . . . . 231 8.5.2 SYNOPSIS...................................... . . . . . . 231 8.5.3 DESCRIPTION ................................... . . . . . . 231 8.5.4 AUTHOR........................................ . . . . . 232 8.5.5 REPORTINGBUGS ................................. . . . . . . 232 CONTENTS 6 8.5.6 COPYRIGHT..................................... . . . . . . 232 8.6 TWISTD.1 ......................................... ....... 233 8.6.1 NAME .......................................... . . . . 233 8.6.2 SYNOPSIS...................................... . . . . . . 233 8.6.3 DESCRIPTION ................................... . . . . . . 233 8.6.4 OPTIONS....................................... . . . . . . 233 8.6.5 SIGNALS....................................... . . . . . . 234 8.6.6 AUTHOR........................................ . . . . . 234 8.6.7 REPORTINGBUGS ................................. . . . . . . 234 8.6.8 COPYRIGHT..................................... . . . . . . 234 9 Appendix 235 9.1 TwistedGlossary ................................. ............ 235 9.2 Banana Protocol Specifications . . . . . . . . . . . . . . . . . . . . . ................. 237 9.2.1 Introduction.................................. .......... 237 9.2.2 BananaEncodings ............................... ......... 237 9.2.3 ElementTypes .................................. ........ 238 9.2.4 Profiles ....................................... ....... 238 9.2.5 Protocol Handshake and Behaviour . . . . . . . . . . . . . . . . . .............. 240 Chapter 1 Introduction 1.1 The Vision For Twisted Many other documents in this repository are dedicated to defining what Twisted is. Here, I will attempt to explain not what Twisted is, but what it should be, once I’ve met my goals with it. First, Twisted should be fun. It began as a game, it is being used commercially in games, and it will be, I hope, an interactive and entertaining experience for the end-user. Twisted is a platform for developing internet applications. While python, by itself, is a very powerful language, there are many facilities it lacks which other languages have spent great attention to adding. It can do this now; Twisted is a good (if somewhat idiosyncratic) pure-python framework or library, depending on how you treat it, and it continues to improve. As a platform, Twisted should be focused on integration. Ideally, all functionality will be accessible through all protocols. Failing that, all functionality should be configurable through at least one protocol, with a seamless and consistent user-interface. The next phase of development will be focusing strongly on a configuration system which will unify many disparate pieces of the current infrastructure, and allow them to be tacked together by a non- programmer. 1.2 High-Level Overview of Twisted 7 CHAPTER 1. INTRODUCTION 8 1.3 Overview of Twisted Internet Twisted Internet is a collection of compatible event-loops for Python. It contains the code to dispatch events to interested observers and a portable API so that observers need not care about which event loop is running. Thus, it is possible to use the same code for different loops, from Twisted’s basic, yet portable, select-based loop to the loops of various GUI toolkits like GTK+ or Tk. Twisted Internet contains the various interfaces to the reactor API, whose usage is documented in the low-level chapter. Those APIs are IReactorCore, IReactorTCP, IReactorSSL, IReactorUNIX, IReactorUDP, IReactorTime, IReactorProcess, IReactorMulticast and IReactorThreads. The reactor APIs allow non-persistent calls to be made. Twisted Internet also covers the interfaces for the various transports, in ITransport and friends. These inter- faces allow Twisted network code to be written without regard to the underlying implementation of the transport. The IProtocolFactory dictates how factories, which are usually a large part of third party code, are written. Chapter 2 Tutorial 2.1 Writing Servers 2.1.1 Overview Twisted is a framework designed to be very flexible and let you write powerful servers. The cost of this flexibility is a few layers in the way to writing your server. This document describes the Protocol layer, where you implement protocol parsing and handling. If you are implementing an application then you should read this document second, after first reading the top level overview of how to begin writing your Twisted application, in Writing Plug-Ins for Twisted (page 146). This document is only relevant to TCP, SSL and Unix socket servers, there is a separate document (page 94) for UDP. Your protocol handling class will usually subclass twisted.internet.protocol.Protocol. Most pro- tocol handlers inherit either from this class or from one of its convenience children. An instance of the protocol class might be instantiated per-connection, on demand, and might go away when the connection is finished. This means that persistent configuration is not saved in the Protocol. The persistent configuration is kept in a Factory class, which usually inherits from twisted.internet. protocol.Factory. The default factory class just instantiates each Protocol, and then sets on it an attribute called factory which points to itself. This lets every Protocol access, and possibly modify, the persistent con- figuration. It is usually useful to be able to offer the same service on multiple ports or network addresses. This is why the Factory does not listen to connections, and in fact does not know anything about the network. See twisted. internet.interfaces.IReactorTCP.listenTCP, and the other IReactor*.listen* APIs for more information. This document will explain each step of the way. 2.1.2 Protocols As mentioned above, this, along with auxiliary classes and functions, is where most of the code is. A Twisted protocol handles data in an asynchronous manner. What this means is that the protocol never waits for an event, but rather responds to events as they arrive from the network. Here is a simple example: from twisted.internet.protocol import Protocol class Echo(Protocol): def dataReceived(self, data): self.transport.write(data) This is one of the simplest protocols. It simply writes back whatever is written to it, and does not respond to all events. Here is an example of a Protocol responding to another event: from twisted.internet.protocol import Protocol 9 CHAPTER 2. TUTORIAL 10 class QOTD(Protocol): def connectionMade(self): self.transport.write("An apple a day keeps the doctor away\r\n") self.transport.loseConnection() This protocol responds to the initial connection with a well known quote, and then terminates the connection. The connectionMade event is usually where set up of the connection object happens, as well as any initial greetings (as in the QOTD protocol above, which is actually based on RFC 865). The connectionLost event is where tearing down of any connection-specific objects is done. Here is an example: from twisted.internet.protocol import Protocol class Echo(Protocol): def connectionMade(self): self.factory.numProtocols = self.factory.numProtocols+1 if self.factory.numProtocols > 100: self.transport.write("Too many connections, try later") self.transport.loseConnection() def connectionLost(self, reason): self.factory.numProtocols = self.factory.numProtocols-1 def dataReceived(self, data): self.transport.write(data) Here connectionMade and connectionLost cooperate to keep a count of the active protocols in the factory. connectionMade immediately closes the connection if there are too many active protocols. Using the Protocol In this section, I will explain how to test your protocol easily. (In order to see how you should write a production-grade Twisted server, though, you should read the Writing Plug-Ins for Twisted (page 146) HOWTO as well). Here is code that will run the QOTD server discussed earlier from twisted.internet.protocol import Protocol, Factory from twisted.internet import reactor class QOTD(Protocol): def connectionMade(self): self.transport.write("An apple a day keeps the doctor away\r\n") self.transport.loseConnection() # Next lines are magic: factory = Factory() factory.protocol = QOTD # 8007 is the port you want to run under. Choose something >1024 reactor.listenTCP(8007, factory) reactor.run() Don’t worry about the last 6 magic lines – you will understand what they do later in the document. Helper Protocols Many protocols build upon similar lower-level abstraction. The most popular in internet protocols is being line-based. Lines are usually terminated with a CR-LF combinations. CHAPTER 2. TUTORIAL 11 However, quite a few protocols are mixed - they have line-based sections and then raw data sections. Examples include HTTP/1.1 and the Freenet protocol. For those cases, there is the LineReceiver protocol. This protocol dispatches to two different event handlers - lineReceived and rawDataReceived. By default, only lineReceived will be called, once for each line. However, if setRawMode is called, the protocol will call rawDataReceived until setLineMode is called, which returns it to using lineReceived. Here is an example for a simple use of the line receiver: from twisted.protocols.basic import LineReceiver class Answer(LineReceiver): answers = {’How are you?’: ’Fine’, None : "I don’t know what you mean"} def lineReceived(self, line): if self.answers.has_key(line): self.sendLine(self.answers[line]) else: self.sendLine(self.answers[None]) Note that the delimiter is not part of the line. Several other, less popular, helpers exist, such as a netstring based protocol and a prefixed-message-length protocol. State Machines Many Twisted protocol handlers need to write a state machine to record the state they are at. Here are some pieces of advice which help to write state machines: • Don’t write big state machines. Prefer to write a state machine which deals with one level of abstraction at a time. • Use Python’s dynamicity to create open ended state machines. See, for example, the code for the SMTP client. • Don’t mix application-specific code with Protocol handling code. When the protocol handler has to make an application-specific call, keep it as a method call. 2.1.3 Factories As mentioned before, usually the class twisted.internet.protocol.Factory works, and there is no need to subclass it. However, sometimes there can be factory-specific configuration of the protocols, or other considerations. In those cases, there is a need to subclass Factory. For a factory which simply instantiates instances of a specific protocol class, simply instantiate Factory, and sets its protocol attribute: from twisted.internet.protocol import Factory from twisted.protocols.wire import Echo myFactory = Factory() myFactory.protocol = Echo If there is a need to easily construct factories for a specific configuration, a factory function is often useful: from twisted.internet.protocol import Factory, Protocol class QOTD(Protocol): def connectionMade(self): self.transport.write(self.factory.quote+’\r\n’) self.transport.loseConnection() CHAPTER 2. TUTORIAL 12 def makeQOTDFactory(quote=None): factory = Factory() factory.protocol = QOTD factory.quote = quote or ’An apple a day keeps the doctor away’ return factory A Factory has two methods to perform application-specific building up and tearing down (since a Factory is frequently persisted, it is often not appropriate to do them in init or del , and would frequently be too early or too late). Here is an example of a factory which allows its Protocols to write to a special log-file: from twisted.internet.protocol import Factory from twisted.protocols.basic import LineReceiver class LoggingProtocol(LineReceiver): def lineReceived(self, line): self.factory.fp.write(line+’\n’) class LogfileFactory(Factory): protocol = LoggingProtocol def __init__(self, fileName): self.file = fileName def startFactory(self): self.fp = open(self.file, ’a’) def stopFactory(self): self.fp.close() Putting it All Together So, you know what factories are, and want to run the QOTD with configurable quote server, do you? No problems, here is an example. from twisted.internet.protocol import Factory, Protocol from twisted.internet import reactor class QOTD(Protocol): def connectionMade(self): self.transport.write(self.factory.quote+’\r\n’) self.transport.loseConnection() class QOTDFactory(Factory): protocol = QOTD def __init__(self, quote=None): self.quote = quote or ’An apple a day keeps the doctor away’ reactor.listenTCP(8007, QOTDFactory("configurable quote")) reactor.run() CHAPTER 2. TUTORIAL 13 The only lines you might not understand are the last two. listenTCP is the method which connects a Factory to the network. It uses the reactor interface, which lets many different loops handle the networking code, without modifying end-user code, like this. As mentioned above, if you want to write your code to be a production-grade Twisted server, and not a mere 20-line hack, you will want to use the Application object (page 163). 2.2 Writing Clients 2.2.1 Overview Twisted is a framework designed to be very flexible, and let you write powerful clients. The cost of this flexibility is a few layers in the way to writing your client. This document covers creating clients that can be used for TCP, SSL and Unix sockets, UDP is covered in a different document (page 94). At the base, the place where you actually implement the protocol parsing and handling, is the Protocol class. This class will usually be decended from twisted.internet.protocol.Protocol. Most protocol handlers inherit either from this class or from one of its convenience children. An instance of the protocol class will be instantiated when you connect to the server, and will go away when the connection is finished. This means that persistent configuration is not saved in the Protocol. The persistent configuration is kept in a Factory class, which usually inherits from twisted.internet. protocol.ClientFactory. The default factory class just instantiate the Protocol, and then sets on it an attribute called factory which points to itself. This let the Protocol access, and possibly modify, the persistent configuration. 2.2.2 Protocol As mentioned above, this, and auxiliary classes and functions, is where most of the code is. A Twisted protocol handles data in an asynchronous manner. What this means is that the protocol never waits for an event, but rather responds to events as they arrive from the network. Here is a simple example: from twisted.internet.protocol import Protocol from sys import stdout class Echo(Protocol): def dataReceived(self, data): stdout.write(data) This is one of the simplest protocols. It simply writes to standard output whatever it reads from the connection. There are many events it does not respond to. Here is an example of a Protocol responding to another event. from twisted.internet.protocol import Protocol class WelcomeMessage(Protocol): def connectionMade(self): self.transport.write("Hello server, I am the client!\r\n") self.transport.loseConnection() This protocol connects to the server, sends it a welcome message, and then terminates the connection. The connectionMade event is usually where set up of the Protocol object happens, as well as any initial greetings (as in the WelcomeMessage protocol above). Any tearing down of Protocol-specific objects is done in connectionLost. 2.2.3 Simple, single-use clients In many cases, the protocol only needs to connect to the server once, and the code just wants to get a connected instance of the protocol. In those cases twisted.internet.protocol.ClientCreator provides the appropriate API. from twisted.internet import reactor from twisted.internet.protocol import Protocol, ClientCreator CHAPTER 2. TUTORIAL 14 class Greeter(Protocol): def sendMessage(self, msg): self.transport.write("MESSAGE %s\n" % msg) def gotProtocol(p): p.sendMessage("Hello") reactor.callLater(1, p.sendMessage, "This is sent in a second") reactor.callLater(2, p.transport.loseConnection) c = ClientCreator(reactor, Greeter) c.connectTCP("localhost", 1234).addCallback(gotProtocol) 2.2.4 ClientFactory We use reactor.connect* and a ClientFactory. The ClientFactory is in charge of creating the Protocol, and also receives events relating to the connection state. This allows it to do things like reconnect on the event of a connection error. Here is an example of a simple ClientFactory that uses the Echo protocol (above) and also prints what state the connection is in. from twisted.internet.protocol import Protocol, ClientFactory from sys import stdout class Echo(Protocol): def dataReceived(self, data): stdout.write(data) class EchoClientFactory(ClientFactory): def startedConnecting(self, connector): print ’Started to connect.’ def buildProtocol(self, addr): print ’Connected.’ return Echo() def clientConnectionLost(self, connector, reason): print ’Lost connection. Reason:’, reason def clientConnectionFailed(self, connector, reason): print ’Connection failed. Reason:’, reason To connect this EchoClientFactory to a server, you could use this code: from twisted.internet import reactor reactor.connectTCP(host, port, EchoClientFactory()) reactor.run() Note that clientConnectionFailed is called when a connection could not be established, and that client ConnectionLost is called when a connection was made and then disconnected. Reconnection Many times, the connection of a client will be lost unintentionally due to network errors. One way to reconnect after a disconnection would be to call connector.connect() when the connection is lost: from twisted.internet.protocol import ClientFactory class EchoClientFactory(ClientFactory): def clientConnectionLost(self, connector, reason): connector.connect() CHAPTER 2. TUTORIAL 15 The connector passed as the first argument is the interface between a connection and a protocol. When the con- nection fails and the factory receives the clientConnectionLost event, the factory can call connector.connect() to start the connection over again from scratch. However, most programs that want this functionality should implement ReconnectingClientFactory in- stead, which tries to reconnect if a connection is lost or fails, and which exponentially delays repeated reconnect attempts. Here is the Echo protocol implemented with a ReconnectingClientFactory: from twisted.internet.protocol import Protocol, ReconnectingClientFactory from sys import stdout class Echo(Protocol): def dataReceived(self, data): stdout.write(data) class EchoClientFactory(ReconnectingClientFactory): def startedConnecting(self, connector): print ’Started to connect.’ def buildProtocol(self, addr): print ’Connected.’ print ’Resetting reconnection delay’ self.resetDelay() return Echo() def clientConnectionLost(self, connector, reason): print ’Lost connection. Reason:’, reason ReconnectingClientFactory.clientConnectionLost(self, connector, reason) def clientConnectionFailed(self, connector, reason): print ’Connection failed. Reason:’, reason ReconnectingClientFactory.clientConnectionFailed(self, connector, reason) 2.2.5 A Higher-Level Example: ircLogBot Overview of ircLogBot The clients so far have been fairly simple. A more complicated example comes with Twisted Words in the doc/examples directory. # twisted imports from twisted.words.protocols import irc from twisted.internet import reactor, protocol from twisted.python import log # system imports import time, sys class MessageLogger: """ An independent logger class (because separation of application and protocol logic is a good thing). """ def __init__(self, file): self.file = file CHAPTER 2. TUTORIAL 16 def log(self, message): """Write a message to the file.""" timestamp = time.strftime("[%H:%M:%S]", time.localtime(time.time())) self.file.write(’%s %s\n’ % (timestamp, message)) self.file.flush() def close(self): self.file.close() class LogBot(irc.IRCClient): """A logging IRC bot.""" nickname = "twistedbot" def connectionMade(self): irc.IRCClient.connectionMade(self) self.logger = MessageLogger(open(self.factory.filename, "a")) self.logger.log("[connected at %s]" % time.asctime(time.localtime(time.time()))) def connectionLost(self, reason): irc.IRCClient.connectionLost(self, reason) self.logger.log("[disconnected at %s]" % time.asctime(time.localtime(time.time()))) self.logger.close() # callbacks for events def signedOn(self): """Called when bot has succesfully signed on to server.""" self.join(self.factory.channel) def joined(self, channel): """This will get called when the bot joins the channel.""" self.logger.log("[I have joined %s]" % channel) def privmsg(self, user, channel, msg): """This will get called when the bot receives a message.""" user = user.split(’!’, 1)[0] self.logger.log("<%s> %s" % (user, msg)) # Check to see if they’re sending me a private message if channel == self.nickname: msg = "It isn’t nice to whisper! Play nice with the group." self.msg(user, msg) return # Otherwise check to see if it is a message directed at me if msg.startswith(self.nickname + ":"): msg = "%s: I am a log bot" % user self.msg(channel, msg) self.logger.log("<%s> %s" % (self.nickname, msg)) def action(self, user, channel, msg): """This will get called when the bot sees someone do an action.""" CHAPTER 2. TUTORIAL 17 user = user.split(’!’, 1)[0] self.logger.log("*%s %s" % (user, msg)) # irc callbacks def irc_NICK(self, prefix, params): """Called when an IRC user changes their nickname.""" old_nick = prefix.split(’!’)[0] new_nick = params[0] self.logger.log("%s is now known as %s" % (old_nick, new_nick)) # For fun, override the method that determines how a nickname is changed on # collisions. The default method appends an underscore. def alterCollidedNick(self, nickname): """ Generate an altered version of a nickname that caused a collision in an effort to create an unused related name for subsequent registration. """ return nickname + ’ˆ’ class LogBotFactory(protocol.ClientFactory): """A factory for LogBots. A new protocol instance will be created each time we connect to the server. """ # the class of the protocol to build when new connection is made protocol = LogBot def __init__(self, channel, filename): self.channel = channel self.filename = filename def clientConnectionLost(self, connector, reason): """If we get disconnected, reconnect to server.""" connector.connect() def clientConnectionFailed(self, connector, reason): print "connection failed:", reason reactor.stop() if __name__ == ’__main__’: # initialize logging log.startLogging(sys.stdout) # create factory protocol and application f = LogBotFactory(sys.argv[1], sys.argv[2]) # connect factory to this host and port reactor.connectTCP("irc.freenode.net", 6667, f) # run bot reactor.run() CHAPTER 2. TUTORIAL 18 Source listing — ircLogBot.py ircLogBot.py connects to an IRC server, joins a channel, and logs all traffic on it to a file. It demonstrates some of the connection-level logic of reconnecting on a lost connection, as well as storing persistent data in the Factory. Persistent Data in the Factory Since the Protocol instance is recreated each time the connection is made, the client needs some way to keep track of data that should be persisted. In the case of the logging bot, it needs to know which channel it is logging, and where to log it to. from twisted.internet import protocol from twisted.protocols import irc class LogBot(irc.IRCClient): def connectionMade(self): irc.IRCClient.connectionMade(self) self.logger = MessageLogger(open(self.factory.filename, "a")) self.logger.log("[connected at %s]" % time.asctime(time.localtime(time.time()))) def signedOn(self): self.join(self.factory.channel) class LogBotFactory(protocol.ClientFactory): protocol = LogBot def __init__(self, channel, filename): self.channel = channel self.filename = filename When the protocol is created, it gets a reference to the factory as self.factory. It can then access attributes of the factory in its logic. In the case of LogBot, it opens the file and connects to the channel stored in the factory. 2.2.6 Further Reading The Protocol class used throughout this document is a base implementation of IProtocol used in most Twisted applications for convenience. To learn about the completeIProtocol interface, see the API documentation for IProtocol. The transport attribute used in some examples in this document provides the ITCPTransport interface. To learn about the complete interface, see the API documentation for ITCPTransport. Interface classes are a way of specifying what methods and attributes an object has and how they behave. See the Components: Interfaces and Adapters (page 151) document. 2.3 Setting up the TwistedQuotes application 2.3.1 Goal This document describes how to set up the TwistedQuotes application used in a number of other documents, such as designing Twisted applications (page 20). 2.3.2 Setting up the TwistedQuotes project directory In order to run the Twisted Quotes example, you will need to do the following: CHAPTER 2. TUTORIAL 19 1. Make a TwistedQuotes directory on your system 2. Place the following files in the TwistedQuotes directory: •""" Twisted Quotes """ Source listing — init .py (this file marks it as a package, see this section1 of the Python tutorial for more on packages); • from random import choice from zope.interface import implements from TwistedQuotes import quoteproto class StaticQuoter: """ Return a static quote. """ implements(quoteproto.IQuoter) def __init__(self, quote): self.quote = quote def getQuote(self): return self.quote class FortuneQuoter: """ Load quotes from a fortune-format file. """ implements(quoteproto.IQuoter) def __init__(self, filenames): self.filenames = filenames def getQuote(self): quoteFile = file(choice(self.filenames)) quotes = quoteFile.read().split(’\n%\n’) quoteFile.close() return choice(quotes) Source listing — quoters.py ; • from zope.interface import Interface from twisted.internet.protocol import Factory, Protocol 1http://docs.python.org/tutorial/modules.html#packages CHAPTER 2. TUTORIAL 20 class IQuoter(Interface): """ An object that returns quotes. """ def getQuote(): """ Return a quote. """ class QOTD(Protocol): def connectionMade(self): self.transport.write(self.factory.quoter.getQuote()+’\r\n’) self.transport.loseConnection() class QOTDFactory(Factory): """ A factory for the Quote of the Day protocol. @type quoter: L{IQuoter} provider @ivar quoter: An object which provides L{IQuoter} which will be used by the L{QOTD} protocol to get quotes to emit. """ protocol = QOTD def __init__(self, quoter): self.quoter = quoter Source listing — quoteproto.py ; 3. Add the TwistedQuotes directory’s parent to your Python path. For example, if the TwistedQuotes direc- tory’s path is /tmp/TwistedQuotes add /tmp to your Python path. On UNIX this would be export PYTHONPATH=/my/stuff:$PYTHONPATH, on Microsoft Windows change the PYTHONPATH variable through the Systems Properites dialog to add /my/stuff; at the beginning. 4. Test your package by trying to import it in the Python interpreter: Python 2.1.3 (#1, Apr 20 2002, 22:45:31) [GCC 2.95.4 20011002 (Debian prerelease)] on linux2 Type "copyright", "credits" or "license" for more information. >>> import TwistedQuotes >>> # No traceback means you’re fine. 2.4 Designing Twisted Applications 2.4.1 Goals This document describes how a good Twisted application is structured. It should be useful for beginning Twisted developers who want to structure their code in a clean, maintainable way that reflects current best practices. Readers will want to be familiar with writing servers (page 9) and clients (page 13) using Twisted. CHAPTER 2. TUTORIAL 21 2.4.2 Example of a modular design: TwistedQuotes TwistedQuotes is a very simple plugin which is a great demonstration of Twisted’s power. It will export a small kernel of functionality – Quote of the Day – which can be accessed through every interface that Twisted supports: web pages, e-mail, instant messaging, a specific Quote of the Day protocol, and more. Set up the project directory See the description of setting up the TwistedQuotes example (page 18). A Look at the Heart of the Application from random import choice from zope.interface import implements from TwistedQuotes import quoteproto class StaticQuoter: """ Return a static quote. """ implements(quoteproto.IQuoter) def __init__(self, quote): self.quote = quote def getQuote(self): return self.quote class FortuneQuoter: """ Load quotes from a fortune-format file. """ implements(quoteproto.IQuoter) def __init__(self, filenames): self.filenames = filenames def getQuote(self): quoteFile = file(choice(self.filenames)) quotes = quoteFile.read().split(’\n%\n’) quoteFile.close() return choice(quotes) Twisted Quotes Central Abstraction — quoters.py This code listing shows us what the Twisted Quotes system is all about. The code doesn’t have any way of talking to the outside world, but it provides a library which is a clear and uncluttered abstraction: “give me the quote of the day”. CHAPTER 2. TUTORIAL 22 Note that this module does not import any Twisted functionality at all! The reason for doing things this way is integration. If your “business objects” are not stuck to your user interface, you can make a module that can integrate those objects with different protocols, GUIs, and file formats. Having such classes provides a way to decouple your components from each other, by allowing each to be used independently. In this manner, Twisted itself has minimal impact on the logic of your program. Although the Twisted “dot products” are highly interoperable, they also follow this approach. You can use them independently because they are not stuck to each other. They communicate in well-defined ways, and only when that communication provides some additional feature. Thus, you can use twisted.web with twisted.enterprise, but neither requires the other, because they are integrated around the concept of Deferreds (page 104). Your Twisted applications should follow this style as much as possible. Have (at least) one module which imple- ments your specific functionality, independent of any user-interface code. Next, we’re going to need to associate this abstract logic with some way of displaying it to the user. We’ll do this by writing a Twisted server protocol, which will respond to the clients that connect to it by sending a quote to the client and then closing the connection. Note: don’t get too focused on the details of this – different ways to interface with the user are 90% of what Twisted does, and there are lots of documents describing the different ways to do it. from zope.interface import Interface from twisted.internet.protocol import Factory, Protocol class IQuoter(Interface): """ An object that returns quotes. """ def getQuote(): """ Return a quote. """ class QOTD(Protocol): def connectionMade(self): self.transport.write(self.factory.quoter.getQuote()+’\r\n’) self.transport.loseConnection() class QOTDFactory(Factory): """ A factory for the Quote of the Day protocol. @type quoter: L{IQuoter} provider @ivar quoter: An object which provides L{IQuoter} which will be used by the L{QOTD} protocol to get quotes to emit. """ protocol = QOTD def __init__(self, quoter): self.quoter = quoter Twisted Quotes Protocol Implementation — quoteproto.py This is a very straightforward Protocol implementation, and the pattern described above is repeated here. The Protocol contains essentially no logic of its own, just enough to tie together an object which can generate quotes (a CHAPTER 2. TUTORIAL 23 Quoter) and an object which can relay bytes to a TCP connection (a Transport). When a client connects to this server, a QOTD instance is created, and its connectionMade method is called. The QOTDFactory’s role is to specify to the Twisted framework how to create a Protocol instance that will handle the connection. Twisted will not instantiate a QOTDFactory; you will do that yourself later, in a twistd plug-in. Note: you can read more specifics of Protocol and Factory in the Writing Servers (page 9) HOWTO. Once we have an abstraction – a Quoter – and we have a mechanism to connect it to the network – the QOTD protocol – the next thing to do is to put the last link in the chain of functionality between abstraction and user. This last link will allow a user to choose a Quoter and configure the protocol. Writing this configuration is covered in the Application HOWTO (page 163). 2.5 Twisted from Scratch, or The Evolution of Finger 2.5.1 Introduction Twisted is a big system. People are often daunted when they approach it. It’s hard to know where to start looking. This guide builds a full-fledged Twisted application from the ground up, using most of the important bits of the framework. There is a lot of code, but don’t be afraid. The application we are looking at is a “finger” service, along the lines of the familiar service traditionally provided by UNIX servers. We will extend this service slightly beyond the standard, in order to demonstrate some of Twisted’s higher-level features. Each section of the tutorial dives straight into applications for various Twisted topics. These topics have their own introductory howtos listed in the core howto index (page ??) and in the documentation for other Twisted projects like Twisted Web and Twisted Words. There are at least three ways to use this tutorial: you may find it useful to read through the rest of the topics listed in the core howto index (page ??) before working through the finger tutorial, work through the finger tutorial and then go back and hit the introductory material that is relevant to the Twisted project you’re working on, or read the introductory material one piece at a time as it comes up in the finger tutorial. 2.5.2 Contents This tutorial is split into eleven parts: 1. The Evolution of Finger: building a simple finger service (page 24) 2. The Evolution of Finger: adding features to the finger service (page 31) 3. The Evolution of Finger: cleaning up the finger code (page 41) 4. The Evolution of Finger: moving to a component based architecture (page 43) 5. The Evolution of Finger: pluggable backends (page 53) 6. The Evolution of Finger: a web frontend (page 64) 7. The Evolution of Finger: Twisted client support using Perspective Broker (page 68) 8. The Evolution of Finger: using a single factory for multiple protocols (page 74) 9. The Evolution of Finger: a Twisted finger client (page 80) 10. The Evolution of Finger: making a finger library (page 82) 11. The Evolution of Finger: configuration and packaging of the finger service (page 85) CHAPTER 2. TUTORIAL 24 2.6 The Evolution of Finger: building a simple finger service 2.6.1 Introduction This is the first part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger (this page). If you’re not familiar with ’finger’ it’s probably because it’s not used as much nowadays as it used to be. Basically, if you run finger nail or finger nail@example.com the target computer spits out some information about the user named nail. For instance: Login: nail Name: Nail Sharp Directory: /home/nail Shell: /usr/bin/sh Last login Wed Mar 31 18:32 2004 (PST) New mail received Thu Apr 1 10:50 2004 (PST) Unread since Thu Apr 1 10:50 2004 (PST) No Plan. If the target computer does not have the fingerd daemon (page 235) running you’ll get a ”Connection Refused” error. Paranoid sysadmins keep fingerd off or limit the output to hinder crackers and harassers. The above format is the standard fingerd default, but an alternate implementation can output anything it wants, such as automated responsibility status for everyone in an organization. You can also define pseudo ”users”, which are essentially key- words. This portion of the tutorial makes use of factories and protocols as introduced in the Writing a TCP Server howto (page 9) and deferreds as introduced in Using Deferreds (page 104) and Generating Deferreds (page 113). Services and applications are discussed in Using the Twisted Application Framework (page 163). By the end of this section of the tutorial, our finger server will answer TCP finger requests on port 1079, and will read data from the web. 2.6.2 Refuse Connections from twisted.internet import reactor reactor.run() Source listing — finger01.py This example only runs the reactor. It will consume almost no CPU resources. As it is not listening on any port, it can’t respond to network requests nothing at all will happen until we interrupt the program. At this point if you run finger nail or telnet localhost 1079, you’ll get a ”Connection refused” error since there’s no daemon running to respond. Not very useful, perhaps but this is the skeleton inside which the Twisted program will grow. As implied above, at various points in this tutorial you’ll want to observe the behavior of the server being devel- oped. Unless you have a finger program which can use an alternate port, the easiest way to do this is with a telnet client. telnet localhost 1079 will connect to the local host on port 1079, where a finger server will eventually be listening. The Reactor You don’t call Twisted, Twisted calls you. The reactor is Twisted’s main event loop, similar to the main loop in other toolkits available in Python (Qt, wx, and Gtk). There is exactly one reactor in any running Twisted application. Once started it loops over and over again, responding to network events and making scheduled calls to code. Note that there are actually several different reactors to choose from; from twisted.internet import reactor returns the current reactor. If you haven’t chosen a reactor class yet, it automatically chooses the default. See the Reactor Basics HOWTO (page 93) for more information. 2.6.3 Do Nothing from twisted.internet import protocol, reactor class FingerProtocol(protocol.Protocol): pass CHAPTER 2. TUTORIAL 25 class FingerFactory(protocol.ServerFactory): protocol = FingerProtocol reactor.listenTCP(1079, FingerFactory()) reactor.run() Source listing — finger02.py Here, reactor.listenTCP opens port 1079. (The number 1079 is a reminder that eventually we want to run on port 79, the standard port for finger servers.) The specified factory, FingerFactory, is used to handle incoming requests on that port. Specifically, for each request, the reactor calls the factory’s buildProtocol method, which in this case causes FingerProtocol to be instantiated. Since the protocol defined here does not actually respond to any events, connections to 1079 will be accepted, but the input ignored. A Factory is the proper place for data that you want to make available to the protocol instances, since the protocol instances are garbage collected when the connection is closed. 2.6.4 Drop Connections from twisted.internet import protocol, reactor class FingerProtocol(protocol.Protocol): def connectionMade(self): self.transport.loseConnection() class FingerFactory(protocol.ServerFactory): protocol = FingerProtocol reactor.listenTCP(1079, FingerFactory()) reactor.run() Source listing — finger03.py Here we add to the protocol the ability to respond to the event of beginning a connection by terminating it. Perhaps not an interesting behavior, but it is already close to behaving according to the letter of the standard finger protocol. After all, there is no requirement to send any data to the remote connection in the standard. The only problem, as far as the standard is concerned, is that we terminate the connection too soon. A client which is slow enough will see his send() of the username result in an error. 2.6.5 Read Username, Drop Connections from twisted.internet import protocol, reactor from twisted.protocols import basic class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): self.transport.loseConnection() class FingerFactory(protocol.ServerFactory): protocol = FingerProtocol reactor.listenTCP(1079, FingerFactory()) reactor.run() Source listing — finger04.py CHAPTER 2. TUTORIAL 26 Here we make FingerProtocol inherit from LineReceiver, so that we get data-based events on a line-by- line basis. We respond to the event of receiving the line with shutting down the connection. If you use a telnet client to interact with this server, the result will look something like this: $ telnet localhost 1079 Trying 127.0.0.1... Connected to localhost.localdomain. alice Connection closed by foreign host. Congratulations, this is the first standard-compliant version of the code. However, usually people actually expect some data about users to be transmitted. 2.6.6 Read Username, Output Error, Drop Connections from twisted.internet import protocol, reactor from twisted.protocols import basic class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): self.transport.write("No such user\r\n") self.transport.loseConnection() class FingerFactory(protocol.ServerFactory): protocol = FingerProtocol reactor.listenTCP(1079, FingerFactory()) reactor.run() Source listing — finger05.py Finally, a useful version. Granted, the usefulness is somewhat limited by the fact that this version only prints out a “No such user” message. It could be used for devastating effect in honey-pots (decoy servers), of course. 2.6.7 Output From Empty Factory # Read username, output from empty factory, drop connections from twisted.internet import protocol, reactor from twisted.protocols import basic class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): self.transport.write(self.factory.getUser(user)+"\r\n") self.transport.loseConnection() class FingerFactory(protocol.ServerFactory): protocol = FingerProtocol def getUser(self, user): return "No such user" reactor.listenTCP(1079, FingerFactory()) reactor.run() Source listing — finger06.py CHAPTER 2. TUTORIAL 27 The same behavior, but finally we see what usefulness the factory has: as something that does not get constructed for every connection, it can be in charge of the user database. In particular, we won’t have to change the protocol if the user database back-end changes. 2.6.8 Output from Non-empty Factory # Read username, output from non-empty factory, drop connections from twisted.internet import protocol, reactor from twisted.protocols import basic class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): self.transport.write(self.factory.getUser(user)+"\r\n") self.transport.loseConnection() class FingerFactory(protocol.ServerFactory): protocol = FingerProtocol def __init__(self, **kwargs): self.users = kwargs def getUser(self, user): return self.users.get(user, "No such user") reactor.listenTCP(1079, FingerFactory(moshez=’Happy and well’)) reactor.run() Source listing — finger07.py Finally, a really useful finger database. While it does not supply information about logged in users, it could be used to distribute things like office locations and internal office numbers. As hinted above, the factory is in charge of keeping the user database: note that the protocol instance has not changed. This is starting to look good: we really won’t have to keep tweaking our protocol. 2.6.9 Use Deferreds # Read username, output from non-empty factory, drop connections # Use deferreds, to minimize synchronicity assumptions from twisted.internet import protocol, reactor, defer from twisted.protocols import basic class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) def onError(err): return ’Internal error in server’ d.addErrback(onError) def writeResponse(message): self.transport.write(message + ’\r\n’) self.transport.loseConnection() d.addCallback(writeResponse) class FingerFactory(protocol.ServerFactory): CHAPTER 2. TUTORIAL 28 protocol = FingerProtocol def __init__(self, **kwargs): self.users = kwargs def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) reactor.listenTCP(1079, FingerFactory(moshez=’Happy and well’)) reactor.run() Source listing — finger08.py But, here we tweak it just for the hell of it. Yes, while the previous version worked, it did assume the result of getUser is always immediately available. But what if instead of an in-memory database, we would have to fetch the result from a remote Oracle server? By allowing getUser to return a Deferred, we make it easier for the data to be retrieved asynchronously so that the CPU can be used for other tasks in the meanwhile. As described in the Deferred HOWTO (page 104), Deferreds allow a program to be driven by events. For instance, if one task in a program is waiting on data, rather than have the CPU (and the program!) idly waiting for that data (a process normally called ’blocking’), the program can perform other operations in the meantime, and waits for some signal that data is ready to be processed before returning to that process. In brief, the code in FingerFactory above creates a Deferred, to which we start to attach callbacks. The de- ferred action in FingerFactory is actually a fast-running expression consisting of one dictionary method, get. Since this action can execute without delay, FingerFactory.getUser uses defer.succeed to create a De- ferred which already has a result, meaning its return value will be passed immediately to the first callback function, which turns out to be FingerProtocol.writeResponse. We’ve also defined an errback (appropriately named FingerProtocol.onError) that will be called instead of writeResponse if something goes wrong. 2.6.10 Run ’finger’ Locally # Read username, output from factory interfacing to OS, drop connections from twisted.internet import protocol, reactor, defer, utils from twisted.protocols import basic class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) def onError(err): return ’Internal error in server’ d.addErrback(onError) def writeResponse(message): self.transport.write(message + ’\r\n’) self.transport.loseConnection() d.addCallback(writeResponse) class FingerFactory(protocol.ServerFactory): protocol = FingerProtocol def getUser(self, user): return utils.getProcessOutput("finger", [user]) reactor.listenTCP(1079, FingerFactory()) reactor.run() CHAPTER 2. TUTORIAL 29 Source listing — finger09.py This example also makes use of a Deferred. twisted.internet.utils.getProcessOutput is a non- blocking version of Python’s commands.getoutput: it runs a shell command (finger, in this case) and captures its standard output. However, getProcessOutput returns a Deferred instead of the output itself. Since Finger Protocol.lineReceived is already expecting a Deferred to be returned by getUser, it doesn’t need to be changed, and it returns the standard output as the finger result. Note that in this case the shell’s built-in finger command is simply run with whatever arguments it is given. This is probably insecure, so you probably don’t want a real server to do this without a lot more validation of the user input. This will do exactly what the standard version of the finger server does. 2.6.11 Read Status from the Web The web. That invention which has infiltrated homes around the world finally gets through to our invention. In this case we use the built-in Twisted web client via twisted.web.client.getPage, a non-blocking version of Python’s urllib2.urlopen(URL).read(). Like getProcessOutput it returns a Deferred which will be called back with a string, and can thus be used as a drop-in replacement. Thus, we have examples of three different database back-ends, none of which change the protocol class. In fact, we will not have to change the protocol again until the end of this tutorial: we have achieved, here, one truly usable class. # Read username, output from factory interfacing to web, drop connections from twisted.internet import protocol, reactor, defer, utils from twisted.protocols import basic from twisted.web import client class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) def onError(err): return ’Internal error in server’ d.addErrback(onError) def writeResponse(message): self.transport.write(message + ’\r\n’) self.transport.loseConnection() d.addCallback(writeResponse) class FingerFactory(protocol.ServerFactory): protocol = FingerProtocol def __init__(self, prefix): self.prefix=prefix def getUser(self, user): return client.getPage(self.prefix+user) reactor.listenTCP(1079, FingerFactory(prefix=’http://livejournal.com/˜’)) reactor.run() Source listing — finger10.py CHAPTER 2. TUTORIAL 30 2.6.12 Use Application Up until now, we faked. We kept using port 1079, because really, who wants to run a finger server with root privileges? Well, the common solution is “privilege shedding”: after binding to the network, become a different, less privileged user. We could have done it ourselves, but Twisted has a built-in way to do it. We will create a snippet as above, but now we will define an application object. That object will have uid and gid attributes. When running it (later we will see how) it will bind to ports, shed privileges and then run. Read on to find out how to run this code using the twistd utility. 2.6.13 twistd This is how to run “Twisted Applications” files which define an ’application’. A daemon is expected to adhere to certain behavioral standards so that standard tools can stop/start/query them. If a Twisted application is run via twistd, the TWISTed Daemonizer, all this behavioral stuff will be handled for you. twistd does everything a daemon can be expected to shuts down stdin/stdout/stderr, disconnects from the terminal and can even change runtime directory, or even the root filesystems. In short, it does everything so the Twisted application developer can concentrate on writing his networking code. root% twistd -ny finger11.tac # just like before root% twistd -y finger11.tac # daemonize, keep pid in twistd.pid root% twistd -y finger11.tac --pidfile=finger.pid root% twistd -y finger11.tac --rundir=/ root% twistd -y finger11.tac --chroot=/var root% twistd -y finger11.tac -l /var/log/finger.log root% twistd -y finger11.tac --syslog # just log to syslog root% twistd -y finger11.tac --syslog --prefix=twistedfinger # use given prefix There are several ways to tell twistd where your application is; here we show how it is done using the application global variable in a Python source file (a Twisted Application Configuration (page ??) file). # Read username, output from non-empty factory, drop connections # Use deferreds, to minimize synchronicity assumptions # Write application. Save in ’finger.tpy’ from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.protocols import basic class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) def onError(err): return ’Internal error in server’ d.addErrback(onError) def writeResponse(message): self.transport.write(message + ’\r\n’) self.transport.loseConnection() d.addCallback(writeResponse) class FingerFactory(protocol.ServerFactory): protocol = FingerProtocol def __init__(self, **kwargs): self.users = kwargs def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) CHAPTER 2. TUTORIAL 31 application = service.Application(’finger’, uid=1, gid=1) factory = FingerFactory(moshez=’Happy and well’) internet.TCPServer(79, factory).setServiceParent( service.IServiceCollection(application)) Source listing — finger11.tac Instead of using reactor.listenTCP as in the above examples, here we are using its application-aware coun- terpart, internet.TCPServer. Notice that when it is instantiated, the application object itself does not reference either the protocol or the factory. Any services (such as TCPServer) which have the application as their parent will be started when the application is started by twistd. The application object is more useful for returning an object that supports the IService, IServiceCollection, IProcess, and sob.IPersistable interfaces with the given parameters; we’ll be seeing these in the next part of the tutorial. As the parent of the TCPServer we opened, the application lets us manage the TCPServer. With the daemon running on the standard finger port, you can test it with the standard finger command: finger moshez. 2.7 The Evolution of Finger: adding features to the finger service 2.7.1 Introduction This is the second part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger (page 23). In this section of the tutorial, our finger server will continue to sprout features: the ability for users to set finger announces, and using our finger service to send those announcements on the web, on IRC and over XML-RPC. Resources and XML-RPC are introduced in the Web Applications portion of the Twisted Web howto (page ??). More examples using twisted.words.protocols.irc can be found in Writing a TCP Client (page 13) and the Twisted Words examples (page ??). 2.7.2 Setting Message By Local Users Now that port 1079 is free, maybe we can use it with a different server, one which will let people set their messages. It does no access control, so anyone who can login to the machine can set any message. We assume this is the desired behavior in our case. Testing it can be done by simply: % nc localhost 1079 # or telnet localhost 1079 moshez Giving a tutorial now, sorry! ˆD # But let’s try and fix setting away messages, shall we? from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.protocols import basic class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) def onError(err): return ’Internal error in server’ d.addErrback(onError) def writeResponse(message): self.transport.write(message + ’\r\n’) self.transport.loseConnection() d.addCallback(writeResponse) CHAPTER 2. TUTORIAL 32 class FingerFactory(protocol.ServerFactory): protocol = FingerProtocol def __init__(self, **kwargs): self.users = kwargs def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) class FingerSetterProtocol(basic.LineReceiver): def connectionMade(self): self.lines = [] def lineReceived(self, line): self.lines.append(line) def connectionLost(self, reason): user = self.lines[0] status = self.lines[1] self.factory.setUser(user, status) class FingerSetterFactory(protocol.ServerFactory): protocol = FingerSetterProtocol def __init__(self, fingerFactory): self.fingerFactory = fingerFactory def setUser(self, user, status): self.fingerFactory.users[user] = status ff = FingerFactory(moshez=’Happy and well’) fsf = FingerSetterFactory(ff) application = service.Application(’finger’, uid=1, gid=1) serviceCollection = service.IServiceCollection(application) internet.TCPServer(79,ff).setServiceParent(serviceCollection) internet.TCPServer(1079,fsf).setServiceParent(serviceCollection) Source listing — finger12.tac This program has two protocol-factory-TCPServer pairs, which are both child services of the application. Specifi- cally, the setServiceParent method is used to define the two TCPServer services as children of application, which implements IServiceCollection. Both services are thus started with the application. 2.7.3 Use Services to Make Dependencies Sane The previous version had the setter poke at the innards of the finger factory. This strategy is usually not a good idea: this version makes both factories symmetric by making them both look at a single object. Services are useful for when an object is needed which is not related to a specific network server. Here, we define a common service class with methods that will create factories on the fly. The service also contains methods the factories will depend on. The factory-creation methods, getFingerFactory and getFingerSetterFactory, follow this pattern: 1. Instantiate a generic server factory, twisted.internet.protocol.ServerFactory. 2. Set the protocol class, just like our factory class would have. CHAPTER 2. TUTORIAL 33 3. Copy a service method to the factory as a function attribute. The function won’t have access to the factory’s self, but that’s OK because as a bound method it has access to the service’s self, which is what it needs. For getUser, a custom method defined in the service gets copied. For setUser, a standard method of the users dictionary is copied. Thus, we stopped subclassing: the service simply puts useful methods and attributes inside the factories. We are getting better at protocol design: none of our protocol classes had to be changed, and neither will have to change until the end of the tutorial. As an application service , this new finger service implements the IService interface and can be started and stopped in a standardized manner. We’ll make use of this in the next example. # Fix asymmetry from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.protocols import basic class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) def onError(err): return ’Internal error in server’ d.addErrback(onError) def writeResponse(message): self.transport.write(message + ’\r\n’) self.transport.loseConnection() d.addCallback(writeResponse) class FingerSetterProtocol(basic.LineReceiver): def connectionMade(self): self.lines = [] def lineReceived(self, line): self.lines.append(line) def connectionLost(self,reason): user = self.lines[0] status = self.lines[1] self.factory.setUser(user, status) class FingerService(service.Service): def __init__(self, **kwargs): self.users = kwargs def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def setUser(self, user, status): self.users[user] = status def getFingerFactory(self): f = protocol.ServerFactory() f.protocol = FingerProtocol f.getUser = self.getUser return f def getFingerSetterFactory(self): CHAPTER 2. TUTORIAL 34 f = protocol.ServerFactory() f.protocol = FingerSetterProtocol f.setUser = self.setUser return f application = service.Application(’finger’, uid=1, gid=1) f = FingerService(moshez=’Happy and well’) serviceCollection = service.IServiceCollection(application) internet.TCPServer(79,f.getFingerFactory() ).setServiceParent(serviceCollection) internet.TCPServer(1079,f.getFingerSetterFactory() ).setServiceParent(serviceCollection) Source listing — finger13.tac 2.7.4 Read Status File This version shows how, instead of just letting users set their messages, we can read those from a centrally managed file. We cache results, and every 30 seconds we refresh it. Services are useful for such scheduled tasks. moshez: happy and well shawn: alive sample /etc/users file — etc.users # Read from file from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.protocols import basic class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) def onError(err): return ’Internal error in server’ d.addErrback(onError) def writeResponse(message): self.transport.write(message + ’\r\n’) self.transport.loseConnection() d.addCallback(writeResponse) class FingerService(service.Service): def __init__(self, filename): self.users = {} self.filename = filename self._read() def _read(self): for line in file(self.filename): user, status = line.split(’:’, 1) user = user.strip() status = status.strip() self.users[user] = status self.call = reactor.callLater(30, self._read) CHAPTER 2. TUTORIAL 35 def startService(self): self._read() service.Service.startService(self) def stopService(self): service.Service.stopService(self) self.call.cancel() def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getFingerFactory(self): f = protocol.ServerFactory() f.protocol = FingerProtocol f.getUser = self.getUser return f application = service.Application(’finger’, uid=1, gid=1) f = FingerService(’/etc/users’) finger = internet.TCPServer(79, f.getFingerFactory()) finger.setServiceParent(service.IServiceCollection(application)) f.setServiceParent(service.IServiceCollection(application)) Source listing — finger14.tac Since this verison is reading data from a file (and refreshing the data every 30 seconds), there is no Finger SetterFactory and thus nothing listening on port 1079. Here we override the standard startService and stopService hooks in the Finger service, which is set up as a child service of the application in the last line of the code. startService calls read, the function responsible for reading the data; reactor.callLater is then used to schedule it to run again after thirty seconds every time it is called. reactor.callLater returns an object that lets us cancel the scheduled run in stopService using its cancel method. 2.7.5 Announce on Web, Too The same kind of service can also produce things useful for other protocols. For example, in twisted.web, the factory itself (Site) is almost never subclassed instead, it is given a resource, which represents the tree of resources available via URLs. That hierarchy is navigated by Site and overriding it dynamically is possible with getChild. To integrate this into the Finger application (just because we can), we set up a new TCPServer that calls the Site factory and retrieves resources via a new function of FingerService named getResource. This function specifically returns a Resource object with an overridden getChild method. # Read from file, announce on the web! from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.protocols import basic from twisted.web import resource, server, static import cgi class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) def onError(err): return ’Internal error in server’ CHAPTER 2. TUTORIAL 36 d.addErrback(onError) def writeResponse(message): self.transport.write(message + ’\r\n’) self.transport.loseConnection() d.addCallback(writeResponse) class FingerResource(resource.Resource): def __init__(self, users): self.users = users resource.Resource.__init__(self) # we treat the path as the username def getChild(self, username, request): """ ’username’ is a string. ’request’ is a ’twisted.web.server.Request’. """ messagevalue = self.users.get(username) username = cgi.escape(username) if messagevalue is not None: messagevalue = cgi.escape(messagevalue) text = ’

%s

%s

’ % (username,messagevalue) else: text = ’

%s

No such user

’ % username return static.Data(text, ’text/html’) class FingerService(service.Service): def __init__(self, filename): self.filename = filename self.users = {} self._read() def _read(self): self.users.clear() for line in file(self.filename): user, status = line.split(’:’, 1) user = user.strip() status = status.strip() self.users[user] = status self.call = reactor.callLater(30, self._read) def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getFingerFactory(self): f = protocol.ServerFactory() f.protocol = FingerProtocol f.getUser = self.getUser return f def getResource(self): r = FingerResource(self.users) return r application = service.Application(’finger’, uid=1, gid=1) CHAPTER 2. TUTORIAL 37 f = FingerService(’/etc/users’) serviceCollection = service.IServiceCollection(application) internet.TCPServer(79, f.getFingerFactory() ).setServiceParent(serviceCollection) internet.TCPServer(8000, server.Site(f.getResource()) ).setServiceParent(serviceCollection) Source listing — finger15.tac 2.7.6 Announce on IRC, Too This is the first time there is client code. IRC clients often act a lot like servers: responding to events from the network. The reconnecting client factory will make sure that severed links will get re-established, with intelligent tweaked exponential back-off algorithms. The IRC client itself is simple: the only real hack is getting the nickname from the factory in connectionMade. # Read from file, announce on the web, irc from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.words.protocols import irc from twisted.protocols import basic from twisted.web import resource, server, static import cgi class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) def onError(err): return ’Internal error in server’ d.addErrback(onError) def writeResponse(message): self.transport.write(message + ’\r\n’) self.transport.loseConnection() d.addCallback(writeResponse) class IRCReplyBot(irc.IRCClient): def connectionMade(self): self.nickname = self.factory.nickname irc.IRCClient.connectionMade(self) def privmsg(self, user, channel, msg): user = user.split(’!’)[0] if self.nickname.lower() == channel.lower(): d = self.factory.getUser(msg) def onError(err): return ’Internal error in server’ d.addErrback(onError) def writeResponse(message): irc.IRCClient.msg(self, user, msg+’: ’+message) d.addCallback(writeResponse) CHAPTER 2. TUTORIAL 38 class FingerService(service.Service): def __init__(self, filename): self.filename = filename self.users = {} self._read() def _read(self): self.users.clear() for line in file(self.filename): user, status = line.split(’:’, 1) user = user.strip() status = status.strip() self.users[user] = status self.call = reactor.callLater(30, self._read) def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getFingerFactory(self): f = protocol.ServerFactory() f.protocol = FingerProtocol f.getUser = self.getUser return f def getResource(self): r = resource.Resource() r.getChild = (lambda path, request: static.Data(’

%s

%s

’ % tuple(map(cgi.escape, [path,self.users.get(path, "No such user

usage: site/user")])), ’text/html’)) return r def getIRCBot(self, nickname): f = protocol.ReconnectingClientFactory() f.protocol = IRCReplyBot f.nickname = nickname f.getUser = self.getUser return f application = service.Application(’finger’, uid=1, gid=1) f = FingerService(’/etc/users’) serviceCollection = service.IServiceCollection(application) internet.TCPServer(79, f.getFingerFactory() ).setServiceParent(serviceCollection) internet.TCPServer(8000, server.Site(f.getResource()) ).setServiceParent(serviceCollection) internet.TCPClient(’irc.freenode.org’, 6667, f.getIRCBot(’fingerbot’) ).setServiceParent(serviceCollection) Source listing — finger16.tac FingerService now has another new function, getIRCbot, which returns the ReconnectingClient Factory. This factory in turn will instantiate the IRCReplyBot protocol. The IRCBot is configured in the last line to connect to irc.freenode.org with a nickname of fingerbot. CHAPTER 2. TUTORIAL 39 By overriding irc.IRCClient.connectionMade, IRCReplyBot can access the nickname attribute of the factory that instantiated it. 2.7.7 Add XML-RPC Support In Twisted, XML-RPC support is handled just as though it was another resource. That resource will still support GET calls normally through render(), but that is usually left unimplemented. Note that it is possible to return deferreds from XML-RPC methods. The client, of course, will not get the answer until the deferred is triggered. # Read from file, announce on the web, irc, xml-rpc from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.words.protocols import irc from twisted.protocols import basic from twisted.web import resource, server, static, xmlrpc import cgi class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) def onError(err): return ’Internal error in server’ d.addErrback(onError) def writeResponse(message): self.transport.write(message + ’\r\n’) self.transport.loseConnection() d.addCallback(writeResponse) class IRCReplyBot(irc.IRCClient): def connectionMade(self): self.nickname = self.factory.nickname irc.IRCClient.connectionMade(self) def privmsg(self, user, channel, msg): user = user.split(’!’)[0] if self.nickname.lower() == channel.lower(): d = self.factory.getUser(msg) def onError(err): return ’Internal error in server’ d.addErrback(onError) def writeResponse(message): irc.IRCClient.msg(self, user, msg+’: ’+message) d.addCallback(writeResponse) class FingerService(service.Service): def __init__(self, filename): self.filename = filename self.users = {} self._read() def _read(self): self.users.clear() for line in file(self.filename): user, status = line.split(’:’, 1) CHAPTER 2. TUTORIAL 40 user = user.strip() status = status.strip() self.users[user] = status self.call = reactor.callLater(30, self._read) def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getFingerFactory(self): f = protocol.ServerFactory() f.protocol = FingerProtocol f.getUser = self.getUser return f def getResource(self): r = resource.Resource() r.getChild = (lambda path, request: static.Data(’

%s

%s

’ % tuple(map(cgi.escape, [path,self.users.get(path, "No such user")])), ’text/html’)) x = xmlrpc.XMLRPC() x.xmlrpc_getUser = self.getUser r.putChild(’RPC2’, x) return r def getIRCBot(self, nickname): f = protocol.ReconnectingClientFactory() f.protocol = IRCReplyBot f.nickname = nickname f.getUser = self.getUser return f application = service.Application(’finger’, uid=1, gid=1) f = FingerService(’/etc/users’) serviceCollection = service.IServiceCollection(application) internet.TCPServer(79, f.getFingerFactory() ).setServiceParent(serviceCollection) internet.TCPServer(8000, server.Site(f.getResource()) ).setServiceParent(serviceCollection) internet.TCPClient(’irc.freenode.org’, 6667, f.getIRCBot(’fingerbot’) ).setServiceParent(serviceCollection) Source listing — finger17.tac Instead of a web browser, we can test the XMLRPC finger using a simple client based on Python’s built-in xmlrpclib, which will access the resource we’ve made available at localhost/RPC2. # testing xmlrpc finger import xmlrpclib server = xmlrpclib.Server(’http://127.0.0.1:8000/RPC2’) print server.getUser(’moshez’) Source listing — fingerXRclient.py CHAPTER 2. TUTORIAL 41 2.8 The Evolution of Finger: cleaning up the finger code 2.8.1 Introduction This is the third part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger (page 23). In this section of the tutorial, we’ll clean up our code so that it is closer to a readable and extendable style. 2.8.2 Write Readable Code The last version of the application had a lot of hacks. We avoided sub-classing, didn’t support things like user listings over the web, and removed all blank lines – all in the interest of code which is shorter. Here we take a step back, subclass what is more naturally a subclass, make things which should take multiple lines take them, etc. This shows a much better style of developing Twisted applications, though the hacks in the previous stages are sometimes used in throw-away prototypes. # Do everything properly from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.words.protocols import irc from twisted.protocols import basic from twisted.web import resource, server, static, xmlrpc import cgi def catchError(err): return "Internal error in server" class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) d.addErrback(catchError) def writeValue(value): self.transport.write(value+’\r\n’) self.transport.loseConnection() d.addCallback(writeValue) class IRCReplyBot(irc.IRCClient): def connectionMade(self): self.nickname = self.factory.nickname irc.IRCClient.connectionMade(self) def privmsg(self, user, channel, msg): user = user.split(’!’)[0] if self.nickname.lower() == channel.lower(): d = self.factory.getUser(msg) d.addErrback(catchError) d.addCallback(lambda m: "Status of %s: %s" % (msg, m)) d.addCallback(lambda m: self.msg(user, m)) class UserStatusTree(resource.Resource): def __init__(self, service): resource.Resource.__init__(self) self.service = service def render_GET(self, request): CHAPTER 2. TUTORIAL 42 d = self.service.getUsers() def formatUsers(users): l = [’
  • %s
  • ’ % (user, user) for user in users] return ’
      ’+’’.join(l)+’
    ’ d.addCallback(formatUsers) d.addCallback(request.write) d.addCallback(lambda _: request.finish()) return server.NOT_DONE_YET def getChild(self, path, request): if path=="": return UserStatusTree(self.service) else: return UserStatus(path, self.service) class UserStatus(resource.Resource): def __init__(self, user, service): resource.Resource.__init__(self) self.user = user self.service = service def render_GET(self, request): d = self.service.getUser(self.user) d.addCallback(cgi.escape) d.addCallback(lambda m: ’

    %s

    ’%self.user+’

    %s

    ’%m) d.addCallback(request.write) d.addCallback(lambda _: request.finish()) return server.NOT_DONE_YET class UserStatusXR(xmlrpc.XMLRPC): def __init__(self, service): xmlrpc.XMLRPC.__init__(self) self.service = service def xmlrpc_getUser(self, user): return self.service.getUser(user) class FingerService(service.Service): def __init__(self, filename): self.filename = filename self.users = {} self._read() def _read(self): self.users.clear() for line in file(self.filename): user, status = line.split(’:’, 1) user = user.strip() status = status.strip() self.users[user] = status CHAPTER 2. TUTORIAL 43 self.call = reactor.callLater(30, self._read) def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getUsers(self): return defer.succeed(self.users.keys()) def getFingerFactory(self): f = protocol.ServerFactory() f.protocol = FingerProtocol f.getUser = self.getUser return f def getResource(self): r = UserStatusTree(self) x = UserStatusXR(self) r.putChild(’RPC2’, x) return r def getIRCBot(self, nickname): f = protocol.ReconnectingClientFactory() f.protocol = IRCReplyBot f.nickname = nickname f.getUser = self.getUser return f application = service.Application(’finger’, uid=1, gid=1) f = FingerService(’/etc/users’) serviceCollection = service.IServiceCollection(application) internet.TCPServer(79, f.getFingerFactory() ).setServiceParent(serviceCollection) internet.TCPServer(8000, server.Site(f.getResource()) ).setServiceParent(serviceCollection) internet.TCPClient(’irc.freenode.org’, 6667, f.getIRCBot(’fingerbot’) ).setServiceParent(serviceCollection) Source listing — finger18.tac 2.9 The Evolution of Finger: moving to a component based architecture 2.9.1 Introduction This is the fourth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger (page 23). In this section of the tutorial, we’ll move our code to a component architecture so that adding new features is trivial. See Interfaces and Adapters (page 151) for a more complete discussion of components. 2.9.2 Write Maintainable Code In the last version, the service class was three times longer than any other class, and was hard to understand. This was because it turned out to have multiple responsibilities. It had to know how to access user information, by rereading the file every half minute, but also how to display itself in a myriad of protocols. Here, we used the component-based architecture that Twisted provides to achieve a separation of concerns. All the service is responsible for, now, is CHAPTER 2. TUTORIAL 44 supporting getUser/getUsers. It declares its support via a call to zope.interface.implements. Then, adapters are used to make this service look like an appropriate class for various things: for supplying a finger factory to TCPServer, for supplying a resource to site’s constructor, and to provide an IRC client factory for TCPClient. All the adapters use are the methods in FingerService they are declared to use: getUser/getUsers. We could, of course, skip the interfaces and let the configuration code use things like FingerFactoryFromService(f) directly. However, using interfaces provides the same flexibility inheritance gives: future subclasses can override the adapters. # Do everything properly, and componentize from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.words.protocols import irc from twisted.protocols import basic from twisted.python import components from twisted.web import resource, server, static, xmlrpc from zope.interface import Interface, implements import cgi class IFingerService(Interface): def getUser(user): """Return a deferred returning a string""" def getUsers(): """Return a deferred returning a list of strings""" class IFingerSetterService(Interface): def setUser(user, status): """Set the user’s status to something""" def catchError(err): return "Internal error in server" class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) d.addErrback(catchError) def writeValue(value): self.transport.write(value+’\r\n’) self.transport.loseConnection() d.addCallback(writeValue) class IFingerFactory(Interface): def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerFactoryFromService(protocol.ServerFactory): implements(IFingerFactory) protocol = FingerProtocol CHAPTER 2. TUTORIAL 45 def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory) class FingerSetterProtocol(basic.LineReceiver): def connectionMade(self): self.lines = [] def lineReceived(self, line): self.lines.append(line) def connectionLost(self, reason): if len(self.lines) == 2: self.factory.setUser(*self.lines) class IFingerSetterFactory(Interface): def setUser(user, status): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerSetterFactoryFromService(protocol.ServerFactory): implements(IFingerSetterFactory) protocol = FingerSetterProtocol def __init__(self, service): self.service = service def setUser(self, user, status): self.service.setUser(user, status) components.registerAdapter(FingerSetterFactoryFromService, IFingerSetterService, IFingerSetterFactory) class IRCReplyBot(irc.IRCClient): def connectionMade(self): self.nickname = self.factory.nickname irc.IRCClient.connectionMade(self) def privmsg(self, user, channel, msg): CHAPTER 2. TUTORIAL 46 user = user.split(’!’)[0] if self.nickname.lower() == channel.lower(): d = self.factory.getUser(msg) d.addErrback(catchError) d.addCallback(lambda m: "Status of %s: %s" % (msg, m)) d.addCallback(lambda m: self.msg(user, m)) class IIRCClientFactory(Interface): """ @ivar nickname """ def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol""" class IRCClientFactoryFromService(protocol.ClientFactory): implements(IIRCClientFactory) protocol = IRCReplyBot nickname = None def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(IRCClientFactoryFromService, IFingerService, IIRCClientFactory) class UserStatusTree(resource.Resource): implements(resource.IResource) def __init__(self, service): resource.Resource.__init__(self) self.service = service self.putChild(’RPC2’, UserStatusXR(self.service)) def render_GET(self, request): d = self.service.getUsers() def formatUsers(users): l = [’
  • %s
  • ’ % (user, user) for user in users] return ’
      ’+’’.join(l)+’
    ’ d.addCallback(formatUsers) d.addCallback(request.write) d.addCallback(lambda _: request.finish()) return server.NOT_DONE_YET CHAPTER 2. TUTORIAL 47 def getChild(self, path, request): if path=="": return UserStatusTree(self.service) else: return UserStatus(path, self.service) components.registerAdapter(UserStatusTree, IFingerService, resource.IResource) class UserStatus(resource.Resource): def __init__(self, user, service): resource.Resource.__init__(self) self.user = user self.service = service def render_GET(self, request): d = self.service.getUser(self.user) d.addCallback(cgi.escape) d.addCallback(lambda m: ’

    %s

    ’%self.user+’

    %s

    ’%m) d.addCallback(request.write) d.addCallback(lambda _: request.finish()) return server.NOT_DONE_YET class UserStatusXR(xmlrpc.XMLRPC): def __init__(self, service): xmlrpc.XMLRPC.__init__(self) self.service = service def xmlrpc_getUser(self, user): return self.service.getUser(user) class FingerService(service.Service): implements(IFingerService) def __init__(self, filename): self.filename = filename self.users = {} self._read() def _read(self): self.users.clear() for line in file(self.filename): user, status = line.split(’:’, 1) user = user.strip() status = status.strip() self.users[user] = status self.call = reactor.callLater(30, self._read) def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) CHAPTER 2. TUTORIAL 48 def getUsers(self): return defer.succeed(self.users.keys()) application = service.Application(’finger’, uid=1, gid=1) f = FingerService(’/etc/users’) serviceCollection = service.IServiceCollection(application) internet.TCPServer(79, IFingerFactory(f) ).setServiceParent(serviceCollection) internet.TCPServer(8000, server.Site(resource.IResource(f)) ).setServiceParent(serviceCollection) i = IIRCClientFactory(f) i.nickname = ’fingerbot’ internet.TCPClient(’irc.freenode.org’, 6667, i ).setServiceParent(serviceCollection) Source listing — finger19.tac 2.9.3 Advantages of Latest Version • Readable – each class is short • Maintainable – each class knows only about interfaces • Dependencies between code parts are minimized • Example: writing a new IFingerService is easy class IFingerSetterService(Interface): def setUser(user, status): """Set the user’s status to something""" # Advantages of latest version class MemoryFingerService(service.Service): implements([IFingerService, IFingerSetterService]) def __init__(self, **kwargs): self.users = kwargs def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getUsers(self): return defer.succeed(self.users.keys()) def setUser(self, user, status): self.users[user] = status f = MemoryFingerService(moshez=’Happy and well’) serviceCollection = service.IServiceCollection(application) internet.TCPServer(1079, IFingerSetterFactory(f), interface=’127.0.0.1’ ).setServiceParent(serviceCollection) CHAPTER 2. TUTORIAL 49 Source listing — finger19a changes.py Full source code here: # Do everything properly, and componentize from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.words.protocols import irc from twisted.protocols import basic from twisted.python import components from twisted.web import resource, server, static, xmlrpc from zope.interface import Interface, implements import cgi class IFingerService(Interface): def getUser(user): """Return a deferred returning a string""" def getUsers(): """Return a deferred returning a list of strings""" class IFingerSetterService(Interface): def setUser(user, status): """Set the user’s status to something""" def catchError(err): return "Internal error in server" class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) d.addErrback(catchError) def writeValue(value): self.transport.write(value+’\r\n’) self.transport.loseConnection() d.addCallback(writeValue) class IFingerFactory(Interface): def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerFactoryFromService(protocol.ServerFactory): implements(IFingerFactory) protocol = FingerProtocol def __init__(self, service): CHAPTER 2. TUTORIAL 50 self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory) class FingerSetterProtocol(basic.LineReceiver): def connectionMade(self): self.lines = [] def lineReceived(self, line): self.lines.append(line) def connectionLost(self, reason): if len(self.lines) == 2: self.factory.setUser(*self.lines) class IFingerSetterFactory(Interface): def setUser(user, status): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerSetterFactoryFromService(protocol.ServerFactory): implements(IFingerSetterFactory) protocol = FingerSetterProtocol def __init__(self, service): self.service = service def setUser(self, user, status): self.service.setUser(user, status) components.registerAdapter(FingerSetterFactoryFromService, IFingerSetterService, IFingerSetterFactory) class IRCReplyBot(irc.IRCClient): def connectionMade(self): self.nickname = self.factory.nickname irc.IRCClient.connectionMade(self) def privmsg(self, user, channel, msg): user = user.split(’!’)[0] if self.nickname.lower() == channel.lower(): CHAPTER 2. TUTORIAL 51 d = self.factory.getUser(msg) d.addErrback(catchError) d.addCallback(lambda m: "Status of %s: %s" % (msg, m)) d.addCallback(lambda m: self.msg(user, m)) class IIRCClientFactory(Interface): """ @ivar nickname """ def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol""" class IRCClientFactoryFromService(protocol.ClientFactory): implements(IIRCClientFactory) protocol = IRCReplyBot nickname = None def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(IRCClientFactoryFromService, IFingerService, IIRCClientFactory) class UserStatusTree(resource.Resource): implements(resource.IResource) def __init__(self, service): resource.Resource.__init__(self) self.service = service self.putChild(’RPC2’, UserStatusXR(self.service)) def render_GET(self, request): d = self.service.getUsers() def formatUsers(users): l = [’
  • %s
  • ’ % (user, user) for user in users] return ’
      ’+’’.join(l)+’
    ’ d.addCallback(formatUsers) d.addCallback(request.write) d.addCallback(lambda _: request.finish()) return server.NOT_DONE_YET def getChild(self, path, request): CHAPTER 2. TUTORIAL 52 if path=="": return UserStatusTree(self.service) else: return UserStatus(path, self.service) components.registerAdapter(UserStatusTree, IFingerService, resource.IResource) class UserStatus(resource.Resource): def __init__(self, user, service): resource.Resource.__init__(self) self.user = user self.service = service def render_GET(self, request): d = self.service.getUser(self.user) d.addCallback(cgi.escape) d.addCallback(lambda m: ’

    %s

    ’%self.user+’

    %s

    ’%m) d.addCallback(request.write) d.addCallback(lambda _: request.finish()) return server.NOT_DONE_YET class UserStatusXR(xmlrpc.XMLRPC): def __init__(self, service): xmlrpc.XMLRPC.__init__(self) self.service = service def xmlrpc_getUser(self, user): return self.service.getUser(user) class MemoryFingerService(service.Service): implements([IFingerService, IFingerSetterService]) def __init__(self, **kwargs): self.users = kwargs def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getUsers(self): return defer.succeed(self.users.keys()) def setUser(self, user, status): self.users[user] = status application = service.Application(’finger’, uid=1, gid=1) f = MemoryFingerService(moshez=’Happy and well’) serviceCollection = service.IServiceCollection(application) internet.TCPServer(79, IFingerFactory(f) ).setServiceParent(serviceCollection) internet.TCPServer(8000, server.Site(resource.IResource(f)) CHAPTER 2. TUTORIAL 53 ).setServiceParent(serviceCollection) i = IIRCClientFactory(f) i.nickname = ’fingerbot’ internet.TCPClient(’irc.freenode.org’, 6667, i ).setServiceParent(serviceCollection) internet.TCPServer(1079, IFingerSetterFactory(f), interface=’127.0.0.1’ ).setServiceParent(serviceCollection) Source listing — finger19a.tac 2.9.4 Aspect-Oriented Programming At last, an example of aspect-oriented programming that isn’t about logging or timing. This code is actually useful! Watch how aspect-oriented programming helps you write less code and have fewer dependencies! 2.10 The Evolution of Finger: pluggable backends 2.10.1 Introduction This is the fifth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger (page 23). In this part we will add new several new backends to our finger service using the component-based architecture developed in The Evolution of Finger: moving to a component based architecture (page 43). This will show just how convenient it is to implement new back-ends when we move to a component based architecture. Note that here we also use an interface we previously wrote, FingerSetterFactory, by supporting one single method. We manage to preserve the service’s ignorance of the network. 2.10.2 Another Back-end from twisted.internet import protocol, reactor, defer, utils import pwd # Another back-end class LocalFingerService(service.Service): implements(IFingerService) def getUser(self, user): # need a local finger daemon running for this to work return utils.getProcessOutput("finger", [user]) def getUsers(self): return defer.succeed([]) f = LocalFingerService() Source listing — finger19b changes.py Full source code here: # Do everything properly, and componentize from twisted.application import internet, service from twisted.internet import protocol, reactor, defer, utils from twisted.words.protocols import irc from twisted.protocols import basic CHAPTER 2. TUTORIAL 54 from twisted.python import components from twisted.web import resource, server, static, xmlrpc from zope.interface import Interface, implements import cgi import pwd class IFingerService(Interface): def getUser(user): """Return a deferred returning a string""" def getUsers(): """Return a deferred returning a list of strings""" class IFingerSetterService(Interface): def setUser(user, status): """Set the user’s status to something""" class IFingerSetterService(Interface): def setUser(user, status): """Set the user’s status to something""" def catchError(err): return "Internal error in server" class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) d.addErrback(catchError) def writeValue(value): self.transport.write(value+’\r\n’) self.transport.loseConnection() d.addCallback(writeValue) class IFingerFactory(Interface): def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerFactoryFromService(protocol.ServerFactory): implements(IFingerFactory) protocol = FingerProtocol def __init__(self, service): self.service = service def getUser(self, user): CHAPTER 2. TUTORIAL 55 return self.service.getUser(user) components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory) class FingerSetterProtocol(basic.LineReceiver): def connectionMade(self): self.lines = [] def lineReceived(self, line): self.lines.append(line) def connectionLost(self, reason): if len(self.lines) == 2: self.factory.setUser(*self.lines) class IFingerSetterFactory(Interface): def setUser(user, status): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerSetterFactoryFromService(protocol.ServerFactory): implements(IFingerSetterFactory) protocol = FingerSetterProtocol def __init__(self, service): self.service = service def setUser(self, user, status): self.service.setUser(user, status) components.registerAdapter(FingerSetterFactoryFromService, IFingerSetterService, IFingerSetterFactory) class IRCReplyBot(irc.IRCClient): def connectionMade(self): self.nickname = self.factory.nickname irc.IRCClient.connectionMade(self) def privmsg(self, user, channel, msg): user = user.split(’!’)[0] if self.nickname.lower() == channel.lower(): d = self.factory.getUser(msg) d.addErrback(catchError) d.addCallback(lambda m: "Status of %s: %s" % (msg, m)) CHAPTER 2. TUTORIAL 56 d.addCallback(lambda m: self.msg(user, m)) class IIRCClientFactory(Interface): """ @ivar nickname """ def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol""" class IRCClientFactoryFromService(protocol.ClientFactory): implements(IIRCClientFactory) protocol = IRCReplyBot nickname = None def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(IRCClientFactoryFromService, IFingerService, IIRCClientFactory) class UserStatusTree(resource.Resource): implements(resource.IResource) def __init__(self, service): resource.Resource.__init__(self) self.service = service self.putChild(’RPC2’, UserStatusXR(self.service)) def render_GET(self, request): d = self.service.getUsers() def formatUsers(users): l = [’
  • %s
  • ’ % (user, user) for user in users] return ’
      ’+’’.join(l)+’
    ’ d.addCallback(formatUsers) d.addCallback(request.write) d.addCallback(lambda _: request.finish()) return server.NOT_DONE_YET def getChild(self, path, request): if path=="": return UserStatusTree(self.service) else: CHAPTER 2. TUTORIAL 57 return UserStatus(path, self.service) components.registerAdapter(UserStatusTree, IFingerService, resource.IResource) class UserStatus(resource.Resource): def __init__(self, user, service): resource.Resource.__init__(self) self.user = user self.service = service def render_GET(self, request): d = self.service.getUser(self.user) d.addCallback(cgi.escape) d.addCallback(lambda m: ’

    %s

    ’%self.user+’

    %s

    ’%m) d.addCallback(request.write) d.addCallback(lambda _: request.finish()) return server.NOT_DONE_YET class UserStatusXR(xmlrpc.XMLRPC): def __init__(self, service): xmlrpc.XMLRPC.__init__(self) self.service = service def xmlrpc_getUser(self, user): return self.service.getUser(user) class FingerService(service.Service): implements(IFingerService) def __init__(self, filename): self.filename = filename self.users = {} self._read() def _read(self): self.users.clear() for line in file(self.filename): user, status = line.split(’:’, 1) user = user.strip() status = status.strip() self.users[user] = status self.call = reactor.callLater(30, self._read) def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getUsers(self): return defer.succeed(self.users.keys()) # Another back-end CHAPTER 2. TUTORIAL 58 class LocalFingerService(service.Service): implements(IFingerService) def getUser(self, user): # need a local finger daemon running for this to work return utils.getProcessOutput("finger", [user]) def getUsers(self): return defer.succeed([]) application = service.Application(’finger’, uid=1, gid=1) f = LocalFingerService() serviceCollection = service.IServiceCollection(application) internet.TCPServer(79, IFingerFactory(f) ).setServiceParent(serviceCollection) internet.TCPServer(8000, server.Site(resource.IResource(f)) ).setServiceParent(serviceCollection) i = IIRCClientFactory(f) i.nickname = ’fingerbot’ internet.TCPClient(’irc.freenode.org’, 6667, i ).setServiceParent(serviceCollection) Source listing — finger19b.tac We’ve already written this, but now we get more for less work: the network code is completely separate from the back-end. 2.10.3 Yet Another Back-end: Doing the Standard Thing from twisted.internet import protocol, reactor, defer, utils import pwd import os # Yet another back-end class LocalFingerService(service.Service): implements(IFingerService) def getUser(self, user): user = user.strip() try: entry = pwd.getpwnam(user) except KeyError: return defer.succeed("No such user") try: f = file(os.path.join(entry[5],’.plan’)) except (IOError, OSError): return defer.succeed("No such user") data = f.read() data = data.strip() f.close() return defer.succeed(data) CHAPTER 2. TUTORIAL 59 def getUsers(self): return defer.succeed([]) f = LocalFingerService() Source listing — finger19c changes.py Full source code here: # Do everything properly, and componentize from twisted.application import internet, service from twisted.internet import protocol, reactor, defer, utils from twisted.words.protocols import irc from twisted.protocols import basic from twisted.python import components from twisted.web import resource, server, static, xmlrpc from zope.interface import Interface, implements import cgi import pwd import os class IFingerService(Interface): def getUser(user): """Return a deferred returning a string""" def getUsers(): """Return a deferred returning a list of strings""" class IFingerSetterService(Interface): def setUser(user, status): """Set the user’s status to something""" class IFingerSetterService(Interface): def setUser(user, status): """Set the user’s status to something""" def catchError(err): return "Internal error in server" class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) d.addErrback(catchError) def writeValue(value): self.transport.write(value+’\r\n’) self.transport.loseConnection() d.addCallback(writeValue) class IFingerFactory(Interface): CHAPTER 2. TUTORIAL 60 def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerFactoryFromService(protocol.ServerFactory): implements(IFingerFactory) protocol = FingerProtocol def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory) class FingerSetterProtocol(basic.LineReceiver): def connectionMade(self): self.lines = [] def lineReceived(self, line): self.lines.append(line) def connectionLost(self, reason): if len(self.lines) == 2: self.factory.setUser(*self.lines) class IFingerSetterFactory(Interface): def setUser(user, status): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerSetterFactoryFromService(protocol.ServerFactory): implements(IFingerSetterFactory) protocol = FingerSetterProtocol def __init__(self, service): self.service = service def setUser(self, user, status): self.service.setUser(user, status) CHAPTER 2. TUTORIAL 61 components.registerAdapter(FingerSetterFactoryFromService, IFingerSetterService, IFingerSetterFactory) class IRCReplyBot(irc.IRCClient): def connectionMade(): self.nickname = self.factory.nickname irc.IRCClient.connectionMade(self) def privmsg(self, user, channel, msg): user = user.split(’!’)[0] if self.nickname.lower() == channel.lower(): d = self.factory.getUser(msg) d.addErrback(catchError) d.addCallback(lambda m: "Status of %s: %s" % (msg, m)) d.addCallback(lambda m: self.msg(user, m)) class IIRCClientFactory(Interface): """ @ivar nickname """ def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol""" class IRCClientFactoryFromService(protocol.ClientFactory): implements(IIRCClientFactory) protocol = IRCReplyBot nickname = None def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(IRCClientFactoryFromService, IFingerService, IIRCClientFactory) class UserStatusTree(resource.Resource): implements(resource.IResource) def __init__(self, service): resource.Resource.__init__(self) CHAPTER 2. TUTORIAL 62 self.service = service self.putChild(’RPC2’, UserStatusXR(self.service)) def render_GET(self, request): d = self.service.getUsers() def formatUsers(users): l = [’
  • %s
  • ’ % (user, user) for user in users] return ’
      ’+’’.join(l)+’
    ’ d.addCallback(formatUsers) d.addCallback(request.write) d.addCallback(lambda _: request.finish()) return server.NOT_DONE_YET def getChild(self, path, request): if path=="": return UserStatusTree(self.service) else: return UserStatus(path, self.service) components.registerAdapter(UserStatusTree, IFingerService, resource.IResource) class UserStatus(resource.Resource): def __init__(self, user, service): resource.Resource.__init__(self) self.user = user self.service = service def render_GET(self, request): d = self.service.getUser(self.user) d.addCallback(cgi.escape) d.addCallback(lambda m: ’

    %s

    ’%self.user+’

    %s

    ’%m) d.addCallback(request.write) d.addCallback(lambda _: request.finish()) return server.NOT_DONE_YET class UserStatusXR(xmlrpc.XMLRPC): def __init__(self, service): xmlrpc.XMLRPC.__init__(self) self.service = service def xmlrpc_getUser(self, user): return self.service.getUser(user) class FingerService(service.Service): implements(IFingerService) def __init__(self, filename): self.filename = filename self.users = {} CHAPTER 2. TUTORIAL 63 self._read() def _read(self): self.users.clear() for line in file(self.filename): user, status = line.split(’:’, 1) user = user.strip() status = status.strip() self.users[user] = status self.call = reactor.callLater(30, self._read) def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getUsers(self): return defer.succeed(self.users.keys()) # Yet another back-end class LocalFingerService(service.Service): implements(IFingerService) def getUser(self, user): user = user.strip() try: entry = pwd.getpwnam(user) except KeyError: return defer.succeed("No such user") try: f = file(os.path.join(entry[5],’.plan’)) except (IOError, OSError): return defer.succeed("No such user") data = f.read() data = data.strip() f.close() return defer.succeed(data) def getUsers(self): return defer.succeed([]) application = service.Application(’finger’, uid=1, gid=1) f = LocalFingerService() serviceCollection = service.IServiceCollection(application) internet.TCPServer(79, IFingerFactory(f) ).setServiceParent(serviceCollection) internet.TCPServer(8000, server.Site(resource.IResource(f)) ).setServiceParent(serviceCollection) i = IIRCClientFactory(f) i.nickname = ’fingerbot’ internet.TCPClient(’irc.freenode.org’, 6667, i ).setServiceParent(serviceCollection) Source listing — finger19c.tac Not much to say except that now we can be churn out backends like crazy. Feel like doing a back-end for Advogato, CHAPTER 2. TUTORIAL 64 for example? Dig out the XML-RPC client support Twisted has, and get to work! 2.11 The Evolution of Finger: a web frontend 2.11.1 Introduction This is the sixth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger (page 23). In this part, we demonstrate adding a web frontend using simple twisted.web.resource.Resource ob- jects: UserStatusTree, which will produce a listing of all users at the base URL (/) of our site; UserStatus, which gives the status of each user at the locaton /username; and UserStatusXR, which exposes an XMLRPC interface to getUser and getUsers functions at the URL /RPC2. In this example we construct HTML segments manually. If the web interface was less trivial, we would want to use more sophisticated web templating and design our system so that HTML rendering and logic were clearly separated. # Do everything properly, and componentize from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.words.protocols import irc from twisted.protocols import basic from twisted.python import components from twisted.web import resource, server, static, xmlrpc, microdom from zope.interface import Interface, implements import cgi class IFingerService(Interface): def getUser(user): """Return a deferred returning a string""" def getUsers(): """Return a deferred returning a list of strings""" class IFingerSetterService(Interface): def setUser(user, status): """Set the user’s status to something""" def catchError(err): return "Internal error in server" class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) d.addErrback(catchError) def writeValue(value): self.transport.write(value+’\r\n’) self.transport.loseConnection() d.addCallback(writeValue) class IFingerFactory(Interface): def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): CHAPTER 2. TUTORIAL 65 """Return a protocol returning a string""" class FingerFactoryFromService(protocol.ServerFactory): implements(IFingerFactory) protocol = FingerProtocol def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory) class FingerSetterProtocol(basic.LineReceiver): def connectionMade(self): self.lines = [] def lineReceived(self, line): self.lines.append(line) def connectionLost(self, reason): if len(self.lines) == 2: self.factory.setUser(*self.lines) class IFingerSetterFactory(Interface): def setUser(user, status): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerSetterFactoryFromService(protocol.ServerFactory): implements(IFingerSetterFactory) protocol = FingerSetterProtocol def __init__(self, service): self.service = service def setUser(self, user, status): self.service.setUser(user, status) components.registerAdapter(FingerSetterFactoryFromService, IFingerSetterService, IFingerSetterFactory) CHAPTER 2. TUTORIAL 66 class IRCReplyBot(irc.IRCClient): def connectionMade(self): self.nickname = self.factory.nickname irc.IRCClient.connectionMade(self) def privmsg(self, user, channel, msg): user = user.split(’!’)[0] if self.nickname.lower() == channel.lower(): d = self.factory.getUser(msg) d.addErrback(catchError) d.addCallback(lambda m: "Status of %s: %s" % (msg, m)) d.addCallback(lambda m: self.msg(user, m)) class IIRCClientFactory(Interface): """ @ivar nickname """ def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol""" class IRCClientFactoryFromService(protocol.ClientFactory): implements(IIRCClientFactory) protocol = IRCReplyBot nickname = None def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(IRCClientFactoryFromService, IFingerService, IIRCClientFactory) class UserStatusTree(resource.Resource): def __init__(self, service): resource.Resource.__init__(self) self.service=service # add a specific child for the path "RPC2" self.putChild("RPC2", UserStatusXR(self.service)) # need to do this for resources at the root of the site self.putChild("", self) CHAPTER 2. TUTORIAL 67 def _cb_render_GET(self, users, request): userOutput = ’’.join(["
  • %s
  • " % (user, user) for user in users]) request.write(""" Users

    Users

      %s
    """ % userOutput) request.finish() def render_GET(self, request): d = self.service.getUsers() d.addCallback(self._cb_render_GET, request) # signal that the rendering is not complete return server.NOT_DONE_YET def getChild(self, path, request): return UserStatus(user=path, service=self.service) components.registerAdapter(UserStatusTree, IFingerService, resource.IResource) class UserStatus(resource.Resource): def __init__(self, user, service): resource.Resource.__init__(self) self.user = user self.service = service def _cb_render_GET(self, status, request): request.write("""%s

    %s

    %s

    """ % (self.user, self.user, status)) request.finish() def render_GET(self, request): d = self.service.getUser(self.user) d.addCallback(self._cb_render_GET, request) # signal that the rendering is not complete return server.NOT_DONE_YET class UserStatusXR(xmlrpc.XMLRPC): def __init__(self, service): xmlrpc.XMLRPC.__init__(self) self.service = service def xmlrpc_getUser(self, user): return self.service.getUser(user) def xmlrpc_getUsers(self): return self.service.getUsers() CHAPTER 2. TUTORIAL 68 class FingerService(service.Service): implements(IFingerService) def __init__(self, filename): self.filename = filename self.users = {} self._read() def _read(self): self.users.clear() for line in file(self.filename): user, status = line.split(’:’, 1) user = user.strip() status = status.strip() self.users[user] = status self.call = reactor.callLater(30, self._read) def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getUsers(self): return defer.succeed(self.users.keys()) application = service.Application(’finger’, uid=1, gid=1) f = FingerService(’/etc/users’) serviceCollection = service.IServiceCollection(application) internet.TCPServer(79, IFingerFactory(f) ).setServiceParent(serviceCollection) internet.TCPServer(8000, server.Site(resource.IResource(f)) ).setServiceParent(serviceCollection) i = IIRCClientFactory(f) i.nickname = ’fingerbot’ internet.TCPClient(’irc.freenode.org’, 6667, i ).setServiceParent(serviceCollection) Source listing — finger20.tac 2.12 The Evolution of Finger: Twisted client support using Perspective Bro- ker 2.12.1 Introduction This is the seventh part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger (page 23). In this part, we add a Perspective Broker service to the finger application so that Twisted clients can access the finger server. Perspective Broker is introduced in depth in its own section (page ??) of the core howto index. 2.12.2 Use Perspective Broker We add support for perspective broker, Twisted’s native remote object protocol. Now, Twisted clients will not have to go through XML-RPCish contortions to get information about users. CHAPTER 2. TUTORIAL 69 # Do everything properly, and componentize from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.words.protocols import irc from twisted.protocols import basic from twisted.python import components from twisted.web import resource, server, static, xmlrpc, microdom from twisted.spread import pb from zope.interface import Interface, implements import cgi class IFingerService(Interface): def getUser(user): """Return a deferred returning a string""" def getUsers(): """Return a deferred returning a list of strings""" class IFingerSetterService(Interface): def setUser(user, status): """Set the user’s status to something""" def catchError(err): return "Internal error in server" class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) d.addErrback(catchError) def writeValue(value): self.transport.write(value+’\r\n’) self.transport.loseConnection() d.addCallback(writeValue) class IFingerFactory(Interface): def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerFactoryFromService(protocol.ServerFactory): implements(IFingerFactory) protocol = FingerProtocol def __init__(self, service): self.service = service def getUser(self, user): CHAPTER 2. TUTORIAL 70 return self.service.getUser(user) components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory) class FingerSetterProtocol(basic.LineReceiver): def connectionMade(self): self.lines = [] def lineReceived(self, line): self.lines.append(line) def connectionLost(self, reason): if len(self.lines) == 2: self.factory.setUser(*self.lines) class IFingerSetterFactory(Interface): def setUser(user, status): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerSetterFactoryFromService(protocol.ServerFactory): implements(IFingerSetterFactory) protocol = FingerSetterProtocol def __init__(self, service): self.service = service def setUser(self, user, status): self.service.setUser(user, status) components.registerAdapter(FingerSetterFactoryFromService, IFingerSetterService, IFingerSetterFactory) class IRCReplyBot(irc.IRCClient): def connectionMade(self): self.nickname = self.factory.nickname irc.IRCClient.connectionMade(self) def privmsg(self, user, channel, msg): user = user.split(’!’)[0] if self.nickname.lower() == channel.lower(): d = self.factory.getUser(msg) d.addErrback(catchError) d.addCallback(lambda m: "Status of %s: %s" % (msg, m)) CHAPTER 2. TUTORIAL 71 d.addCallback(lambda m: self.msg(user, m)) class IIRCClientFactory(Interface): """ @ivar nickname """ def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol""" class IRCClientFactoryFromService(protocol.ClientFactory): implements(IIRCClientFactory) protocol = IRCReplyBot nickname = None def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(IRCClientFactoryFromService, IFingerService, IIRCClientFactory) class UserStatusTree(resource.Resource): def __init__(self, service): resource.Resource.__init__(self) self.service=service # add a specific child for the path "RPC2" self.putChild("RPC2", UserStatusXR(self.service)) # need to do this for resources at the root of the site self.putChild("", self) def _cb_render_GET(self, users, request): userOutput = ’’.join(["
  • %s
  • " % (user, user) for user in users]) request.write(""" Users

    Users

      %s
    """ % userOutput) request.finish() def render_GET(self, request): CHAPTER 2. TUTORIAL 72 d = self.service.getUsers() d.addCallback(self._cb_render_GET, request) # signal that the rendering is not complete return server.NOT_DONE_YET def getChild(self, path, request): return UserStatus(user=path, service=self.service) components.registerAdapter(UserStatusTree, IFingerService, resource.IResource) class UserStatus(resource.Resource): def __init__(self, user, service): resource.Resource.__init__(self) self.user = user self.service = service def _cb_render_GET(self, status, request): request.write("""%s

    %s

    %s

    """ % (self.user, self.user, status)) request.finish() def render_GET(self, request): d = self.service.getUser(self.user) d.addCallback(self._cb_render_GET, request) # signal that the rendering is not complete return server.NOT_DONE_YET class UserStatusXR(xmlrpc.XMLRPC): def __init__(self, service): xmlrpc.XMLRPC.__init__(self) self.service = service def xmlrpc_getUser(self, user): return self.service.getUser(user) def xmlrpc_getUsers(self): return self.service.getUsers() class IPerspectiveFinger(Interface): def remote_getUser(username): """return a user’s status""" def remote_getUsers(): """return a user’s status""" class PerspectiveFingerFromService(pb.Root): implements(IPerspectiveFinger) CHAPTER 2. TUTORIAL 73 def __init__(self, service): self.service = service def remote_getUser(self, username): return self.service.getUser(username) def remote_getUsers(self): return self.service.getUsers() components.registerAdapter(PerspectiveFingerFromService, IFingerService, IPerspectiveFinger) class FingerService(service.Service): implements(IFingerService) def __init__(self, filename): self.filename = filename self.users = {} self._read() def _read(self): self.users.clear() for line in file(self.filename): user, status = line.split(’:’, 1) user = user.strip() status = status.strip() self.users[user] = status self.call = reactor.callLater(30, self._read) def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getUsers(self): return defer.succeed(self.users.keys()) application = service.Application(’finger’, uid=1, gid=1) f = FingerService(’/etc/users’) serviceCollection = service.IServiceCollection(application) internet.TCPServer(79, IFingerFactory(f) ).setServiceParent(serviceCollection) internet.TCPServer(8000, server.Site(resource.IResource(f)) ).setServiceParent(serviceCollection) i = IIRCClientFactory(f) i.nickname = ’fingerbot’ internet.TCPClient(’irc.freenode.org’, 6667, i ).setServiceParent(serviceCollection) internet.TCPServer(8889, pb.PBServerFactory(IPerspectiveFinger(f)) ).setServiceParent(serviceCollection) Source listing — finger21.tac A simple client to test the perspective broker finger: # test the PB finger on port 8889 CHAPTER 2. TUTORIAL 74 # this code is essentially the same as # the first example in howto/pb-usage from twisted.spread import pb from twisted.internet import reactor def gotObject(object): print "got object:", object object.callRemote("getUser","moshez").addCallback(gotData) # or # object.callRemote("getUsers").addCallback(gotData) def gotData(data): print ’server sent:’, data reactor.stop() def gotNoObject(reason): print "no object:",reason reactor.stop() factory = pb.PBClientFactory() reactor.connectTCP("127.0.0.1",8889, factory) factory.getRootObject().addCallbacks(gotObject,gotNoObject) reactor.run() Source listing — fingerPBclient.py 2.13 The Evolution of Finger: using a single factory for multiple protocols 2.13.1 Introduction This is the eighth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger (page 23). In this part, we add HTTPS support to our web frontend, showing how to have a single factory listen on multiple ports. More information on using SSL in Twisted can be found in the SSL howto (page ??). 2.13.2 Support HTTPS All we need to do to code an HTTPS site is just write a context factory (in this case, which loads the certificate from a certain file) and then use the twisted.application.internet.SSLServer method. Note that one factory (in this case, a site) can listen on multiple ports with multiple protocols. # Do everything properly, and componentize from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.words.protocols import irc from twisted.protocols import basic from twisted.python import components from twisted.web import resource, server, static, xmlrpc, microdom from twisted.spread import pb from zope.interface import Interface, implements from OpenSSL import SSL import cgi class IFingerService(Interface): CHAPTER 2. TUTORIAL 75 def getUser(user): """Return a deferred returning a string""" def getUsers(): """Return a deferred returning a list of strings""" class IFingerSetterService(Interface): def setUser(user, status): """Set the user’s status to something""" def catchError(err): return "Internal error in server" class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) d.addErrback(catchError) def writeValue(value): self.transport.write(value+’\r\n’) self.transport.loseConnection() d.addCallback(writeValue) class IFingerFactory(Interface): def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerFactoryFromService(protocol.ServerFactory): implements(IFingerFactory) protocol = FingerProtocol def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory) class FingerSetterProtocol(basic.LineReceiver): def connectionMade(self): self.lines = [] def lineReceived(self, line): self.lines.append(line) CHAPTER 2. TUTORIAL 76 def connectionLost(self, reason): if len(self.lines) == 2: self.factory.setUser(*self.lines) class IFingerSetterFactory(Interface): def setUser(user, status): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerSetterFactoryFromService(protocol.ServerFactory): implements(IFingerSetterFactory) protocol = FingerSetterProtocol def __init__(self, service): self.service = service def setUser(self, user, status): self.service.setUser(user, status) components.registerAdapter(FingerSetterFactoryFromService, IFingerSetterService, IFingerSetterFactory) class IRCReplyBot(irc.IRCClient): def connectionMade(self): self.nickname = self.factory.nickname irc.IRCClient.connectionMade(self) def privmsg(self, user, channel, msg): user = user.split(’!’)[0] if self.nickname.lower() == channel.lower(): d = self.factory.getUser(msg) d.addErrback(catchError) d.addCallback(lambda m: "Status of %s: %s" % (msg, m)) d.addCallback(lambda m: self.msg(user, m)) class IIRCClientFactory(Interface): """ @ivar nickname """ def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): CHAPTER 2. TUTORIAL 77 """Return a protocol""" class IRCClientFactoryFromService(protocol.ClientFactory): implements(IIRCClientFactory) protocol = IRCReplyBot nickname = None def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(IRCClientFactoryFromService, IFingerService, IIRCClientFactory) class UserStatusTree(resource.Resource): def __init__(self, service): resource.Resource.__init__(self) self.service=service # add a specific child for the path "RPC2" self.putChild("RPC2", UserStatusXR(self.service)) # need to do this for resources at the root of the site self.putChild("", self) def _cb_render_GET(self, users, request): userOutput = ’’.join(["
  • %s
  • " % (user, user) for user in users]) request.write(""" Users

    Users

      %s
    """ % userOutput) request.finish() def render_GET(self, request): d = self.service.getUsers() d.addCallback(self._cb_render_GET, request) # signal that the rendering is not complete return server.NOT_DONE_YET def getChild(self, path, request): return UserStatus(user=path, service=self.service) components.registerAdapter(UserStatusTree, IFingerService, resource.IResource) class UserStatus(resource.Resource): CHAPTER 2. TUTORIAL 78 def __init__(self, user, service): resource.Resource.__init__(self) self.user = user self.service = service def _cb_render_GET(self, status, request): request.write("""%s

    %s

    %s

    """ % (self.user, self.user, status)) request.finish() def render_GET(self, request): d = self.service.getUser(self.user) d.addCallback(self._cb_render_GET, request) # signal that the rendering is not complete return server.NOT_DONE_YET class UserStatusXR(xmlrpc.XMLRPC): def __init__(self, service): xmlrpc.XMLRPC.__init__(self) self.service = service def xmlrpc_getUser(self, user): return self.service.getUser(user) def xmlrpc_getUsers(self): return self.service.getUsers() class IPerspectiveFinger(Interface): def remote_getUser(username): """return a user’s status""" def remote_getUsers(): """return a user’s status""" class PerspectiveFingerFromService(pb.Root): implements(IPerspectiveFinger) def __init__(self, service): self.service = service def remote_getUser(self, username): return self.service.getUser(username) def remote_getUsers(self): return self.service.getUsers() components.registerAdapter(PerspectiveFingerFromService, IFingerService, IPerspectiveFinger) CHAPTER 2. TUTORIAL 79 class FingerService(service.Service): implements(IFingerService) def __init__(self, filename): self.filename = filename self.users = {} self._read() def _read(self): self.users.clear() for line in file(self.filename): user, status = line.split(’:’, 1) user = user.strip() status = status.strip() self.users[user] = status self.call = reactor.callLater(30, self._read) def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getUsers(self): return defer.succeed(self.users.keys()) class ServerContextFactory: def getContext(self): """Create an SSL context. This is a sample implementation that loads a certificate from a file called ’server.pem’.""" ctx = SSL.Context(SSL.SSLv23_METHOD) ctx.use_certificate_file(’server.pem’) ctx.use_privatekey_file(’server.pem’) return ctx application = service.Application(’finger’, uid=1, gid=1) f = FingerService(’/etc/users’) serviceCollection = service.IServiceCollection(application) internet.TCPServer(79, IFingerFactory(f) ).setServiceParent(serviceCollection) site = server.Site(resource.IResource(f)) internet.TCPServer(8000, site ).setServiceParent(serviceCollection) internet.SSLServer(443, site, ServerContextFactory() ).setServiceParent(serviceCollection) i = IIRCClientFactory(f) i.nickname = ’fingerbot’ internet.TCPClient(’irc.freenode.org’, 6667, i ).setServiceParent(serviceCollection) internet.TCPServer(8889, pb.PBServerFactory(IPerspectiveFinger(f)) ).setServiceParent(serviceCollection) CHAPTER 2. TUTORIAL 80 Source listing — finger22.py 2.14 The Evolution of Finger: a Twisted finger client 2.14.1 Introduction This is the ninth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger (page 23). In this part, we develop a client for the finger server: a proxy finger server which forwards requests to another finger server. 2.14.2 Finger Proxy Writing new clients with Twisted is much like writing new servers. We implement the protocol, which just gathers up all the data, and give it to the factory. The factory keeps a deferred which is triggered if the connection either fails or succeeds. When we use the client, we first make sure the deferred will never fail, by producing a message in that case. Implementing a wrapper around client which just returns the deferred is a common pattern. While less flexible than using the factory directly, it’s also more convenient. # finger proxy from twisted.application import internet, service from twisted.internet import defer, protocol, reactor from twisted.protocols import basic from twisted.python import components from zope.interface import Interface, implements def catchError(err): return "Internal error in server" class IFingerService(Interface): def getUser(user): """Return a deferred returning a string""" def getUsers(): """Return a deferred returning a list of strings""" class IFingerFactory(Interface): def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) d.addErrback(catchError) def writeValue(value): self.transport.write(value) self.transport.loseConnection() CHAPTER 2. TUTORIAL 81 d.addCallback(writeValue) class FingerFactoryFromService(protocol.ClientFactory): implements(IFingerFactory) protocol = FingerProtocol def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory) class FingerClient(protocol.Protocol): def connectionMade(self): self.transport.write(self.factory.user+"\r\n") self.buf = [] def dataReceived(self, data): self.buf.append(data) def connectionLost(self, reason): self.factory.gotData(’’.join(self.buf)) class FingerClientFactory(protocol.ClientFactory): protocol = FingerClient def __init__(self, user): self.user = user self.d = defer.Deferred() def clientConnectionFailed(self, _, reason): self.d.errback(reason) def gotData(self, data): self.d.callback(data) def finger(user, host, port=79): f = FingerClientFactory(user) reactor.connectTCP(host, port, f) return f.d class ProxyFingerService(service.Service): implements(IFingerService) CHAPTER 2. TUTORIAL 82 def getUser(self, user): try: user, host = user.split(’@’, 1) except: user = user.strip() host = ’127.0.0.1’ ret = finger(user, host) ret.addErrback(lambda _: "Could not connect to remote host") return ret def getUsers(self): return defer.succeed([]) application = service.Application(’finger’, uid=1, gid=1) f = ProxyFingerService() internet.TCPServer(7779, IFingerFactory(f)).setServiceParent( service.IServiceCollection(application)) Source listing — fingerproxy.tac 2.15 The Evolution of Finger: making a finger library 2.15.1 Introduction This is the tenth part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger (page 23). In this part, we separate the application code that launches a finger service from the library code which defines a finger service, placing the application in a Twisted Application Configuration (.tac) file. We also move configuration (such as HTML templates) into separate files. Configuration and deployment with .tac and twistd are introduced in Using the Twisted Application Framework (page 163). 2.15.2 Organization Now this code, while quite modular and well-designed, isn’t properly organized. Everything above the application= belongs in a module, and the HTML templates all belong in separate files. We can use the templateFile and templateDirectory attributes to indicate what HTML template file to use for each Page, and where to look for it. # organized-finger.tac # eg: twistd -ny organized-finger.tac import finger from twisted.internet import protocol, reactor, defer from twisted.spread import pb from twisted.web import resource, server from twisted.application import internet, service, strports from twisted.python import log application = service.Application(’finger’, uid=1, gid=1) f = finger.FingerService(’/etc/users’) serviceCollection = service.IServiceCollection(application) internet.TCPServer(79, finger.IFingerFactory(f) ).setServiceParent(serviceCollection) site = server.Site(resource.IResource(f)) CHAPTER 2. TUTORIAL 83 internet.TCPServer(8000, site ).setServiceParent(serviceCollection) internet.SSLServer(443, site, finger.ServerContextFactory() ).setServiceParent(serviceCollection) i = finger.IIRCClientFactory(f) i.nickname = ’fingerbot’ internet.TCPClient(’irc.freenode.org’, 6667, i ).setServiceParent(serviceCollection) internet.TCPServer(8889, pb.PBServerFactory(finger.IPerspectiveFinger(f)) ).setServiceParent(serviceCollection) Source listing — organized-finger.tac Note that our program is now quite separated. We have: • Code (in the module) • Configuration (file above) • Presentation (templates) • Content (/etc/users) • Deployment (twistd) Prototypes don’t need this level of separation, so our earlier examples all bunched together. However, real applications do. Thankfully, if we write our code correctly, it is easy to achieve a good separation of parts. 2.15.3 Easy Configuration We can also supply easy configuration for common cases with a makeService method that will also help build .tap files later: # Easy configuration # makeService from finger module def makeService(config): # finger on port 79 s = service.MultiService() f = FingerService(config[’file’]) h = internet.TCPServer(79, IFingerFactory(f)) h.setServiceParent(s) # website on port 8000 r = resource.IResource(f) r.templateDirectory = config[’templates’] site = server.Site(r) j = internet.TCPServer(8000, site) j.setServiceParent(s) # ssl on port 443 if config.get(’ssl’): k = internet.SSLServer(443, site, ServerContextFactory()) k.setServiceParent(s) # irc fingerbot CHAPTER 2. TUTORIAL 84 if config.has_key(’ircnick’): i = IIRCClientFactory(f) i.nickname = config[’ircnick’] ircserver = config[’ircserver’] b = internet.TCPClient(ircserver, 6667, i) b.setServiceParent(s) # Pespective Broker on port 8889 if config.has_key(’pbport’): m = internet.TCPServer( int(config[’pbport’]), pb.PBServerFactory(IPerspectiveFinger(f))) m.setServiceParent(s) return s Source listing — finger config.py And we can write simpler files now: # simple-finger.tac # eg: twistd -ny simple-finger.tac from twisted.application import service import finger options = { ’file’: ’/etc/users’, ’templates’: ’/usr/share/finger/templates’, ’ircnick’: ’fingerbot’, ’ircserver’: ’irc.freenode.net’, ’pbport’: 8889, ’ssl’: ’ssl=0’ } ser = finger.makeService(options) application = service.Application(’finger’, uid=1, gid=1) ser.setServiceParent(service.IServiceCollection(application)) Source listing — simple-finger.tac % twisted -ny simple-finger.tac Note: the finger user still has ultimate power: he can use makeService, or he can use the lower-level interface if he has specific needs (maybe an IRC server on some other port? maybe we want the non-SSL webserver to listen only locally? etc. etc.) This is an important design principle: never force a layer of abstraction: allow usage of layers of abstractions. The pasta theory of design: • Spaghetti: each piece of code interacts with every other piece of code [can be implemented with GOTO, func- tions, objects] • Lasagna: code has carefully designed layers. Each layer is, in theory independent. However low-level layers usually cannot be used easily, and high-level layers depend on low-level layers. • Ravioli: each part of the code is useful by itself. There is a thin layer of interfaces between various parts [the sauce]. Each part can be usefully be used elsewhere. •...but sometimes, the user just wants to order “Ravioli”, so one coarse-grain easily definable layer of abstraction on top of it all can be useful. CHAPTER 2. TUTORIAL 85 2.16 The Evolution of Finger: configuration and packaging of the finger service 2.16.1 Introduction This is the eleventh part of the Twisted tutorial Twisted from Scratch, or The Evolution of Finger (page 23). In this part, we make it easier for non-programmers to configure a finger server and show how to package it in the .deb and RPM package formats. Plugins are discussed further in the Twisted Plugin System (page 146) howto. .tap files are covered in Writing a twistd Plugin (page 148), and .tac applications are covered in Using the Twisted Application Framework (page 163). 2.16.2 Plugins So far, the user had to be somewhat of a programmer to be able to configure stuff. Maybe we can eliminate even that? Move old code to finger/ init .py and... Full source code for finger module here: # finger.py module from zope.interface import Interface, implements from twisted.application import internet, service from twisted.internet import protocol, reactor, defer from twisted.words.protocols import irc from twisted.protocols import basic from twisted.python import components, log from twisted.web import resource, server, xmlrpc from twisted.spread import pb from OpenSSL import SSL class IFingerService(Interface): def getUser(user): """Return a deferred returning a string""" def getUsers(): """Return a deferred returning a list of strings""" class IFingerSetterService(Interface): def setUser(user, status): """Set the user’s status to something""" def catchError(err): return "Internal error in server" class FingerProtocol(basic.LineReceiver): def lineReceived(self, user): d = self.factory.getUser(user) d.addErrback(catchError) def writeValue(value): self.transport.write(value+’\n’) self.transport.loseConnection() d.addCallback(writeValue) CHAPTER 2. TUTORIAL 86 class IFingerFactory(Interface): def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerFactoryFromService(protocol.ServerFactory): implements(IFingerFactory) protocol = FingerProtocol def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(FingerFactoryFromService, IFingerService, IFingerFactory) class FingerSetterProtocol(basic.LineReceiver): def connectionMade(self): self.lines = [] def lineReceived(self, line): self.lines.append(line) def connectionLost(self, reason): if len(self.lines) == 2: self.factory.setUser(*self.lines) class IFingerSetterFactory(Interface): def setUser(user, status): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol returning a string""" class FingerSetterFactoryFromService(protocol.ServerFactory): implements(IFingerSetterFactory) protocol = FingerSetterProtocol def __init__(self, service): self.service = service def setUser(self, user, status): CHAPTER 2. TUTORIAL 87 self.service.setUser(user, status) components.registerAdapter(FingerSetterFactoryFromService, IFingerSetterService, IFingerSetterFactory) class IRCReplyBot(irc.IRCClient): def connectionMade(self): self.nickname = self.factory.nickname irc.IRCClient.connectionMade(self) def privmsg(self, user, channel, msg): user = user.split(’!’)[0] if self.nickname.lower() == channel.lower(): d = self.factory.getUser(msg) d.addErrback(catchError) d.addCallback(lambda m: "Status of %s: %s" % (msg, m)) d.addCallback(lambda m: self.msg(user, m)) class IIRCClientFactory(Interface): """ @ivar nickname """ def getUser(user): """Return a deferred returning a string""" def buildProtocol(addr): """Return a protocol""" class IRCClientFactoryFromService(protocol.ClientFactory): implements(IIRCClientFactory) protocol = IRCReplyBot nickname = None def __init__(self, service): self.service = service def getUser(self, user): return self.service.getUser(user) components.registerAdapter(IRCClientFactoryFromService, IFingerService, IIRCClientFactory) class UserStatusTree(resource.Resource): template = """Users

    Users

      CHAPTER 2. TUTORIAL 88 %(users)s
    """ def __init__(self, service): resource.Resource.__init__(self) self.service = service def getChild(self, path, request): if path == ’’: return self elif path == ’RPC2’: return UserStatusXR(self.service) else: return UserStatus(path, self.service) def render_GET(self, request): users = self.service.getUsers() def cbUsers(users): request.write(self.template % {’users’: ’’.join([ # Name should be quoted properly these uses. ’
  • %s
  • ’ % (name, name) for name in users])}) request.finish() users.addCallback(cbUsers) def ebUsers(err): log.err(err, "UserStatusTree failed") request.finish() users.addErrback(ebUsers) return server.NOT_DONE_YET components.registerAdapter(UserStatusTree, IFingerService, resource.IResource) class UserStatus(resource.Resource): template=’’’%(title)s

    %(name)s

    %(status)s

    ’’’ def __init__(self, user, service): resource.Resource.__init__(self) self.user = user self.service = service def render_GET(self, request): status = self.service.getUser(self.user) def cbStatus(status): request.write(self.template % { ’title’: self.user, ’name’: self.user, ’status’: status}) request.finish() status.addCallback(cbStatus) def ebStatus(err): log.err(err, "UserStatus failed") request.finish() CHAPTER 2. TUTORIAL 89 status.addErrback(ebStatus) return server.NOT_DONE_YET class UserStatusXR(xmlrpc.XMLRPC): def __init__(self, service): xmlrpc.XMLRPC.__init__(self) self.service = service def xmlrpc_getUser(self, user): return self.service.getUser(user) def xmlrpc_getUsers(self): return self.service.getUsers() class IPerspectiveFinger(Interface): def remote_getUser(username): """return a user’s status""" def remote_getUsers(): """return a user’s status""" class PerspectiveFingerFromService(pb.Root): implements(IPerspectiveFinger) def __init__(self, service): self.service = service def remote_getUser(self, username): return self.service.getUser(username) def remote_getUsers(self): return self.service.getUsers() components.registerAdapter(PerspectiveFingerFromService, IFingerService, IPerspectiveFinger) class FingerService(service.Service): implements(IFingerService) def __init__(self, filename): self.filename = filename self._read() def _read(self): self.users = {} for line in file(self.filename): user, status = line.split(’:’, 1) user = user.strip() status = status.strip() CHAPTER 2. TUTORIAL 90 self.users[user] = status self.call = reactor.callLater(30, self._read) def getUser(self, user): return defer.succeed(self.users.get(user, "No such user")) def getUsers(self): return defer.succeed(self.users.keys()) class ServerContextFactory: def getContext(self): """Create an SSL context. This is a sample implementation that loads a certificate from a file called ’server.pem’.""" ctx = SSL.Context(SSL.SSLv23_METHOD) ctx.use_certificate_file(’server.pem’) ctx.use_privatekey_file(’server.pem’) return ctx # Easy configuration def makeService(config): # finger on port 79 s = service.MultiService() f = FingerService(config[’file’]) h = internet.TCPServer(1079, IFingerFactory(f)) h.setServiceParent(s) # website on port 8000 r = resource.IResource(f) r.templateDirectory = config[’templates’] site = server.Site(r) j = internet.TCPServer(8000, site) j.setServiceParent(s) # ssl on port 443 # if config.get(’ssl’): # k = internet.SSLServer(443, site, ServerContextFactory()) # k.setServiceParent(s) # irc fingerbot if config.has_key(’ircnick’): i = IIRCClientFactory(f) i.nickname = config[’ircnick’] ircserver = config[’ircserver’] b = internet.TCPClient(ircserver, 6667, i) b.setServiceParent(s) # Pespective Broker on port 8889 if config.has_key(’pbport’): CHAPTER 2. TUTORIAL 91 m = internet.TCPServer( int(config[’pbport’]), pb.PBServerFactory(IPerspectiveFinger(f))) m.setServiceParent(s) return s finger module — finger.py # finger/tap.py from twisted.application import internet, service from twisted.internet import interfaces from twisted.python import usage import finger class Options(usage.Options): optParameters = [ [’file’, ’f’, ’/etc/users’], [’templates’, ’t’, ’/usr/share/finger/templates’], [’ircnick’, ’n’, ’fingerbot’], [’ircserver’, None, ’irc.freenode.net’], [’pbport’, ’p’, 8889], ] optFlags = [[’ssl’, ’s’]] def makeService(config): return finger.makeService(config) finger/tap.py — tap.py And register it all: from twisted.application.service import ServiceMaker finger = ServiceMaker( ’finger’, ’finger.tap’, ’Run a finger service’, ’finger’) twisted/plugins/finger tutorial.py — finger tutorial.py And now, the following works % sudo twistd -n finger --file=/etc/users --ircnick=fingerbot For more details about this, see the twistd plugin documentation (page 148). 2.16.3 OS Integration If we already have the “finger” package installed in PYTHONPATH (e.g. we added it to site-packages), we can achieve easy integration: Debian % tap2deb --unsigned -m "Foo " --type=python finger.tac % sudo dpkg -i .build/*.deb CHAPTER 2. TUTORIAL 92 Red Hat / Mandrake % tap2rpm --type=python finger.tac #[maybe other options needed] % sudo rpm -i .build/*.rpm Will properly register the tap/tac, init.d scripts, etc. for the given file. If it doesn’t work on your favorite OS: patches accepted! Chapter 3 Low-Level Twisted 3.1 Reactor Overview This HOWTO introduces the Twisted reactor, describes the basics of the reactor and links to the various reactor interfaces. 3.1.1 Reactor Basics The reactor is the core of the event loop within Twisted – the loop which drives applications using Twisted. The event loop is a programming construct that waits for and dispatches events or messages in a program. It works by calling some internal or external ”event provider”, which generally blocks until an event has arrived, and then calls the relevant event handler (”dispatches the event”). The reactor provides basic interfaces to a number of services, including network communications, threading, and event dispatching. For information about using the reactor and the Twisted event loop, see: • the event dispatching howtos: Scheduling (page 137) and Using Deferreds (page 104); • the communication howtos: TCP servers (page 9), TCP clients (page 13), UDP networking (page 94) and Using processes (page 96); and • Using threads (page 139). There are multiple implementations of the reactor, each modified to provide better support for specialized features over the default implementation. More information about these and how to use a particular implementation is available via Choosing a Reactor (page 140). Twisted applications can use the interfaces in twisted.application.service to configure and run the application instead of using boilerplate reactor code. See Using Application (page 163) for an introduction to Appli- cation. 3.1.2 Using the reactor object You can get to the reactor object using the following code: from twisted.internet import reactor The reactor usually implements a set of interfaces, but depending on the chosen reactor and the platform, some of the interfaces may not be implemented: • IReactorCore: Core (required) functionality. • IReactorFDSet: Use FileDescriptor objects. • IReactorProcess: Process management. Read the Using Processes (page 96) document for more informa- tion. • IReactorSSL: SSL networking support. 93 CHAPTER 3. LOW-LEVEL TWISTED 94 • IReactorTCP: TCP networking support. More information can be found in the Writing Servers (page 9) and Writing Clients (page 13) documents. • IReactorThreads: Threading use and management. More information can be found within Threading In Twisted (page 139). • IReactorTime: Scheduling interface. More information can be found within Scheduling Tasks (page 137). • IReactorUDP: UDP networking support. More information can be found within UDP Networking (this page). • IReactorUNIX: UNIX socket support. 3.2 UDP Networking 3.2.1 Overview Unlike TCP, UDP has no notion of connections. A UDP socket can receive datagrams from any server on the network, and send datagrams to any host on the network. In addition, datagrams may arrive in any order, never arrive at all, or be duplicated in transit. Since there are no multiple connections, we only use a single object, a protocol, for each UDP socket. We then use the reactor to connect this protocol to a UDP transport, using the twisted.internet.interfaces.IReactor UDP reactor API. 3.2.2 DatagramProtocol At the base, the place where you actually implement the protocol parsing and handling, is the DatagramProtocol class. This class will usually be decended from twisted.internet.protocol.DatagramProtocol. Most protocol handlers inherit either from this class or from one of its convenience children. The DatagramProtocol class receives datagrams, and can send them out over the network. Received datagrams include the address they were sent from, and when sending datagrams the address to send to must be specified. Here is a simple example: from twisted.internet.protocol import DatagramProtocol from twisted.internet import reactor class Echo(DatagramProtocol): def datagramReceived(self, data, (host, port)): print "received %r from %s:%d" % (data, host, port) self.transport.write(data, (host, port)) reactor.listenUDP(9999, Echo()) reactor.run() As you can see, the protocol is registed with the reactor. This means it may be persisted if it’s added to an application, and thus it has twisted.internet.protocol.DatagramProtocol.startProtocol and twisted.internet.protocol.DatagramProtocol.stopProtocol methods that will get called when the protocol is connected and disconnected from a UDP socket. The protocol’s transport attribute will implement the twisted.internet.interfaces. IUDPTransport interface. Notice that the host argument should be an IP, not a hostname. If you only have the hostname use reactor.resolve() to resolve the address (see twisted.internet.interfaces. IReactorCore.resolve). 3.2.3 Connected UDP A connected UDP socket is slighly different from a standard one - it can only send and receive datagrams to/from a single address, but this does not in any way imply a connection. Datagrams may still arrive in any order, and the port on the other side may have no one listening. The benefit of the connected UDP socket is that it it may provide CHAPTER 3. LOW-LEVEL TWISTED 95 notification of undelivered packages. This depends on many factors, almost all of which are out of the control of the application, but it still presents certain benefits which occassionally make it useful. Unlike a regular UDP protocol, we do not need to specify where to send datagrams to, and are not told where they came from since they can only come from address the socket is ’connected’ to. from twisted.internet.protocol import DatagramProtocol from twisted.internet import reactor class Helloer(DatagramProtocol): def startProtocol(self): self.transport.connect("192.168.1.1", 1234) print "we can only send to %s now" % str((host, port)) self.transport.write("hello") # no need for address def datagramReceived(self, data, (host, port)): print "received %r from %s:%d" % (data, host, port) # Possibly invoked if there is no server listening on the # address to which we are sending. def connectionRefused(self): print "No one listening" # 0 means any port, we don’t care in this case reactor.listenUDP(0, Helloer()) reactor.run() Note that connect(), like write() will only accept IP addresses, not unresolved domain names. To obtain the IP of a domain name use reactor.resolve(), e.g.: from twisted.internet import reactor def gotIP(ip): print "IP of ’example.com’ is", ip reactor.resolve(’example.com’).addCallback(gotIP) Connecting to a new address after a previous connection, or making a connected port unconnected are not currently supported, but will likely be supported in the future. 3.2.4 Multicast UDP A multicast UDP socket can send and receive datagrams from multiple clients. The interesting and useful feature of the multicast is that a client can contact multiple servers with a single packet, without knowing the specific IP of any of the hosts. from twisted.internet.protocol import DatagramProtocol from twisted.internet import reactor from twisted.application.internet import MulticastServer class MulticastServerUDP(DatagramProtocol): def startProtocol(self): print ’Started Listening’ # Join a specific multicast group, which is the IP we will respond to self.transport.joinGroup(’224.0.0.1’) def datagramReceived(self, datagram, address): # The uniqueID check is to ensure we only service requests from # ourselves CHAPTER 3. LOW-LEVEL TWISTED 96 if datagram == ’UniqueID’: print "Server Received:" + repr(datagram) self.transport.write("data", address) # Note that the join function is picky about having a unique object # on which to call join. To avoid using startProtocol, the following is # sufficient: #reactor.listenMulticast(8005, MulticastServerUDP()).join(’224.0.0.1’) # Listen for multicast on 224.0.0.1:8005 reactor.listenMulticast(8005, MulticastServerUDP()) reactor.run() Source listing — MulticastServer.py The server protocol is very simple, and closely resembles a normal listenUDP implementation. The main difference is that instead of listenUDP, listenMulticast is called with a specified port number. The server must also call joinGroup to specify on which multicast IP address it will service requests. Another item of interest is the contents of the datagram. Many different applications use multicast as a way of device discovery, which leads to an abundance of packets flying around. Checking the payload can ensure that we only service requests from our specific clients. from twisted.internet.protocol import DatagramProtocol from twisted.internet import reactor from twisted.application.internet import MulticastServer class MulticastClientUDP(DatagramProtocol): def datagramReceived(self, datagram, address): print "Received:" + repr(datagram) # Send multicast on 224.0.0.1:8005, on our dynamically allocated port reactor.listenUDP(0, MulticastClientUDP()).write(’UniqueID’, (’224.0.0.1’, 8005)) reactor.run() MulticastServer.py — MulticastClient.py This is a mirror implementation of a standard UDP client. The only difference is that the destination IP is the multicast address. This datagram will be distributed to every server listening on 224.0.0.1 and port 8005. Note that the client port is specified as 0, as we have no need to keep track of what port the client is listening on. 3.2.5 Acknowledgements Thank you to all contributors to this document, including: • Kyle Robertson, author of the explanation and examples of multicast 3.3 Using Processes 3.3.1 Overview Along with connection to servers across the internet, Twisted also connects to local processes with much the same API. The API is described in more detail in the documentation of: • twisted.internet.interfaces.IReactorProcess • twisted.internet.interfaces.IProcessTransport • twisted.internet.interfaces.IProcessProtocol CHAPTER 3. LOW-LEVEL TWISTED 97 3.3.2 Running Another Process Processes are run through the reactor, using reactor.spawnProcess. Pipes are created to the child process, and added to the reactor core so that the application will not block while sending data into or pulling data out of the new process. reactor.spawnProcess requires two arguments, processProtocol and executable, and optionally takes several more: arguments, environment, path, userID, groupID, usePTY, and childFDs. Not all of these are available on Windows. from twisted.internet import reactor processProtocol = MyProcessProtocol() reactor.spawnProcess(processProtocol, executable, args=[program, arg1, arg2], env={’HOME’: os.environ[’HOME’]}, path, uid, gid, usePTY, childFDs) • processProtocol should be an instance of a subclass of twisted.internet.protocol.Process Protocol. The interface is described below. • executable is the full path of the program to run. It will be connected to processProtocol. • args is a list of command line arguments to be passed to the process. args[0] should be the name of the process. • env is a dictionary containing the environment to pass through to the process. • path is the directory to run the process in. The child will switch to the given directory just before starting the new program. The default is to stay in the current directory. • uid and gid are the user ID and group ID to run the subprocess as. Of course, changing identities will be more likely to succeed if you start as root. • usePTY specifies whether the child process should be run with a pty, or if it should just get a pair of pipes. Whether a program needs to be run with a PTY or not depends on the particulars of that program. Often, programs which primarily interact with users via a terminal do need a PTY. • childFDs lets you specify how the child’s file descriptors should be set up. Each key is a file descriptor number (an integer) as seen by the child. 0, 1, and 2 are usually stdin, stdout, and stderr, but some programs may be instructed to use additional fds through command-line arguments or environment variables. Each value is either an integer specifying one of the parent’s current file descriptors, the string “r” which creates a pipe that the parent can read from, or the string “w” which creates a pipe that the parent can write to. If childFDs is not provided, a default is used which creates the usual stdin-writer, stdout-reader, and stderr-reader pipes. args and env have empty default values, but many programs depend upon them to be set correctly. At the very least, args[0] should probably be the same as executable. If you just provide os.environ for env, the child program will inherit the environment from the current process, which is usually the civilized thing to do (unless you want to explicitly clean the environment as a security precaution). The default is to give an empty env to the child. reactor.spawnProcess returns an instance that implements IProcessTransport. 3.3.3 Writing a ProcessProtocol The ProcessProtocol you pass to spawnProcess is your interaction with the process. It has a very similar signature to a regular Protocol, but it has several extra methods to deal with events specific to a process. In our example, we will interface with ’wc’ to create a word count of user-given text. First, we’ll start by importing the required modules, and writing the initialization for our ProcessProtocol. from twisted.internet import protocol class WCProcessProtocol(protocol.ProcessProtocol): def __init__(self, text): self.text = text CHAPTER 3. LOW-LEVEL TWISTED 98 When the ProcessProtocol is connected to the protocol, it has the connectionMade method called. In our protocol, we will write our text to the standard input of our process and then close standard input, to the let the process know we are done writing to it. def connectionMade(self): self.transport.write(self.text) self.transport.closeStdin() At this point, the process has receieved the data, and it’s time for us to read the results. Instead of being received in dataReceived, data from standard output is received in outReceived. This is to distinguish it from data on standard error. def outReceived(self, data): fieldLength = len(data) / 3 lines = int(data[:fieldLength]) words = int(data[fieldLength:fieldLength*2]) chars = int(data[fieldLength*2:]) self.transport.loseConnection() self.receiveCounts(lines, words, chars) Now, the process has parsed the output, and ended the connection to the process. Then it sends the results on to the final method, receiveCounts. This is for users of the class to override, so as to do other things with the data. For our demonstration, we will just print the results. def receiveCounts(self, lines, words, chars): print ’Received counts from wc.’ print ’Lines:’, lines print ’Words:’, words print ’Characters:’, chars We’re done! To use our WCProcessProtocol, we create an instance, and pass it to spawnProcess. from twisted.internet import reactor wcProcess = WCProcessProtocol("accessing protocols through Twisted is fun!\n") reactor.spawnProcess(wcProcess, ’wc’, [’wc’]) reactor.run() 3.3.4 Things that can happen to your ProcessProtocol These are the methods that you can usefully override in your subclass of ProcessProtocol: •.connectionMade(): This is called when the program is started, and makes a good place to write data into the stdin pipe (using self.transport.write). •.outReceived(data): This is called with data that was received from the process’ stdout pipe. Pipes tend to provide data in larger chunks than sockets (one kilobyte is a common buffer size), so you may not experience the “random dribs and drabs” behavior typical of network sockets, but regardless you should be prepared to deal if you don’t get all your data in a single call. To do it properly, outReceived ought to simply accumulate the data and put off doing anything with it until the process has finished. •.errReceived(data): This is called with data from the process’ stderr pipe. It behaves just like out Received. •.inConnectionLost: This is called when the reactor notices that the process’ stdin pipe has closed. Pro- grams don’t typically close their own stdin, so this will probably get called when your ProcessProtocol has shut down the write side with self.transport.loseConnection. •.outConnectionLost: This is called when the program closes its stdout pipe. This usually happens when the program terminates. •.errConnectionLost: Same as outConnectionLost, but for stderr instead of stdout. CHAPTER 3. LOW-LEVEL TWISTED 99 •.processExited(status): This is called when the child process has been reaped, and receives infor- mation about the process’ exit status. The status is passed in the form of a Failure instance, created with a .value that either holds a ProcessDone object if the process terminated normally (it died of natural causes instead of receiving a signal, and if the exit code was 0), or a ProcessTerminated object (with an .exit Code attribute) if something went wrong. •.processEnded(status): This is called when all the file descriptors associated with the child process have been closed and the process has been reaped. This means it is the last callback which will be made onto a ProcessProtocol. The status parameter has the same meaning as it does for processExited. The base-class definitions of most of these functions are no-ops. This will result in all stdout and stderr being thrown away. Note that it is important for data you don’t care about to be thrown away: if the pipe were not read, the child process would eventually block as it tried to write to a full pipe. 3.3.5 Things you can do from your ProcessProtocol The following are the basic ways to control the child process: • self.transport.write(data): Stuff some data in the stdin pipe. Note that this write method will queue any data that can’t be written immediately. Writing will resume in the future when the pipe becomes writable again. • self.transport.closeStdin: Close the stdin pipe. Programs which act as filters (reading from stdin, modifying the data, writing to stdout) usually take this as a sign that they should finish their job and terminate. For these programs, it is important to close stdin when you’re done with it, otherwise the child process will never quit. • self.transport.closeStdout: Not usually called, since you’re putting the process into a state where any attempt to write to stdout will cause a SIGPIPE error. This isn’t a nice thing to do to the poor process. • self.transport.closeStderr: Not usually called, same reason as closeStdout. • self.transport.loseConnection: Close all three pipes. • self.transport.signalProcess(’KILL’): Kill the child process. This will eventually result in processEnded being called. 3.3.6 Verbose Example Here is an example that is rather verbose about exactly when all the methods are called. It writes a number of lines into the wc program and then parses the output. #!/usr/bin/env python # Copyright (c) 2009-2010 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import protocol from twisted.internet import reactor import re class MyPP(protocol.ProcessProtocol): def __init__(self, verses): self.verses = verses self.data = "" def connectionMade(self): print "connectionMade!" for i in range(self.verses): self.transport.write("Aleph-null bottles of beer on the wall,\n" + "Aleph-null bottles of beer,\n" + CHAPTER 3. LOW-LEVEL TWISTED 100 "Take one down and pass it around,\n" + "Aleph-null bottles of beer on the wall.\n") self.transport.closeStdin() # tell them we’re done def outReceived(self, data): print "outReceived! with %d bytes!" % len(data) self.data = self.data + data def errReceived(self, data): print "errReceived! with %d bytes!" % len(data) def inConnectionLost(self): print "inConnectionLost! stdin is closed! (we probably did it)" def outConnectionLost(self): print "outConnectionLost! The child closed their stdout!" # now is the time to examine what they wrote #print "I saw them write:", self.data (dummy, lines, words, chars, file) = re.split(r’\s+’, self.data) print "I saw %s lines" % lines def errConnectionLost(self): print "errConnectionLost! The child closed their stderr." def processExited(self, reason): print "processExited, status %d" % (reason.value.exitCode,) def processEnded(self, reason): print "processEnded, status %d" % (reason.value.exitCode,) print "quitting" reactor.stop() pp = MyPP(10) reactor.spawnProcess(pp, "wc", ["wc"], {}) reactor.run() Source listing — process.py The exact output of this program depends upon the relative timing of some un-synchronized events. In particular, the program may observe the child process close its stderr pipe before or after it reads data from the stdout pipe. One possible transcript would look like this: % ./process.py connectionMade! inConnectionLost! stdin is closed! (we probably did it) errConnectionLost! The child closed their stderr. outReceived! with 24 bytes! outConnectionLost! The child closed their stdout! I saw 40 lines processEnded, status 0 quitting Main loop terminated. % 3.3.7 Doing it the Easy Way Frequently, one just needs a simple way to get all the output from a program. In the blocking world, you might use commands.getoutput from the standard library, but using that in an event-driven program will cause everything else to stall until the command finishes. (in addition, the SIGCHLD handler used by that function does not play well with Twisted’s own signal handling). For these cases, the twisted.internet.utils.getProcessOutput function can be used. Here is a simple example: from twisted.internet import protocol, utils, reactor from twisted.python import failure from cStringIO import StringIO CHAPTER 3. LOW-LEVEL TWISTED 101 class FortuneQuoter(protocol.Protocol): fortune = ’/usr/games/fortune’ def connectionMade(self): output = utils.getProcessOutput(self.fortune) output.addCallbacks(self.writeResponse, self.noResponse) def writeResponse(self, resp): self.transport.write(resp) self.transport.loseConnection() def noResponse(self, err): self.transport.loseConnection() if __name__ == ’__main__’: f = protocol.Factory() f.protocol = FortuneQuoter reactor.listenTCP(10999, f) reactor.run() Source listing — quotes.py If you only need the final exit code (like commands.getstatusoutput(cmd)[0]), the twisted. internet.utils.getProcessValue function is useful. Here is an example: from twisted.internet import utils, reactor def printTrueValue(val): print "/bin/true exits with rc=%d" % val output = utils.getProcessValue(’/bin/false’) output.addCallback(printFalseValue) def printFalseValue(val): print "/bin/false exits with rc=%d" % val reactor.stop() output = utils.getProcessValue(’/bin/true’) output.addCallback(printTrueValue) reactor.run() Source listing — trueandfalse.py 3.3.8 Mapping File Descriptors “stdin”, “stdout”, and “stderr” are just conventions. Programs which operate as filters generally accept input on fd0, write their output on fd1, and emit error messages on fd2. This is common enough that the standard C library provides macros like “stdin” to mean fd0, and shells interpret the pipe character “—” to mean “redirect fd1 from one command into fd0 of the next command”. But these are just conventions, and programs are free to use additional file descriptors or even ignore the standard three entirely. The “childFDs” argument allows you to specify exactly what kind of files descriptors the child process should be given. Each child FD can be put into one of three states: CHAPTER 3. LOW-LEVEL TWISTED 102 • Mapped to a parent FD: this causes the child’s reads and writes to come from or go to the same source/destination as the parent. • Feeding into a pipe which can be read by the parent. • Feeding from a pipe which the parent writes into. Mapping the child FDs to the parent’s is very commonly used to send the child’s stderr output to the same place as the parent’s. When you run a program from the shell, it will typically leave fds 0, 1, and 2 mapped to the shell’s 0, 1, and 2, allowing you to see the child program’s output on the same terminal you used to launch the child. Likewise, inetd will typically map both stdin and stdout to the network socket, and may map stderr to the same socket or to some kind of logging mechanism. This allows the child program to be implemented with no knowledge of the network: it merely speaks its protocol by doing reads on fd0 and writes on fd1. Feeding into a parent’s read pipe is used to gather output from the child, and is by far the most common way of interacting with child processes. Feeding from a parent’s write pipe allows the parent to control the child. Programs like “bc” or “ftp” can be controlled this way, by writing commands into their stdin stream. The “childFDs” dictionary maps file descriptor numbers (as will be seen by the child process) to one of these three states. To map the fd to one of the parent’s fds, simply provide the fd number as the value. To map it to a read pipe, use the string “r” as the value. To map it to a write pipe, use the string “w”. For example, the default mapping sets up the standard stdin/stdout/stderr pipes. It is implemented with the follow- ing dictionary: childFDs = { 0: "w", 1: "r", 2: "r" } To launch a process which reads and writes to the same places that the parent python program does, use this: childFDs = { 0: 0, 1: 1, 2: 2} To write into an additional fd (say it is fd number 4), use this: childFDs = { 0: "w", 1: "r", 2: "r" , 4: "w"} ProcessProtocols with extra file descriptors When you provide a “childFDs” dictionary with more than the normal three fds, you need addtional methods to access those pipes. These methods are more generalized than the .outReceived ones described above. In fact, those methods (outReceived and errReceived) are actually just wrappers left in for compatibility with older code, written before this generalized fd mapping was implemented. The new list of things that can happen to your ProcessProtocol is as follows: •.connectionMade: This is called when the program is started. •.childDataReceived(childFD, data): This is called with data that was received from one of the process’ output pipes (i.e. where the childFDs value was “r”. The actual file number (from the point of view of the child process) is in “childFD”. For compatibility, the default implementation of .dataReceived dis- patches to .outReceived or .errReceived when “childFD” is 1 or 2. •.childConnectionLost(childFD): This is called when the reactor notices that one of the process’ pipes has been closed. This either means you have just closed down the parent’s end of the pipe (with .transport. closeChildFD), the child closed the pipe explicitly (sometimes to indicate EOF), or the child process has terminated and the kernel has closed all of its pipes. The “childFD” argument tells you which pipe was closed. Note that you can only find out about file descriptors which were mapped to pipes: when they are mapped to existing fds the parent has no way to notice when they’ve been closed. For compatibility, the default implemen- tation dispatches to .inConnectionLost,.outConnectionLost, or .errConnectionLost. •.processEnded(status): This is called when the child process has been reaped, and all pipes have been closed. This insures that all data written by the child prior to its death will be received before .process Ended is invoked. In addition to those methods, there are other methods available to influence the child process: CHAPTER 3. LOW-LEVEL TWISTED 103 • self.transport.writeToChild(childFD, data): Stuff some data into an input pipe. .write simply writes to childFD=0. • self.transport.closeChildFD(childFD): Close one of the child’s pipes. Closing an input pipe is a common way to indicate EOF to the child process. Closing an output pipe is neither very friendly nor very useful. Examples GnuPG, the encryption program, can use additional file descriptors to accept a passphrase and emit status output. These are distinct from stdin (used to accept the crypttext), stdout (used to emit the plaintext), and stderr (used to emit human-readable status/warning messages). The passphrase FD reads until the pipe is closed and uses the resulting string to unlock the secret key that performs the actual decryption. The status FD emits machine-parseable status messages to indicate the validity of the signature, which key the message was encrypted to, etc. gpg accepts command-line arguments to specify what these fds are, and then assumes that they have been opened by the parent before the gpg process is started. It simply performs reads and writes to these fd numbers. To invoke gpg in decryption/verification mode, you would do something like the following: class GPGProtocol(ProcessProtocol): def __init__(self, crypttext): self.crypttext = crypttext self.plaintext = "" self.status = "" def connectionMade(self): self.transport.writeToChild(3, self.passphrase) self.transport.closeChildFD(3) self.transport.writeToChild(0, self.crypttext) self.transport.closeChildFD(0) def childDataReceived(self, childFD, data): if childFD == 1: self.plaintext += data if childFD == 4: self.status += data def processEnded(self, status): rc = status.value.exitCode if rc == 0: self.deferred.callback(self) else: self.deferred.errback(rc) def decrypt(crypttext): gp = GPGProtocol(crypttext) gp.deferred = Deferred() cmd = ["gpg", "--decrypt", "--passphrase-fd", "3", "--status-fd", "4", "--batch"] p = reactor.spawnProcess(gp, cmd[0], cmd, env=None, childFDs={0:"w", 1:"r", 2:2, 3:"w", 4:"r"}) return gp.deferred In this example, the status output could be parsed after the fact. It could, of course, be parsed on the fly, as it is a simple line-oriented protocol. Methods from LineReceiver could be mixed in to make this parsing more convenient. The stderr mapping (“2:2”) used will cause any GPG errors to be emitted by the parent program, just as if those errors had caused in the parent itself. This is sometimes desireable (it roughly corresponds to letting exceptions propagate upwards), especially if you do not expect to encounter errors in the child process and want them to be more visible to the end user. The alternative is to map stderr to a read-pipe and handle any such output from within the ProcessProtocol (roughly corresponding to catching the exception locally). CHAPTER 3. LOW-LEVEL TWISTED 104 3.4 Deferred Reference This document is a guide to the behaviour of the twisted.internet.defer.Deferred object, and to various ways you can use them when they are returned by functions. This document assumes that you are familiar with the basic principle that the Twisted framework is structured around: asynchronous, callback-based programming, where instead of having blocking code in your program or using threads to run blocking code, you have functions that return immediately and then begin a callback chain when data is available. After reading this document, the reader should expect to be able to deal with most simple APIs in Twisted and Twisted-using code that return Deferreds. • what sorts of things you can do when you get a Deferred from a function call; and • how you can write your code to robustly handle errors in Deferred code. 3.4.1 Deferreds Twisted uses the Deferred object to manage the callback sequence. The client application attaches a series of functions to the deferred to be called in order when the results of the asychronous request are available (this series of functions is known as a series of callbacks, or a callback chain), together with a series of functions to be called if there is an error in the asychronous request (known as a series of errbacks or an errback chain). The asychronous library code calls the first callback when the result is available, or the first errback when an error occurs, and the Deferred object then hands the results of each callback or errback function to the next function in the chain. 3.4.2 Callbacks A twisted.internet.defer.Deferred is a promise that a function will at some point have a result. We can attach callback functions to a Deferred, and once it gets a result these callbacks will be called. In addition Deferreds allow the developer to register a callback for an error, with the default behavior of logging the error. The deferred mechanism standardizes the application programmer’s interface with all sorts of blocking or delayed operations. from twisted.internet import reactor, defer def getDummyData(x): """ This function is a dummy which simulates a delayed result and returns a Deferred which will fire with that result. Don’t try too hard to understand this. """ d = defer.Deferred() # simulate a delayed result by asking the reactor to fire the # Deferred in 2 seconds time with the result x * 3 reactor.callLater(2, d.callback, x * 3) return d def printData(d): """ Data handling function to be added as a callback: handles the data by printing the result """ print d d = getDummyData(3) d.addCallback(printData) # manually set up the end of the process by asking the reactor to # stop itself in 4 seconds time reactor.callLater(4, reactor.stop) CHAPTER 3. LOW-LEVEL TWISTED 105 # start up the Twisted reactor (event loop handler) manually reactor.run() Multiple callbacks Multiple callbacks can be added to a Deferred. The first callback in the Deferred’s callback chain will be called with the result, the second with the result of the first callback, and so on. Why do we need this? Well, consider a Deferred returned by twisted.enterprise.adbapi - the result of a SQL query. A web widget might add a callback that converts this result into HTML, and pass the Deferred onwards, where the callback will be used by twisted to return the result to the HTTP client. The callback chain will be bypassed in case of errors or exceptions. from twisted.internet import reactor, defer class Getter: def gotResults(self, x): """ The Deferred mechanism provides a mechanism to signal error conditions. In this case, odd numbers are bad. This function demonstrates a more complex way of starting the callback chain by checking for expected results and choosing whether to fire the callback or errback chain """ if x % 2 == 0: self.d.callback(x*3) else: self.d.errback(ValueError("You used an odd number!")) def _toHTML(self, r): """ This function converts r to HTML. It is added to the callback chain by getDummyData in order to demonstrate how a callback passes its own result to the next callback """ return "Result: %s" % r def getDummyData(self, x): """ The Deferred mechanism allows for chained callbacks. In this example, the output of gotResults is first passed through _toHTML on its way to printData. Again this function is a dummy, simulating a delayed result using callLater, rather than using a real asynchronous setup. """ self.d = defer.Deferred() # simulate a delayed result by asking the reactor to schedule # gotResults in 2 seconds time reactor.callLater(2, self.gotResults, x) self.d.addCallback(self._toHTML) return self.d def printData(d): print d CHAPTER 3. LOW-LEVEL TWISTED 106 def printError(failure): import sys sys.stderr.write(str(failure)) # this series of callbacks and errbacks will print an error message g = Getter() d = g.getDummyData(3) d.addCallback(printData) d.addErrback(printError) # this series of callbacks and errbacks will print "Result: 12" g = Getter() d = g.getDummyData(4) d.addCallback(printData) d.addErrback(printError) reactor.callLater(4, reactor.stop); reactor.run() Visual Explanation 1. Requesting method (data sink) requests data, gets Deferred object. 2. Requesting method attaches callbacks to Deferred object. CHAPTER 3. LOW-LEVEL TWISTED 107 CHAPTER 3. LOW-LEVEL TWISTED 108 1. When the result is ready, give it to the Deferred object. .callback(result) if the operation succeeded, .errback(failure) if it failed. Note that failure is typically an instance of a twisted.python. failure.Failure instance. 2. Deferred object triggers previously-added (call/err)back with the result or failure. Execution then follows the following rules, going down the chain of callbacks to be processed. • Result of the callback is always passed as the first argument to the next callback, creating a chain of processors. • If a callback raises an exception, switch to errback. • An unhandled failure gets passed down the line of errbacks, this creating an asynchronous analog to a series to a series of except: statements. • If an errback doesn’t raise an exception or return a twisted.python.failure.Failure instance, switch to callback. 3.4.3 Errbacks Deferred’s error handling is modeled after Python’s exception handling. In the case that no errors occur, all the callbacks run, one after the other, as described above. If the errback is called instead of the callback (e.g. because a DB query raised an error), then a twisted. python.failure.Failure is passed into the first errback (you can add multiple errbacks, just like with call- backs). You can think of your errbacks as being like except blocks of ordinary Python code. Unless you explicitly raise an error in except block, the Exception is caught and stops propagating, and normal execution continues. The same thing happens with errbacks: unless you explicitly return a Failure or (re-)raise an exception, the error stops propagating, and normal callbacks continue executing from that point (using the value returned from the errback). If the errback does returns a Failure or raise an exception, then that is passed to the next errback, and so on. Note: If an errback doesn’t return anything, then it effectively returns None, meaning that callbacks will continue to be executed after this errback. This may not be what you expect to happen, so be careful. Make sure your errbacks return a Failure (probably the one that was passed to it), or a meaningful return value for the next callback. Also, twisted.python.failure.Failure instances have a useful method called trap, allowing you to effectively do the equivalent of: try: # code that may throw an exception cookSpamAndEggs() except (SpamException, EggException): # Handle SpamExceptions and EggExceptions ... You do this by: def errorHandler(failure): failure.trap(SpamException, EggException) # Handle SpamExceptions and EggExceptions d.addCallback(cookSpamAndEggs) d.addErrback(errorHandler) If none of arguments passed to failure.trap match the error encapsulated in that Failure, then it re-raises the error. There’s another potential “gotcha” here. There’s a method twisted.internet.defer.Deferred.add Callbacks which is similar to, but not exactly the same as, addCallback followed by addErrback. In partic- ular, consider these two cases: # Case 1 d = getDeferredFromSomewhere() d.addCallback(callback1) # A d.addErrback(errback1) # B CHAPTER 3. LOW-LEVEL TWISTED 109 d.addCallback(callback2) d.addErrback(errback2) # Case 2 d = getDeferredFromSomewhere() d.addCallbacks(callback1, errback1) # C d.addCallbacks(callback2, errback2) If an error occurs in callback1, then for Case 1 errback1 will be called with the failure. For Case 2, errback2 will be called. Be careful with your callbacks and errbacks. What this means in a practical sense is in Case 1, ”A” will handle a success condition from getDeferredFrom Somewhere, and ”B” will handle any errors that occur from either the upstream source, or that occur in ’A’. In Case 2, ”C”’s errback1 will only handle an error condition raised by getDeferredFromSomewhere, it will not do any handling of errors raised in callback1. Unhandled Errors If a Deferred is garbage-collected with an unhandled error (i.e. it would call the next errback if there was one), then Twisted will write the error’s traceback to the log file. This means that you can typically get away with not adding errbacks and still get errors logged. Be careful though; if you keep a reference to the Deferred around, preventing it from being garbage-collected, then you may never see the error (and your callbacks will mysteriously seem to have never been called). If unsure, you should explicitly add an errback after your callbacks, even if all you do is: # Make sure errors get logged from twisted.python import log d.addErrback(log.err) 3.4.4 Handling either synchronous or asynchronous results In some applications, there are functions that might be either asynchronous or synchronous. For example, a user authentication function might be able to check in memory whether a user is authenticated, allowing the authentication function to return an immediate result, or it may need to wait on network data, in which case it should return a Deferred to be fired when that data arrives. However, a function that wants to check if a user is authenticated will then need to accept both immediate results and Deferreds. In this example, the library function authenticateUser uses the application function isValidUser to authenticate a user: def authenticateUser(isValidUser, user): if isValidUser(user): print "User is authenticated" else: print "User is not authenticated" However, it assumes that isValidUser returns immediately, whereas isValidUser may actually authen- ticate the user asynchronously and return a Deferred. It is possible to adapt this trivial user authentication code to accept either a synchronous isValidUser or an asynchronous isValidUser, allowing the library to handle ei- ther type of function. It is, however, also possible to adapt synchronous functions to return Deferreds. This section describes both alternatives: handling functions that might be synchronous or asynchronous in the library function (authenticateUser) or in the application code. Handling possible Deferreds in the library code Here is an example of a synchronous user authentication function that might be passed to authenticateUser: def synchronousIsValidUser(user): ’’’ Return true if user is a valid user, false otherwise ’’’ return user in ["Alice", "Angus", "Agnes"] CHAPTER 3. LOW-LEVEL TWISTED 110 Source listing — synch-validation.py However, here’s an asynchronousIsValidUser function that returns a Deferred: from twisted.internet import reactor def asynchronousIsValidUser(d, user): d = Deferred() reactor.callLater(2, d.callback, user in ["Alice", "Angus", "Agnes"]) return d Our original implementation of authenticateUser expected isValidUser to be synchronous, but now we need to change it to handle both synchronous and asynchronous implementations of isValidUser. For this, we use maybeDeferred to call isValidUser, ensuring that the result of isValidUser is a Deferred, even if is ValidUser is a synchronous function: from twisted.internet import defer def printResult(result): if result: print "User is authenticated" else: print "User is not authenticated" def authenticateUser(isValidUser, user): d = defer.maybeDeferred(isValidUser, user) d.addCallback(printResult) Now isValidUser could be either synchronousIsValidUser or asynchronousIsValidUser. It is also possible to modify synchronousIsValidUser to return a Deferred, see Generating Deferreds (page 113) for more information. 3.4.5 DeferredList Sometimes you want to be notified after several different events have all happened, rather than waiting for each one individually. For example, you may want to wait for all the connections in a list to close. twisted.internet. defer.DeferredList is the way to do this. To create a DeferredList from multiple Deferreds, you simply pass a list of the Deferreds you want it to wait for: # Creates a DeferredList dl = defer.DeferredList([deferred1, deferred2, deferred3]) You can now treat the DeferredList like an ordinary Deferred; you can call addCallbacks and so on. The DeferredList will call its callback when all the deferreds have completed. The callback will be called with a list of the results of the Deferreds it contains, like so: def printResult(result): for (success, value) in result: if success: print ’Success:’, value else: print ’Failure:’, value.getErrorMessage() deferred1 = defer.Deferred() deferred2 = defer.Deferred() deferred3 = defer.Deferred() dl = defer.DeferredList([deferred1, deferred2, deferred3], consumeErrors=True) dl.addCallback(printResult) deferred1.callback(’one’) deferred2.errback(Exception(’bang!’)) CHAPTER 3. LOW-LEVEL TWISTED 111 deferred3.callback(’three’) # At this point, dl will fire its callback, printing: # Success: one # Failure: bang! # Success: three # (note that defer.SUCCESS == True, and defer.FAILURE == False) A standard DeferredList will never call errback, but failures in Deferreds passed to a DeferredList will still errback unless consumeErrors is passed True. See below for more details about this and other flags which modify the behavior of DeferredList. Note: If you want to apply callbacks to the individual Deferreds that go into the DeferredList, you should be careful about when those callbacks are added. The act of adding a Deferred to a DeferredList inserts a callback into that Deferred (when that callback is run, it checks to see if the DeferredList has been completed yet). The important thing to remember is that it is this callback which records the value that goes into the result list handed to the DeferredList’s callback. Therefore, if you add a callback to the Deferred after adding the Deferred to the DeferredList, the value returned by that callback will not be given to the DeferredList’s callback. To avoid confusion, we recommend not adding callbacks to a Deferred once it has been used in a DeferredList. def printResult(result): print result def addTen(result): return result + " ten" # Deferred gets callback before DeferredList is created deferred1 = defer.Deferred() deferred2 = defer.Deferred() deferred1.addCallback(addTen) dl = defer.DeferredList([deferred1, deferred2]) dl.addCallback(printResult) deferred1.callback("one") # fires addTen, checks DeferredList, stores "one ten" deferred2.callback("two") # At this point, dl will fire its callback, printing: # [(1, ’one ten’), (1, ’two’)] # Deferred gets callback after DeferredList is created deferred1 = defer.Deferred() deferred2 = defer.Deferred() dl = defer.DeferredList([deferred1, deferred2]) deferred1.addCallback(addTen) # will run *after* DeferredList gets its value dl.addCallback(printResult) deferred1.callback("one") # checks DeferredList, stores "one", fires addTen deferred2.callback("two") # At this point, dl will fire its callback, printing: # [(1, ’one), (1, ’two’)] Other behaviours DeferredList accepts three keyword arguments that modify its behaviour: fireOnOneCallback, fireOnOne Errback and consumeErrors. If fireOnOneCallback is set, the DeferredList will immediately call its callback as soon as any of its Deferreds call their callback. Similarly, fireOnOneErrback will call errback as soon as any of the Deferreds call their errback. Note that DeferredList is still one-shot, like ordinary Deferreds, so after a callback or errback has been called the DeferredList will do nothing further (it will just silently ignore any other results from its Deferreds). The fireOnOneErrback option is particularly useful when you want to wait for all the results if everything succeeds, but also want to know immediately if something fails. CHAPTER 3. LOW-LEVEL TWISTED 112 The consumeErrors argument will stop the DeferredList from propagating any errors along the callback chains of any Deferreds it contains (usually creating a DeferredList has no effect on the results passed along the callbacks and errbacks of their Deferreds). Stopping errors at the DeferredList with this option will prevent “Unhandled error in Deferred” warnings from the Deferreds it contains without needing to add extra errbacks1. 3.4.6 Class Overview This is an overview API reference for Deferred from the point of using a Deferred returned by a function. It is not meant to be a substitute for the docstrings in the Deferred class, but can provide guidelines for its use. There is a parallel overview of functions used by the Deferred’s creator in Generating Deferreds (page 113). Basic Callback Functions • addCallbacks(self, callback[, errback, callbackArgs, callbackKeywords, errbackArgs, errbackKeywords]) This is the method you will use to interact with Deferred. It adds a pair of callbacks “parallel” to each other (see diagram above) in the list of callbacks made when the Deferred is called back to. The signature of a method added using addCallbacks should be myMethod(result, *methodArgs, **methodKeywords). If your method is passed in the callback slot, for example, all arguments in the tuple callbackArgs will be passed as *methodArgs to your method. There are various convenience methods that are derivative of addCallbacks. I will not cover them in detail here, but it is important to know about them in order to create concise code. – addCallback(callback, *callbackArgs, **callbackKeywords) Adds your callback at the next point in the processing chain, while adding an errback that will re-raise its first argument, not affecting further processing in the error case. Note that, while addCallbacks (plural) requires the arguments to be passed in a tuple, addCallback (sin- gular) takes all its remaining arguments as things to be passed to the callback function. The reason is obvious: addCallbacks (plural) cannot tell whether the arguments are meant for the callback or the er- rback, so they must be specifically marked by putting them into a tuple. addCallback (singular) knows that everything is destined to go to the callback, so it can use Python’s “*” and “**” syntax to collect the remaining arguments. – addErrback(errback, *errbackArgs, **errbackKeywords) Adds your errback at the next point in the processing chain, while adding a callback that will return its first argument, not affecting further processing in the success case. – addBoth(callbackOrErrback, *callbackOrErrbackArgs, **callbackOrErrback Keywords) This method adds the same callback into both sides of the processing chain at both points. Keep in mind that the type of the first argument is indeterminate if you use this method! Use it for finally: style blocks. Chaining Deferreds If you need one Deferred to wait on another, all you need to do is return a Deferred from a method added to addCall- backs. Specifically, if you return Deferred B from a method added to Deferred A using A.addCallbacks, Deferred A’s processing chain will stop until Deferred B’s .callback() method is called; at that point, the next callback in A will be passed the result of the last callback in Deferred B’s processing chain at the time. If this seems confusing, don’t worry about it right now – when you run into a situation where you need this behavior, you will probably recognize it immediately and realize why this happens. If you want to chain deferreds manually, there is also a convenience method to help you. • chainDeferred(otherDeferred) 1Unless of course a later callback starts a fresh error but as we’ve already noted, adding callbacks to a Deferred after its used in a DeferredList is confusing and usually avoided. CHAPTER 3. LOW-LEVEL TWISTED 113 Add otherDeferred to the end of this Deferred’s processing chain. When self.callback is called, the result of my processing chain up to this point will be passed to otherDeferred.callback. Further additions to my callback chain do not affect otherDeferred This is the same as self.addCallbacks(otherDeferred.callback, otherDeferred. errback) 3.4.7 See also 1. Generating Deferreds (this page), an introduction to writing asynchronous functions that return Deferreds. 3.5 Generating Deferreds Deferred objects are signals that a function you have called does not yet have the data you want available. When a function returns a Deferred object, your calling function attaches callbacks to it to handle the data when available. This document addresses the other half of the question: writing functions that return Deferreds, that is, constructing Deferred objects, arranging for them to be returned immediately without blocking until data is available, and firing their callbacks when the data is available. This document assumes that you are familiar with the asynchronous model used by Twisted, and with using deferreds returned by functions (page 104) . 3.5.1 Class overview This is an overview API reference for Deferred from the point of creating a Deferred and firing its callbacks and errbacks. It is not meant to be a substitute for the docstrings in the Deferred class, but can provide guidelines for its use. There is a parallel overview of functions used by calling function which the Deferred is returned to at Using Deferreds (page 112). Basic Callback Functions • callback(result) Run success callbacks with the given result. This can only be run once. Later calls to this or errback will raise twisted.internet.defer.AlreadyCalledError. If further callbacks or errbacks are added after this point, addCallbacks will run the callbacks immediately. • errback(failure) Run error callbacks with the given failure. This can only be run once. Later calls to this or callback will raise twisted.internet.defer.AlreadyCalledError. If further callbacks or errbacks are added after this point, addCallbacks will run the callbacks immediately. 3.5.2 What Deferreds don’t do: make your code asynchronous Deferreds do not make the code magically not block. Let’s take this function as an example: from twisted.internet import defer TARGET = 10000 def largeFibonnaciNumber(): # create a Deferred object to return: d = defer.Deferred() # calculate the ten thousandth Fibonnaci number first = 0 CHAPTER 3. LOW-LEVEL TWISTED 114 second = 1 for i in xrange(TARGET - 1): new = first + second first = second second = new if i % 100 == 0: print "Progress: calculating the %dth Fibonnaci number" % i # give the Deferred the answer to pass to the callbacks: d.callback(second) # return the Deferred with the answer: return d import time timeBefore = time.time() # call the function and get our Deferred d = largeFibonnaciNumber() timeAfter = time.time() print "Total time taken for largeFibonnaciNumber call: %0.3f seconds" % \ (timeAfter - timeBefore) # add a callback to it to print the number def printNumber(number): print "The %dth Fibonacci number is %d" % (TARGET, number) print "Adding the callback now." d.addCallback(printNumber) You will notice that despite creating a Deferred in the largeFibonnaciNumber function, these things hap- pened: • the ”Total time taken for largeFibonnaciNumber call” output shows that the function did not return immediately as asynchronous functions are expected to do; and • rather than the callback being added before the result was available and called after the result is available, it isn’t even added until after the calculation has been completed. The function completed its calculation before returning, blocking the process until it had finished, which is exactly what asynchronous functions are not meant to do. Deferreds are not a non-blocking talisman: they are a signal for asynchronous functions to use to pass results onto callbacks, but using them does not guarantee that you have an asynchronous function. 3.5.3 Advanced Processing Chain Control • pause() Cease calling any methods as they are added, and do not respond to callback, until self.unpause() is called. • unpause() If callback has been called on this Deferred already, call all the callbacks that have been added to this Deferred since pause was called. CHAPTER 3. LOW-LEVEL TWISTED 115 Whether it was called or not, this will put this Deferred in a state where further calls to addCallbacks or callback will work as normal. 3.5.4 Returning Deferreds from synchronous functions Sometimes you might wish to return a Deferred from a synchronous function. There are several reasons why, the major two are maintaining API compatibility with another version of your function which returns a Deferred, or allowing for the possiblity that in the future your function might need to be asynchronous. In the Using Deferreds (page 104) reference, we gave the following example of a synchronous function: def synchronousIsValidUser(user): ’’’ Return true if user is a valid user, false otherwise ’’’ return user in ["Alice", "Angus", "Agnes"] Source listing — synch-validation.py While we can require that callers of our function wrap our synchronous result in a Deferred using maybe Deferred, for the sake of API compatibility it is better to return a Deferred ourself using defer.succeed: from twisted.internet import defer def immediateIsValidUser(user): ’’’ Returns a Deferred resulting in true if user is a valid user, false otherwise ’’’ result = user in ["Alice", "Angus", "Agnes"] # return a Deferred object already called back with the value of result return defer.succeed(result) There is an equivalent defer.fail method to return a Deferred with the errback chain already fired. 3.5.5 Integrating blocking code with Twisted At some point, you are likely to need to call a blocking function: many functions in third party libraries will have long running blocking functions. There is no way to ’force’ a function to be asynchronous: it must be written that way specifically. When using Twisted, your own code should be asynchronous, but there is no way to make third party functions asynchronous other than rewriting them. In this case, Twisted provides the ability to run the blocking code in a separate thread rather than letting it block your application. The twisted.internet.threads.deferToThread function will set up a thread to run your blocking function, return a Deferred and later fire that Deferred when the thread completes. Let’s assume our largeFibonnaciNumber function from above is in a third party library (returning the result of the calculation, not a Deferred) and is not easily modifiable to be finished in discrete blocks. This example shows it being called in a thread, unlike in the earlier section we’ll see that the operation does not block our entire program: def largeFibonnaciNumber(): """ Represent a long running blocking function by calculating the TARGETth Fibonnaci number """ TARGET = 10000 first = 0 second = 1 CHAPTER 3. LOW-LEVEL TWISTED 116 for i in xrange(TARGET - 1): new = first + second first = second second = new return second from twisted.internet import threads, reactor def fibonacciCallback(result): """ Callback which manages the largeFibonnaciNumber result by printing it out """ print "largeFibonnaciNumber result =", result # make sure the reactor stops after the callback chain finishes, # just so that this example terminates reactor.stop() def run(): """ Run a series of operations, deferring the largeFibonnaciNumber operation to a thread and performing some other operations after adding the callback """ # get our Deferred which will be called with the largeFibonnaciNumber result d = threads.deferToThread(largeFibonnaciNumber) # add our callback to print it out d.addCallback(fibonacciCallback) print "1st line after the addition of the callback" print "2nd line after the addition of the callback" if __name__ == ’__main__’: run() reactor.run() 3.5.6 Possible sources of error Deferreds greatly simplify the process of writing asynchronous code by providing a standard for registering callbacks, but there are some subtle and sometimes confusing rules that you need to follow if you are going to use them. This mostly applies to people who are writing new systems that use Deferreds internally, and not writers of applications that just add callbacks to Deferreds produced and processed by other systems. Nevertheless, it is good to know. Firing Deferreds more than once is impossible Deferreds are one-shot. You can only call Deferred.callback or Deferred.errback once. The processing chain continues each time you add new callbacks to an already-called-back-to Deferred. Synchronous callback execution If a Deferred already has a result available, addCallback may call the callback synchronously: that is, immediately after it’s been added. In situations where callbacks modify state, it is might be desirable for the chain of processing to halt until all callbacks are added. For this, it is possible to pause and unpause a Deferred’s processing chain while you are adding lots of callbacks. Be careful when you use these methods! If you pause a Deferred, it is your responsibility to make sure that you unpause it. The function adding the callbacks must unpause a paused Deferred, it should never be the responsibility of the code that actually fires the callback chain by calling callback or errback as this would negate its usefulness! CHAPTER 3. LOW-LEVEL TWISTED 117 3.6 Deferreds are beautiful! (A Tutorial) 3.6.1 Introduction Deferreds are quite possibly the single most confusing topic that a newcomer to Twisted has to deal with. I am going to forgo the normal talk about what deferreds are, what they aren’t, and why they’re used in Twisted. Instead, I’m going show you the logic behind what they do. A deferred allows you to encapsulate the logic that you’d normally use to make a series of function calls after receiving a result into a single object. In the examples that follow, I’ll first show you what’s going to go on behind the scenes in the deferred chain, then show you the deferred API calls that set up that chain. All of these examples are runnable code, so feel free to play around with them. 3.6.2 A simple example First, a simple example so that we have something to talk about: #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import defer from twisted.python import failure, util """ Here we have the simplest case, a single callback and a single errback. """ num = 0 def handleFailure(f): print "errback" print "we got an exception: %s" % (f.getTraceback(),) f.trap(RuntimeError) def handleResult(result): global num; num += 1 print "callback %s" % (num,) print "\tgot result: %s" % (result,) return "yay! handleResult was successful!" def behindTheScenes(result): # equivalent to d.callback(result) if not isinstance(result, failure.Failure): # ---- callback try: result = handleResult(result) except: result = failure.Failure() else: # ---- errback pass if not isinstance(result, failure.Failure): # ---- callback pass else: # ---- errback try: CHAPTER 3. LOW-LEVEL TWISTED 118 result = handleFailure(result) except: result = failure.Failure() def deferredExample(): d = defer.Deferred() d.addCallback(handleResult) d.addErrback(handleFailure) d.callback("success") if __name__ == ’__main__’: behindTheScenes("success") print "\n-------------------------------------------------\n" global num; num = 0 deferredExample() Source listing — deferred ex.py And the output: (since both methods in the example produce the same output, it will only be shown once.) callback 1 got result: success Here we have the simplest case. A deferred with a single callback and a single errback. Normally, a function would create a deferred and hand it back to you when you request an operation that needs to wait for an event for completion. The object you called then does d.callback(result) when the results are in. The thing to notice is that there is only one result that is passed from method to method, and that the result returned from a method is the argument to the next method in the chain. In case of an exception, result is set to an instance of Failure that describes the exception. 3.6.3 Errbacks Failure in requested operation Things don’t always go as planned, and sometimes the function that returned the deferred needs to alert the callback chain that an error has occurred. #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import defer from twisted.python import failure, util """ This example is analogous to a function calling .errback(failure) """ class Counter(object): num = 0 def handleFailure(f): print "errback" CHAPTER 3. LOW-LEVEL TWISTED 119 print "we got an exception: %s" % (f.getTraceback(),) f.trap(RuntimeError) def handleResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) return "yay! handleResult was successful!" def failAtHandlingResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) print "\tabout to raise exception" raise RuntimeError, "whoops! we encountered an error" def behindTheScenes(result): if not isinstance(result, failure.Failure): # ---- callback try: result = handleResult(result) except: result = failure.Failure() else: # ---- errback pass if not isinstance(result, failure.Failure): # ---- callback pass else: # ---- errback try: result = handleFailure(result) except: result = failure.Failure() def deferredExample(result): d = defer.Deferred() d.addCallback(handleResult) d.addCallback(failAtHandlingResult) d.addErrback(handleFailure) d.errback(result) if __name__ == ’__main__’: result = None try: raise RuntimeError, "*doh*! failure!" except: result = failure.Failure() behindTheScenes(result) print "\n-------------------------------------------------\n" Counter.num = 0 deferredExample(result) CHAPTER 3. LOW-LEVEL TWISTED 120 Source listing — deferred ex1a.py errback we got an exception: Traceback (most recent call last): --- exception caught here --- File "deferred_ex1a.py", line 73, in ? raise RuntimeError, "*doh*! failure!" exceptions.RuntimeError: *doh*! failure! The important thing to note (as it will come up again in later examples) is that the callback isn’t touched, the failure goes right to the errback. Also note that the errback trap()s the expected exception type. If you don’t trap the exception, an error will be logged when the deferred is garbage-collected. Exceptions raised in callbacks Now let’s see what happens when our callback raises an exception #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import defer from twisted.python import failure, util """ Here we have a slightly more involved case. The deferred is called back with a result. the first callback returns a value, the second callback, however raises an exception, which is handled by the errback. """ class Counter(object): num = 0 def handleFailure(f): print "errback" print "we got an exception: %s" % (f.getTraceback(),) f.trap(RuntimeError) def handleResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) return "yay! handleResult was successful!" def failAtHandlingResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) print "\tabout to raise exception" raise RuntimeError, "whoops! we encountered an error" def behindTheScenes(result): if not isinstance(result, failure.Failure): # ---- callback try: CHAPTER 3. LOW-LEVEL TWISTED 121 result = handleResult(result) except: result = failure.Failure() else: # ---- errback pass if not isinstance(result, failure.Failure): # ---- callback try: result = failAtHandlingResult(result) except: result = failure.Failure() else: # ---- errback pass if not isinstance(result, failure.Failure): # ---- callback pass else: # ---- errback try: result = handleFailure(result) except: result = failure.Failure() def deferredExample(): d = defer.Deferred() d.addCallback(handleResult) d.addCallback(failAtHandlingResult) d.addErrback(handleFailure) d.callback("success") if __name__ == ’__main__’: behindTheScenes("success") print "\n-------------------------------------------------\n" Counter.num = 0 deferredExample() Source listing — deferred ex1b.py And the output: (note, tracebacks will be edited slightly to conserve space) callback 1 got result: success callback 2 got result: yay! handleResult was successful! about to raise exception errback we got an exception: Traceback (most recent call last): --- --- File "/home/slyphon/Projects/Twisted/trunk/twisted/internet/defer.py", line 326, in _runCallbacks self.result = callback(self.result, *args, **kw) File "./deferred_ex1.py", line 32, in failAtHandlingResult raise RuntimeError, "whoops! we encountered an error" exceptions.RuntimeError: whoops! we encountered an error CHAPTER 3. LOW-LEVEL TWISTED 122 If your callback raises an exception, the next method to be called will be the next errback in your chain. Exceptions will only be handled by errbacks If a callback raises an exception the next method to be called will be next errback in the chain. If the chain is started off with a failure, the first method to be called will be the first errback. #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import defer from twisted.python import failure, util """ This example shows an important concept that many deferred newbies (myself included) have trouble understanding. when an error occurs in a callback, the first errback after the error occurs will be the next method called. (in the next example we’ll see what happens in the ’chain’ after an errback). """ class Counter(object): num = 0 def handleFailure(f): print "errback" print "we got an exception: %s" % (f.getTraceback(),) f.trap(RuntimeError) def handleResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) return "yay! handleResult was successful!" def failAtHandlingResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) print "\tabout to raise exception" raise RuntimeError, "whoops! we encountered an error" def behindTheScenes(result): # equivalent to d.callback(result) # now, let’s make the error happen in the first callback if not isinstance(result, failure.Failure): # ---- callback try: result = failAtHandlingResult(result) except: result = failure.Failure() else: # ---- errback CHAPTER 3. LOW-LEVEL TWISTED 123 pass # note: this callback will be skipped because # result is a failure if not isinstance(result, failure.Failure): # ---- callback try: result = handleResult(result) except: result = failure.Failure() else: # ---- errback pass if not isinstance(result, failure.Failure): # ---- callback pass else: # ---- errback try: result = handleFailure(result) except: result = failure.Failure() def deferredExample(): d = defer.Deferred() d.addCallback(failAtHandlingResult) d.addCallback(handleResult) d.addErrback(handleFailure) d.callback("success") if __name__ == ’__main__’: behindTheScenes("success") print "\n-------------------------------------------------\n" Counter.num = 0 deferredExample() Source listing — deferred ex2.py callback 1 got result: success about to raise exception errback we got an exception: Traceback (most recent call last): File "./deferred_ex2.py", line 85, in ? nonDeferredExample("success") --- --- File "./deferred_ex2.py", line 46, in nonDeferredExample result = failAtHandlingResult(result) File "./deferred_ex2.py", line 35, in failAtHandlingResult raise RuntimeError, "whoops! we encountered an error" exceptions.RuntimeError: whoops! we encountered an error CHAPTER 3. LOW-LEVEL TWISTED 124 You can see that our second callback, handleResult was not called because failAtHandlingResult raised an excep- tion Handling an exception and continuing on In this example, we see an errback handle an exception raised in the preceeding callback. Take note that it could just as easily been an exception from any other preceeding method. You’ll see that after the exception is handled in the errback (i.e. the errback does not return a failure or raise an exception) the chain continues on with the next callback. #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import defer from twisted.python import failure, util """ Now we see how an errback can handle errors. if an errback does not raise an exception, the next callback in the chain will be called. """ class Counter(object): num = 0 def handleFailure(f): print "errback" print "we got an exception: %s" % (f.getTraceback(),) f.trap(RuntimeError) return "okay, continue on" def handleResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) return "yay! handleResult was successful!" def failAtHandlingResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) print "\tabout to raise exception" raise RuntimeError, "whoops! we encountered an error" def callbackAfterErrback(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) def behindTheScenes(result): # equivalent to d.callback(result) if not isinstance(result, failure.Failure): # ---- callback try: CHAPTER 3. LOW-LEVEL TWISTED 125 result = handleResult(result) except: result = failure.Failure() else: # ---- errback pass if not isinstance(result, failure.Failure): # ---- callback try: result = failAtHandlingResult(result) except: result = failure.Failure() else: # ---- errback pass if not isinstance(result, failure.Failure): # ---- callback pass else: # ---- errback try: result = handleFailure(result) except: result = failure.Failure() if not isinstance(result, failure.Failure): # ---- callback try: result = callbackAfterErrback(result) except: result = failure.Failure() else: # ---- errback pass def deferredExample(): d = defer.Deferred() d.addCallback(handleResult) d.addCallback(failAtHandlingResult) d.addErrback(handleFailure) d.addCallback(callbackAfterErrback) d.callback("success") if __name__ == ’__main__’: behindTheScenes("success") print "\n-------------------------------------------------\n" Counter.num = 0 deferredExample() Source listing — deferred ex3.py callback 1 got result: success callback 2 CHAPTER 3. LOW-LEVEL TWISTED 126 got result: yay! handleResult was successful! about to raise exception errback we got an exception: Traceback (most recent call last): File "./deferred_ex3.py", line 97, in deferredExample() File "./deferred_ex3.py", line 90, in deferredExample d.callback("success") File "/home/slyphon/Projects/Twisted/trunk/twisted/internet/defer.py", line 243, in callback self._startRunCallbacks(result) File "/home/slyphon/Projects/Twisted/trunk/twisted/internet/defer.py", line 312, in _startRunCallbacks self._runCallbacks() --- --- File "/home/slyphon/Projects/Twisted/trunk/twisted/internet/defer.py", line 328, in _runCallbacks self.result = callback(self.result, *args, **kw) File "./deferred_ex3.py", line 34, in failAtHandlingResult raise RuntimeError, "whoops! we encountered an error" exceptions.RuntimeError: whoops! we encountered an error callback 3 got result: okay, continue on 3.6.4 addBoth: the deferred version of finally Now we see how deferreds do finally, with .addBoth. The callback that gets added as addBoth will be called if the result is a failure or non-failure. We’ll also see in this example, that our doThisNoMatterWhat() method follows a common idiom in deferred callbacks by acting as a passthru, returning the value that it received to allow processing the chain to continue, but appearing transparent in terms of the result. #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import defer from twisted.python import failure, util """ Now we’ll see what happens when you use ’addBoth’. """ class Counter(object): num = 0 def handleFailure(f): print "errback" print "we got an exception: %s" % (f.getTraceback(),) f.trap(RuntimeError) return "okay, continue on" def handleResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) return "yay! handleResult was successful!" def failAtHandlingResult(result): CHAPTER 3. LOW-LEVEL TWISTED 127 Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) print "\tabout to raise exception" raise RuntimeError, "whoops! we encountered an error" def doThisNoMatterWhat(arg): Counter.num += 1 print "both %s" % (Counter.num,) print "\tgot argument %r" % (arg,) print "\tdoing something very important" # we pass the argument we received to the next phase here return arg def behindTheScenes(result): # equivalent to d.callback(result) if not isinstance(result, failure.Failure): # ---- callback try: result = handleResult(result) except: result = failure.Failure() else: # ---- errback pass if not isinstance(result, failure.Failure): # ---- callback try: result = failAtHandlingResult(result) except: result = failure.Failure() else: # ---- errback pass # ---- this is equivalent to addBoth(doThisNoMatterWhat) if not isinstance(result, failure.Failure): try: result = doThisNoMatterWhat(result) except: result = failure.Failure() else: try: result = doThisNoMatterWhat(result) except: result = failure.Failure() if not isinstance(result, failure.Failure): # ---- callback pass else: # ---- errback try: result = handleFailure(result) except: CHAPTER 3. LOW-LEVEL TWISTED 128 result = failure.Failure() def deferredExample(): d = defer.Deferred() d.addCallback(handleResult) d.addCallback(failAtHandlingResult) d.addBoth(doThisNoMatterWhat) d.addErrback(handleFailure) d.callback("success") if __name__ == ’__main__’: behindTheScenes("success") print "\n-------------------------------------------------\n" Counter.num = 0 deferredExample() Source listing — deferred ex4.py callback 1 got result: success callback 2 got result: yay! handleResult was successful! about to raise exception both 3 got argument doing something very important errback we got an exception: Traceback (most recent call last): --- --- File "/home/slyphon/Projects/Twisted/trunk/twisted/internet/defer.py", line 326, in _runCallbacks self.result = callback(self.result, *args, **kw) File "./deferred_ex4.py", line 32, in failAtHandlingResult raise RuntimeError, "whoops! we encountered an error" exceptions.RuntimeError: whoops! we encountered an error You can see that the errback is called, (and consequently, the failure is trapped). This is because doThisNoMatter- What method returned the value it received, a failure. 3.6.5 addCallbacks: decision making based on previous success or failure As we’ve been seeing in the examples, the callback is a pair of callback/errback. Using addCallback or addErrback is actually a special case where one of the pair is a pass statement. If you want to make a decision based on whether or not the previous result in the chain was a failure or not (which is very rare, but included here for completeness), you use addCallbacks. Note that this is not the same thing as an addCallback followed by an addErrback. #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import defer from twisted.python import failure, util CHAPTER 3. LOW-LEVEL TWISTED 129 """ Now comes the more nuanced addCallbacks, which allows us to make a yes/no (branching) decision based on whether the result at a given point is a failure or not. """ class Counter(object): num = 0 def handleFailure(f): print "errback" print "we got an exception: %s" % (f.getTraceback(),) f.trap(RuntimeError) return "okay, continue on" def handleResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) return "yay! handleResult was successful!" def failAtHandlingResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) print "\tabout to raise exception" raise RuntimeError, "whoops! we encountered an error" def yesDecision(result): Counter.num += 1 print "yes decision %s" % (Counter.num,) print "\twasn’t a failure, so we can plow ahead" return "go ahead!" def noDecision(result): Counter.num += 1 result.trap(RuntimeError) print "no decision %s" % (Counter.num,) print "\t*doh*! a failure! quick! damage control!" return "damage control successful!" def behindTheScenes(result): if not isinstance(result, failure.Failure): # ---- callback try: result = failAtHandlingResult(result) except: result = failure.Failure() else: # ---- errback pass # this is equivalent to addCallbacks(yesDecision, noDecision) CHAPTER 3. LOW-LEVEL TWISTED 130 if not isinstance(result, failure.Failure): # ---- callback try: result = yesDecision(result) except: result = failure.Failure() else: # ---- errback try: result = noDecision(result) except: result = failure.Failure() if not isinstance(result, failure.Failure): # ---- callback try: result = handleResult(result) except: result = failure.Failure() else: # ---- errback pass # this is equivalent to addCallbacks(yesDecision, noDecision) if not isinstance(result, failure.Failure): # ---- callback try: result = yesDecision(result) except: result = failure.Failure() else: # ---- errback try: result = noDecision(result) except: result = failure.Failure() if not isinstance(result, failure.Failure): # ---- callback try: result = handleResult(result) except: result = failure.Failure() else: # ---- errback pass if not isinstance(result, failure.Failure): # ---- callback pass else: # ---- errback try: result = handleFailure(result) except: result = failure.Failure() def deferredExample(): d = defer.Deferred() d.addCallback(failAtHandlingResult) d.addCallbacks(yesDecision, noDecision) # noDecision will be called CHAPTER 3. LOW-LEVEL TWISTED 131 d.addCallback(handleResult) # - A - d.addCallbacks(yesDecision, noDecision) # yesDecision will be called d.addCallback(handleResult) d.addErrback(handleFailure) d.callback("success") if __name__ == ’__main__’: behindTheScenes("success") print "\n-------------------------------------------------\n" Counter.num = 0 deferredExample() Source listing — deferred ex5.py callback 1 got result: success about to raise exception no decision 2 *doh*! a failure! quick! damage control! callback 3 got result: damage control successful! yes decision 4 wasn’t a failure, so we can plow ahead callback 5 got result: go ahead! Notice that our errback is never called. The noDecision method returns a non-failure so processing continues with the next callback. If we wanted to skip the callback at ”- A -” because of the error, but do some kind of processing in response to the error, we would have used a passthru, and returned the failure we received, as we see in this next example: #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import defer from twisted.python import failure, util """ Now comes the more nuanced addCallbacks, which allows us to make a yes/no (branching) decision based on whether the result at a given point is a failure or not. here, we return the failure from noDecisionPassthru, the errback argument to the first addCallbacks method invocation, and see what happens. """ class Counter(object): num = 0 def handleFailure(f): print "errback" print "we got an exception: %s" % (f.getTraceback(),) CHAPTER 3. LOW-LEVEL TWISTED 132 f.trap(RuntimeError) return "okay, continue on" def handleResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) return "yay! handleResult was successful!" def failAtHandlingResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) print "\tabout to raise exception" raise RuntimeError, "whoops! we encountered an error" def yesDecision(result): Counter.num += 1 print "yes decision %s" % (Counter.num,) print "\twasn’t a failure, so we can plow ahead" return "go ahead!" def noDecision(result): Counter.num += 1 result.trap(RuntimeError) print "no decision %s" % (Counter.num,) print "\t*doh*! a failure! quick! damage control!" return "damage control successful!" def noDecisionPassthru(result): Counter.num += 1 print "no decision %s" % (Counter.num,) print "\t*doh*! a failure! don’t know what to do, returning failure!" return result def behindTheScenes(result): if not isinstance(result, failure.Failure): # ---- callback try: result = failAtHandlingResult(result) except: result = failure.Failure() else: # ---- errback pass # this is equivalent to addCallbacks(yesDecision, noDecision) if not isinstance(result, failure.Failure): # ---- callback try: result = yesDecision(result) except: result = failure.Failure() else: # ---- errback try: result = noDecisionPassthru(result) CHAPTER 3. LOW-LEVEL TWISTED 133 except: result = failure.Failure() if not isinstance(result, failure.Failure): # ---- callback try: result = handleResult(result) except: result = failure.Failure() else: # ---- errback pass # this is equivalent to addCallbacks(yesDecision, noDecision) if not isinstance(result, failure.Failure): # ---- callback try: result = yesDecision(result) except: result = failure.Failure() else: # ---- errback try: result = noDecision(result) except: result = failure.Failure() if not isinstance(result, failure.Failure): # ---- callback try: result = handleResult(result) except: result = failure.Failure() else: # ---- errback pass if not isinstance(result, failure.Failure): # ---- callback pass else: # ---- errback try: result = handleFailure(result) except: result = failure.Failure() def deferredExample(): d = defer.Deferred() d.addCallback(failAtHandlingResult) # noDecisionPassthru will be called d.addCallbacks(yesDecision, noDecisionPassthru) d.addCallback(handleResult) # - A - # noDecision will be called d.addCallbacks(yesDecision, noDecision) d.addCallback(handleResult) # - B - d.addErrback(handleFailure) CHAPTER 3. LOW-LEVEL TWISTED 134 d.callback("success") if __name__ == ’__main__’: behindTheScenes("success") print "\n-------------------------------------------------\n" Counter.num = 0 deferredExample() Source listing — deferred ex6.py callback 1 got result: success about to raise exception no decision 2 *doh*! a failure! don’t know what to do, returning failure! no decision 3 *doh*! a failure! quick! damage control! callback 4 got result: damage control successful! Two things to note here. First, ”- A -” was skipped, like we wanted it to, and the second thing is that after ”- A -”, noDecision is called, because it is the next errback that exists in the chain. It returns a non-failure, so processing continues with the next callback at ”- B -”, and the errback at the end of the chain is never called 3.6.6 Hints, tips, common mistakes, and miscellaney The deferred callback chain is stateful A deferred that has been called back will call its addCallback and addErrback methods as appropriate in the order they are added, when they are added. So we see in the following example, deferredExample1 and deferredExample2 are equivalent. The first sets up the processing chain beforehand and then executes it, the other executes the chain as it is being constructed. This is because deferreds are stateful. #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import defer from twisted.python import failure, util """ The deferred callback chain is stateful, and can be executed before or after all callbacks have been added to the chain """ class Counter(object): num = 0 def handleFailure(f): print "errback" print "we got an exception: %s" % (f.getTraceback(),) f.trap(RuntimeError) def handleResult(result): Counter.num += 1 CHAPTER 3. LOW-LEVEL TWISTED 135 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) return "yay! handleResult was successful!" def failAtHandlingResult(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) print "\tabout to raise exception" raise RuntimeError, "whoops! we encountered an error" def deferredExample1(): # this is another common idiom, since all add* methods # return the deferred instance, you can just chain your # calls to addCallback and addErrback d = defer.Deferred().addCallback(failAtHandlingResult ).addCallback(handleResult ).addErrback(handleFailure) d.callback("success") def deferredExample2(): d = defer.Deferred() d.callback("success") d.addCallback(failAtHandlingResult) d.addCallback(handleResult) d.addErrback(handleFailure) if __name__ == ’__main__’: deferredExample1() print "\n-------------------------------------------------\n" Counter.num = 0 deferredExample2() Source listing — deferred ex7.py callback 1 got result: success about to raise exception errback we got an exception: Traceback (most recent call last): --- --- File "/home/slyphon/Projects/Twisted/trunk/twisted/internet/defer.py", line 326, in _runCallbacks self.result = callback(self.result, *args, **kw) File "./deferred_ex7.py", line 35, in failAtHandlingResult raise RuntimeError, "whoops! we encountered an error" exceptions.RuntimeError: whoops! we encountered an error ------------------------------------------------- CHAPTER 3. LOW-LEVEL TWISTED 136 callback 1 got result: success about to raise exception errback we got an exception: Traceback (most recent call last): --- --- File "/home/slyphon/Projects/Twisted/trunk/twisted/internet/defer.py", line 326, in _runCallbacks self.result = callback(self.result, *args, **kw) File "./deferred_ex7.py", line 35, in failAtHandlingResult raise RuntimeError, "whoops! we encountered an error" exceptions.RuntimeError: whoops! we encountered an error This example also shows you the common idiom of chaining calls to addCallback and addErrback. Don’t call .callback() on deferreds you didn’t create! It is an error to reinvoke deferreds callback or errback method, therefore if you didn’t create a deferred, do not under any circumstances call its callback or errback. doing so will raise an exception Callbacks can return deferreds If you need to call a method that returns a deferred within your callback chain, just return that deferred, and the result of the secondary deferred’s processing chain will become the result that gets passed to the next callback of the primary deferreds processing chain #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import defer from twisted.python import failure, util class Counter(object): num = 0 let = ’a’ def incrLet(cls): cls.let = chr(ord(cls.let) + 1) incrLet = classmethod(incrLet) def handleFailure(f): print "errback" print "we got an exception: %s" % (f.getTraceback(),) return f def subCb_B(result): print "sub-callback %s" % (Counter.let,) Counter.incrLet() s = " beautiful!" print "\tadding %r to result" % (s,) result += s return result def subCb_A(result): CHAPTER 3. LOW-LEVEL TWISTED 137 print "sub-callback %s" % (Counter.let,) Counter.incrLet() s = " are " print "\tadding %r to result" % (s,) result += s return result def mainCb_1(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) result += " Deferreds " d = defer.Deferred().addCallback(subCb_A ).addCallback(subCb_B) d.callback(result) return d def mainCb_2(result): Counter.num += 1 print "callback %s" % (Counter.num,) print "\tgot result: %s" % (result,) def deferredExample(): d = defer.Deferred().addCallback(mainCb_1 ).addCallback(mainCb_2) d.callback("I hope you’ll agree: ") if __name__ == ’__main__’: deferredExample() Source listing — deferred ex8.py callback 1 got result: I hope you’ll agree: sub-callback a adding ’ are ’ to result sub-callback b adding ’ beautiful!’ to result callback 2 got result: I hope you’ll agree: Deferreds are beautiful! 3.6.7 Conclusion Deferreds can be confusing, but only because they’re so elegant and simple. There is a lot of logical power that can expressed with a deferred’s processing chain, and once you see what’s going on behind the curtain, it’s a lot easier to understand how to make use of what deferreds have to offer. 3.7 Scheduling tasks for the future Let’s say we want to run a task X seconds in the future. The way to do that is defined in the reactor interface twisted. internet.interfaces.IReactorTime: CHAPTER 3. LOW-LEVEL TWISTED 138 from twisted.internet import reactor def f(s): print "this will run 3.5 seconds after it was scheduled: %s" % s reactor.callLater(3.5, f, "hello, world") # f() will only be called if the event loop is started. reactor.run() If the result of the function is important or if it may be necessary to handle exceptions it raises, then the twisted. internet.task.deferLater utility conveniently takes care of creating a Deferred and setting up a delayed call: from twisted.internet import task from twisted.internet import reactor def f(s): return "This will run 3.5 seconds after it was scheduled: %s" % s d = task.deferLater(reactor, 3.5, f, "hello, world") def called(result): print result d.addCallback(called) # f() will only be called if the event loop is started. reactor.run() If we want a task to run every X seconds repeatedly, we can use twisted.internet.task.LoopingCall: from twisted.internet import task from twisted.internet import reactor def runEverySecond(): print "a second has passed" l = task.LoopingCall(runEverySecond) l.start(1.0) # call every second # l.stop() will stop the looping calls reactor.run() If we want to cancel a task that we’ve scheduled: from twisted.internet import reactor def f(): print "I’ll never run." callID = reactor.callLater(5, f) callID.cancel() reactor.run() As with all reactor-based code, in order for scheduling to work the reactor must be started using reactor. run(). CHAPTER 3. LOW-LEVEL TWISTED 139 3.8 Using Threads in Twisted 3.8.1 Running code in a thread-safe manner Most code in Twisted is not thread-safe. For example, writing data to a transport from a protocol is not thread-safe. Therefore, we want a way to schedule methods to be run in the main event loop. This can be done using the function twisted.internet.interfaces.IReactorThreads.callFromThread: from twisted.internet import reactor def notThreadSafe(x): """do something that isn’t thread-safe""" # ... def threadSafeScheduler(): """Run in thread-safe manner.""" reactor.callFromThread(notThreadSafe, 3) # will run ’notThreadSafe(3)’ # in the event loop reactor.run() 3.8.2 Running code in threads Sometimes we may want to run methods in threads - for example, in order to access blocking APIs. Twisted pro- vides methods for doing so using the IReactorThreads API (twisted.internet.interfaces.IReactor Threads). Additional utility functions are provided in twisted.internet.threads. Basically, these meth- ods allow us to queue methods to be run by a thread pool. For example, to run a method in a thread we can do: from twisted.internet import reactor def aSillyBlockingMethod(x): import time time.sleep(2) print x # run method in thread reactor.callInThread(aSillyBlockingMethod, "2 seconds have passed") reactor.run() 3.8.3 Utility Methods The utility methods are not part of the twisted.internet.reactor APIs, but are implemented in twisted. internet.threads. If we have multiple methods to run sequentially within a thread, we can do: from twisted.internet import reactor, threads def aSillyBlockingMethodOne(x): import time time.sleep(2) print x def aSillyBlockingMethodTwo(x): print x # run both methods sequentially in a thread commands = [(aSillyBlockingMethodOne, ["Calling First"], {})] commands.append((aSillyBlockingMethodTwo, ["And the second"], {})) threads.callMultipleInThread(commands) CHAPTER 3. LOW-LEVEL TWISTED 140 reactor.run() For functions whose results we wish to get, we can have the result returned as a Deferred: from twisted.internet import reactor, threads def doLongCalculation(): # .... do long calculation here ... return 3 def printResult(x): print x # run method in thread and get result as defer.Deferred d = threads.deferToThread(doLongCalculation) d.addCallback(printResult) reactor.run() If you wish to call a method in the reactor thread and get its result, you can use blockingCallFromThread: from twisted.internet import threads, reactor, defer from twisted.web.client import getPage from twisted.web.error import Error def inThread(): try: result = threads.blockingCallFromThread( reactor, getPage, "http://twistedmatrix.com/") except Error, exc: print exc else: print result reactor.callFromThread(reactor.stop) reactor.callInThread(inThread) reactor.run() blockingCallFromThread will return the object or raise the exception returned or raised by the function passed to it. If the function passed to it returns a Deferred, it will return the value the Deferred is called back with or raise the exception it is errbacked with. 3.8.4 Managing the Thread Pool The thread pool is implemented by twisted.python.threadpool.ThreadPool. We may want to modify the size of the threadpool, increasing or decreasing the number of threads in use. We can do this do this quite easily: from twisted.internet import reactor reactor.suggestThreadPoolSize(30) The default size of the thread pool depends on the reactor being used; the default reactor uses a minimum size of 5 and a maximum size of 10. Be careful that you understand threads and their resource usage before drastically altering the thread pool sizes. 3.9 Choosing a Reactor and GUI Toolkit Integration 3.9.1 Overview Twisted provides a variety of implementations of the twisted.internet.reactor. The specialized implemen- tations are suited for different purposes and are designed to integrate better with particular platforms. CHAPTER 3. LOW-LEVEL TWISTED 141 The general purpose reactor implementations are: • The select()-based reactor (page 142) • The poll()-based reactor (page 142) Platform-specific reactor implementations exist for: • KQueue for FreeBSD and OS X (page 142) • Win32 (WFMO) (page 142) • Win32 (IOCP) (page 142) • Mac OS X (page 143) • Epoll for Linux 2.6 (page 143) The remaining custom reactor implementations provide support for integrating with the native event loops of various graphical toolkits. This lets your Twisted application use all of the usual Twisted APIs while still being a graphical application. Twisted currently integrates with the following graphical toolkits: • GTK+ 1.2 and 2.0 (page 143) • Tkinter (page 143) • WxPython (page 144) • Win32 (page 142) • CoreFoundation (page 143) • PyUI (page 144) When using applications that runnable using twistd, e.g. TAPs or plugins, there is no need to choose a reactor explicitly, since this can be chosen using twistd’s -r option. In all cases, the event loop is started by calling reactor.run(). In all cases, the event loop should be stopped with reactor.stop(). IMPORTANT: installing a reactor should be the first thing done in the app, since any code that does from twisted.internet import reactor will automatically install the default reactor if the code hasn’t already installed one. 3.9.2 Reactor Functionality Status TCP SSL UDP Threading Processes Scheduling Platforms select() Stable Y Y Y Y Y Y Unix, Win32 poll() Stable Y Y Y Y Y Y Unix Win32 (WFMO) Experimental Y Y Y Y Y Y Win32 Win32 (IOCP) Experimental Y Y N N N Y Win32 CoreFoundation Unmaintained Y Y Y Y Y Y OS X epoll Stable Y Y Y Y Y Y Linux 2.6 Gtk Stable Y Y Y Y Y Y Unix, Win32 wx Experimental Y Y Y Y Y Y Unix, Win32 kqueue Experimental Y Y Y Y Y Y FreeBSD Table 3.1: Summary of reactor features CHAPTER 3. LOW-LEVEL TWISTED 142 3.9.3 General Purpose Reactors Select()-based Reactor The select reactor is currently the default reactor on all platforms. The following code will install it, if no other reactor has been installed: from twisted.internet import reactor In the future, if another reactor becomes the default, but the select reactor is desired, it may be installed via: from twisted.internet import selectreactor selectreactor.install() from twisted.internet import reactor Poll()-based Reactor The PollReactor will work on any platform that provides poll() (while OS X provides poll(), it is not rec- ommended to use the PollReactor on OS X due to bugs in its implementation of the call). With larger numbers of connected sockets, it may provide for better performance than the SelectReactor. from twisted.internet import pollreactor pollreactor.install() from twisted.internet import reactor 3.9.4 Platform-Specific Reactors KQueue The KQueue Reactor allows Twisted to use FreeBSD’s kqueue mechanism for event scheduling. See instructions in the twisted.internet.kqreactor’s docstring for installation notes. from twisted.internet import kqreactor kqreactor.install() from twisted.internet import reactor Win32 (WFMO) The Win32 reactor is not yet complete and has various limitations and issues that need to be addressed. The reactor supports GUI integration with the win32gui module, so it can be used for native Win32 GUI applications. from twisted.internet import win32eventreactor win32eventreactor.install() from twisted.internet import reactor Win32 (IOCP) Windows provides a fast, scalable event notification system known as IO Completion Ports, or IOCP for short. Twisted includes a reactor based on IOCP which is nearly complete. The reactor has a handful of known bugs and lacks SSL support. from twisted.internet import iocpreactor iocpreactor.install() from twisted.internet import reactor CHAPTER 3. LOW-LEVEL TWISTED 143 Epoll-based Reactor The EPollReactor will work on any platform that provides epoll, today only Linux 2.6 and over. The implementation of the epoll reactor currently uses the Level Triggered interface, which is basically like poll() but scales much better. from twisted.internet import epollreactor epollreactor.install() from twisted.internet import reactor 3.9.5 GUI Integration Reactors GTK+ Twisted integrates with PyGTK2, versions 1.2 (gtkreactor) and 2.0 (gtk2reactor). Sample applications using GTK+ and Twisted are available in the Twisted SVN. GTK-2.0 split the event loop out of the GUI toolkit, into a separate module called “glib”. To run an application using the glib event loop, use the glib2reactor. This will be slightly faster than gtk2reactor (and does not require a working X display), but cannot be used to run GUI applications. from twisted.internet import gtkreactor # for gtk-1.2 gtkreactor.install() from twisted.internet import reactor from twisted.internet import gtk2reactor # for gtk-2.0 gtk2reactor.install() from twisted.internet import reactor from twisted.internet import glib2reactor # for non-GUI apps glib2reactor.install() from twisted.internet import reactor CoreFoundation Twisted integrates with PyObjC3, version 1.0. Sample applications using Cocoa and Twisted are available in the examples directory under threadedselect/Cocoa. from twisted.internet import cfreactor cfreactor.install() from twisted.internet import reactor 3.9.6 Non-Reactor GUI Integration Tkinter The support for Tkinter4 doesn’t use a specialized reactor. Instead, there is some specialized support code: from Tkinter import * from twisted.internet import tksupport root = Tk() # Install the Reactor support 2http://www.pygtk.org/ 3http://pyobjc.sf.net/ 4http://www.python.org/topics/tkinter/ CHAPTER 3. LOW-LEVEL TWISTED 144 tksupport.install(root) # at this point build Tk app as usual using the root object, # and start the program with "reactor.run()", and stop it # with "reactor.stop()". wxPython Twisted currently supports two methods of integrating wxPython. Unfortunately, neither method will work on all wxPython platforms (such as GTK2 or Windows). It seems that the only portable way to integrate with wxPython is to run it in a separate thread. One of these methods may be sufficient if your wx app is limited to a single platform. As with Tkinter (page 143), the support for integrating Twisted with a wxPython5 application uses specialized support code rather than a simple reactor. from wxPython.wx import * from twisted.internet import wxsupport, reactor myWxAppInstance = wxApp(0) wxsupport.install(myWxAppInstance) However, this has issues when running on Windows, so Twisted now comes with alternative wxPython support using a reactor. Using this method is probably better. Initialization is done in two stages. In the first, the reactor is installed: from twisted.internet import wxreactor wxreactor.install() Later, once a wxApp instance has been created, but before reactor.run() is called: myWxAppInstance = wxApp(0) reactor.registerWxApp(myWxAppInstance) An example Twisted application that uses WxWindows can be found in doc/examples/wxdemo.py. PyUI As with Tkinter (page 143), the support for integrating Twisted with a PyUI6 application uses specialized support code rather than a simple reactor. from twisted.internet import pyuisupport, reactor pyuisupport.install(args=(640, 480), kw={’renderer’: ’gl’}) An example Twisted application that uses PyUI can be found in doc/examples/pyuidemo.py. 5http://www.wxpython.org 6http://pyui.sourceforge.net Chapter 4 High-Level Twisted 4.1 The Basics 4.1.1 Application Twisted programs usually work with twisted.application.service.Application. This class usually holds all persistent configuration of a running server – ports to bind to, places where connections to must be kept or attempted, periodic actions to do and almost everything else. It is the root object in a tree of services implementing IService. Other HOWTOs describe how to write custom code for Applications, but this one describes how to use already written code (which can be part of Twisted or from a third-party Twisted plugin developer). The Twisted distribution comes with an important tool to deal with Applications, twistd. Applications are just Python objects, which can be created and manipulated in the same ways as any other object. 4.1.2 twistd The Twisted Daemon is a program that knows how to run Applications. This program is twistd(1). Strictly speaking, twistd is not necessary – fetching the application, getting the IService component, calling start Service, scheduling stopService when the reactor shuts down, and then calling reactor.run() could be done manually. twistd(1), however, supplies many options which are highly useful for program set up. twistd supports choosing a reactor (for more on reactors, see Choosing a Reactor (page 140)), logging to a logfile, daemonizing and more. twistd supports all Applications mentioned above – and an additional one. Sometimes it is convenient to write the code for building a class in straight Python. One big source of such Python files is the doc/examples directory. When a straight Python file which defines an Application object called application is used, use the -y option. When twistd runs, it records its process id in a twistd.pid file (this can be configured via a command line switch). In order to shutdown the twistd process, kill that pid (usually you would do kill ‘cat twistd. pid‘). As always, the gory details are in the manual page. 4.1.3 tap2deb For Twisted-based server application developers who want to deploy on Debian, Twisted supplies the tap2deb program. This program wraps a Twisted Application file (of any of the supported formats – Python, source, xml or pickle) in a Debian package, including correct installation and removal scripts and init.d scripts. This frees the installer from manually stopping or starting the service, and will make sure it goes properly up on startup and down on shutdown and that it obeys the init levels. For the more savvy Debian users, the tap2deb also generates the source package, allowing her to modify and polish things which automated software cannot detect (such as dependencies or relationships to virtual packages). In addition, the Twisted team itself intends to produce Debian packages for some common services, such as web servers and an inetd replacement. Those packages will enjoy the best of all worlds – both the consistency which comes from 145 CHAPTER 4. HIGH-LEVEL TWISTED 146 being based on the tap2deb and the delicate manual tweaking of a Debian maintainer, insuring perfect integration with Debian. 4.1.4 tap2rpm tap2rpm is similar to tap2deb, except that it generates RPMs for Redhat and other related platforms. 4.2 The Twisted Plugin System The purpose of this guide is to describe the preferred way to write extensible Twisted applications (and consequently, also to describe how to extend applications written in such a way). This extensibility is achieved through the definition of one or more APIs and a mechanism for collecting code plugins which implement this API to provide some additional functionality. At the base of this system is the twisted.plugin module. Making an application extensible using the plugin system has several strong advantages over other techniques: • It allows third-party developers to easily enhance your software in a way that is loosely coupled: only the plugin API is required to remain stable. • It allows new plugins to be discovered flexibly. For example, plugins can be loaded and saved when a program is first run, or re-discovered each time the program starts up, or they can be polled for repeatedly at runtime (allowing the discovery of new plugins installed after the program has started). 4.2.1 Writing Extensible Programs Taking advantage of twisted.plugin is a two step process: 1. Define an interface which plugins will be required to implement. This is done using the zope.interface package in the same way one would define an interface for any other purpose. A convention for defining interfaces is do so in a file named like ProjectName/projectname/iprojectname.py. The rest of this document will follow that convention: consider the following interface definition be in Matsim/matsim/imatsim.py, an interface definition module for a hypothetical material simulation pack- age. 2. At one or more places in your program, invoke twisted.plugin.getPlugins and iterate over its result. As an example of the first step, consider the following interface definition for a physical modelling system. from zope.interface import Interface, Attribute class IMaterial(Interface): """ An object with specific physical properties """ def yieldStress(temperature): """ Returns the pressure this material can support without fracturing at the given temperature. @type temperature: C{float} @param temperature: Kelvins @rtype: C{float} @return: Pascals """ dielectricConstant = Attribute(""" @type dielectricConstant: C{complex} @ivar dielectricConstant: The relative permittivity, with the CHAPTER 4. HIGH-LEVEL TWISTED 147 real part giving reflective surface properties and the imaginary part giving the radio absorption coefficient. """) In another module, we might have a function that operates on objects providing the IMaterial interface: def displayMaterial(m): print ’A material with yield stress %s at 500 K’ % (m.yieldStress(500),) print ’Also a dielectric constant of %s.’ % (m.dielectricConstant,) The last piece of required code is that which collects IMaterial providers and passes them to the display Material function. from twisted.plugin import getPlugins from matsim import imatsim def displayAllKnownMaterials(): for material in getPlugins(imatsim.IMaterial): displayMaterial(material) Third party developers may now contribute different materials to be used by this modelling system by implement- ing one or more plugins for the IMaterial interface. 4.2.2 Extending an Existing Program The above code demonstrates how an extensible program might be written using Twisted’s plugin system. How do we write plugins for it, though? Essentially, we create objects which provide the required interface and then make them available at a particular location. Consider the following example. from zope.interface import implements from twisted.plugin import IPlugin from matsim import imatsim class SimpleMaterial(object): implements(IPlugin, imatsim.IMaterial) def __init__(self, yieldStressFactor, dielectricConstant): self._yieldStressFactor = yieldStressFactor self.dielectricConstant = dielectricConstant def yieldStress(self, temperature): return self._yieldStressFactor * temperature steelPlate = SimpleMaterial(2.06842719e11, 2.7 + 0.2j) brassPlate = SimpleMaterial(1.03421359e11, 1.4 + 0.5j) steelPlate and brassPlate now provide both IPlugin and IMaterial. All that remains is to make this module available at an appropriate location. For this, there are two options. The first of these is primarily useful during development: if a directory which has been added to sys.path (typically by adding it to the PYTHONPATH environment variable) contains a directory named twisted/plugins/, each .py file in that directory will be loaded as a source of plugins. This directory must not be a Python package: including init .py will cause the directory to be skipped and no plugins loaded from it. Second, each module in the installed version of Twisted’s twisted.plugins package will also be loaded as a source of plugins. Once this plugin is installed in one of these two ways, displayAllKnownMaterials can be run and we will see two pairs of output: one for a steel plate and one for a brass plate. 4.2.3 Alternate Plugin Packages getPlugins takes one additional argument not mentioned above. If passed in, the 2nd argument should be a module or package to be used instead of twisted.plugins as the plugin meta-package. If you are writing a plugin for a CHAPTER 4. HIGH-LEVEL TWISTED 148 Twisted interface, you should never need to pass this argument. However, if you have developed an interface of your own, you may want to mandate that plugins for it are installed in your own plugins package, rather than in Twisted’s. In this case, you probably also want to support yourproject/plugins/ directories for ease of development. To do so, you should make the init .py for that package contain at least the following lines. from twisted.plugin import pluginPackagePaths __path__.extend(pluginPackagePaths(__name__)) __all__ = [] The key behavior here is that interfaces are essentially paired with a particular plugin package. If plugins are installed in a different package than the one the code which relies on the interface they provide, they will not be found when the application goes to load them. 4.2.4 Plugin Caching In the course of using the Twisted plugin system, you may notice dropin.cache files appearing at various locations. These files are used to cache information about what plugins are present in the directory which contains them. At times, this cached information may become out of date. Twisted uses the mtimes of various files involved in the plugin system to determine when this cache may have become invalid. Twisted will try to re-write the cache each time it tries to use it but finds it out of date. For a site-wide install, it may not (indeed, should not) be possible for applications running as normal users to rewrite the cache file. While these applications will still run and find correct plugin information, they may run more slowly than they would if the cache was up to date, and they may also report exceptions if certain plugins have been removed but which the cache still references. For these reasons, when installing or removing software which provides Twisted plugins, the site administrator should be sure the cache is regenerated. Well-behaved package managers for such software should take this task upon themselves, since it is trivially automatable. The canonical way to regenerate the cache is to run the following Python code: from twisted.plugin import IPlugin, getPlugins list(getPlugins(IPlugin)) As mentioned, it is normal for exceptions to be raised once here if plugins have been removed. 4.2.5 Further Reading • Components: Interfaces and Adapters (page 151) 4.3 Writing a twistd Plugin This document describes adding subcommands to the twistd command, as a way to facilitate the deployment of your applications. (This feature was added in Twisted 2.5) The target audience of this document are those that have developed a Twisted application which needs a command line-based deployment mechanism. There are a few prerequisites to understanding this document: • A basic understanding of the Twisted Plugin System (i.e., the twisted.plugin module) is necessary, how- ever, step-by-step instructions will be given. Reading The Twisted Plugin System (page 146) is recommended, in particular the “Extending an Existing Program” section. • The Application (page 163) infrastructure is used in twistd plugins; in particular, you should know how to expose your program’s functionality as a Service. • In order to parse command line arguments, the twistd plugin mechanism relies on twisted.python. usage, which is documented in Using usage.Options (page 168). 4.3.1 Goals After reading this document, the reader should be able to expose their Service-using application as a subcommand of twistd, taking into consideration whatever was passed on the command line. CHAPTER 4. HIGH-LEVEL TWISTED 149 4.3.2 A note on .tap files Readers may be confused about a historical file type associated with Twisted, the .tap file. This was a kind of file that was generated by a program named mktap and which twistd can read. .tap files are deprecated; this document has nothing to do with them, although the technology described herein is very closely related to the old system. Existing plugins that were written for the mktap system are compatible with this twistd plugin system; the following commands, $ mktap [foo] [options...] $ twistd -n -f [foo].tap are equivalent to the command: $ twistd -n [foo] [options...] 4.3.3 Alternatives to twistd plugins The major alternative to the twistd plugin mechanism is the .tac file, which is a simple script to be used with the twistd -y/--python parameter. The twistd plugin mechanism exists to offer a more extensible command- line-driven interface to your application. For more information on .tac files, see the document Using the Twisted Application Framework (page 163). 4.3.4 Creating the plugin The following directory structure is assumed of your project: • MyProject - Top level directory – myproject - Python package ∗ init .py During development of your project, Twisted plugins can be loaded from a special directory in your project, as- suming your top level directory ends up in sys.path. Create a directory named twisted containing a directory named plugins, and add a file named myproject plugin.py to it. This file will contain your plugin. Note that you should not add any init .py files to this directory structure, and the plugin file should not be named myproject. py (because that would conflict with your project’s module name). In this file, define an object which provides the interfaces twisted.plugin.IPlugin and twisted. application.service.IServiceMaker. The tapname attribute of your IServiceMaker provider will be used as the subcommand name in a command like twistd [subcommand] [args...], and the options attribute (which should be a usage.Options subclass) will be used to parse the given args. from zope.interface import implements from twisted.python import usage from twisted.plugin import IPlugin from twisted.application.service import IServiceMaker from twisted.application import internet from myproject import MyFactory class Options(usage.Options): optParameters = [["port", "p", 1235, "The port number to listen on."]] class MyServiceMaker(object): implements(IServiceMaker, IPlugin) tapname = "myproject" description = "Run this! It’ll make your dog happy." CHAPTER 4. HIGH-LEVEL TWISTED 150 options = Options def makeService(self, options): """ Construct a TCPServer from a factory defined in myproject. """ return internet.TCPServer(int(options["port"]), MyFactory()) # Now construct an object which *provides* the relevant interfaces # The name of this variable is irrelevant, as long as there is *some* # name bound to a provider of IPlugin and IServiceMaker. serviceMaker = MyServiceMaker() Now running twistd --help should print myproject in the list of available subcommands, followed by the description that we specified in the plugin. twistd -n myproject would, assuming we defined a MyFactory factory inside myproject, start a listening server on port 1235 with that factory. 4.3.5 Using cred with your TAP Twisted ships with a robust authentication framework to use with your application. If your server needs authentication functionality, and you haven’t read about twisted.cred (page 157) yet, read up on it first. If you are building a twistd plugin and you want to support a wide variety of authentication patterns, Twisted provides an easy-to-use mixin for your Options subclass: strcred.AuthOptionMixin. The following code is an example of using this mixin: from twisted.cred import credentials, portal, strcred from twisted.python import usage from twisted.plugin import IPlugin from twisted.application.service import IServiceMaker from myserver import myservice class ServerOptions(usage.Options, strcred.AuthOptionMixin): # This part is optional; it tells AuthOptionMixin what # kinds of credential interfaces the user can give us. supportedInterfaces = (credentials.IUsernamePassword,) optParameters = [ ["port", "p", 1234, "Server port number"], ["host", "h", "localhost", "Server hostname"]] class MyServerServiceMaker(object): implements(IServiceMaker, IPlugin) tapname = "myserver" description = "This server does nothing productive." options = ServerOptions def makeService(self, options): """Construct a service object.""" # The realm is a custom object that your server defines. realm = myservice.MyServerRealm(options["host"]) # The portal is something Cred can provide, as long as # you have a list of checkers that you’ll support. This # list is provided my AuthOptionMixin. portal = portal.Portal(realm, options["credCheckers"]) CHAPTER 4. HIGH-LEVEL TWISTED 151 # OR, if you know you might get multiple interfaces, and # only want to give your application one of them, you # also have that option with AuthOptionMixin: interface = credentials.IUsernamePassword portal = portal.Portal(realm, options["credInterfaces"][interface]) # The protocol factory is, like the realm, something you implement. factory = myservice.ServerFactory(realm, portal) # Finally, return a service that will listen for connections. return internet.TCPServer(int(options["port"]), factory) # As in our example above, we have to construct an object that # provides the IPlugin and IServiceMaker interfaces. serviceMaker = MyServerServiceMaker() Now that you have your TAP configured to support any authentication we can throw at it, you’re ready to use it. Here is an example of starting your server using the /etc/passwd file for authentication. (Clearly, this won’t work on servers with shadow passwords.) $ twistd myserver --auth passwd:/etc/passwd For a full list of cred plugins supported, see twisted.plugins, or use the command-line help: $ twistd myserver --help-auth $ twistd myserver --help-auth-type passwd 4.3.6 Conclusion You should now be able to • Create a twistd plugin • Incorporate authentication into your plugin • Use it from your development environment • Install it correctly and use it in deployment 4.4 Components: Interfaces and Adapters Object oriented programming languages allow programmers to reuse portions of existing code by creating new “classes” of objects which subclass another class. When a class subclasses another, it is said to inherit all of its behaviour. The subclass can then “override” and “extend” the behavior provided to it by the superclass. Inheritance is very useful in many situations, but because it is so convenient to use, often becomes abused in large software systems, especially when multiple inheritance is involved. One solution is to use delegation instead of “inheritance” where ap- propriate. Delegation is simply the act of asking another object to perform a task for an object. To support this design pattern, which is often referred to as the components pattern because it involves many small interacting components, interfaces and adapters were created by the Zope 3 team. “Interfaces” are simply markers which objects can use to say “I implement this interface”. Other objects may then make requests like “Please give me an object which implements interface X for object type Y”. Objects which implement an interface for another object type are called “adapters”. The superclass-subclass relationship is said to be an is-a relationship. When designing object hierarchies, object modellers use subclassing when they can say that the subclass is the same class as the superclass. For example: class Shape: sideLength = 0 def getSideLength(self): CHAPTER 4. HIGH-LEVEL TWISTED 152 return self.sideLength def setSideLength(self, sideLength): self.sideLength = sideLength def area(self): raise NotImplementedError, "Subclasses must implement area" class Triangle(Shape): def area(self): return (self.sideLength * self.sideLength) / 2 class Square(Shape): def area(self): return self.sideLength * self.sideLength In the above example, a Triangle is-a Shape, so it subclasses Shape, and a Square is-a Shape, so it also subclasses Shape. However, subclassing can get complicated, especially when Multiple Inheritance enters the picture. Multiple In- heritance allows a class to inherit from more than one base class. Software which relies heavily on inheritance often ends up having both very wide and very deep inheritance trees, meaning that one class inherits from many superclasses spread throughout the system. Since subclassing with Multiple Inheritance means implementation inheritance, locat- ing a method’s actual implementation and ensuring the correct method is actually being invoked becomes a challenge. For example: class Area: sideLength = 0 def getSideLength(self): return self.sideLength def setSideLength(self, sideLength): self.sideLength = sideLength def area(self): raise NotImplementedError, "Subclasses must implement area" class Color: color = None def setColor(self, color): self.color = color def getColor(self): return self.color class Square(Area, Color): def area(self): return self.sideLength * self.sideLength The reason programmers like using implementation inheritance is because it makes code easier to read since the implementation details of Area are in a separate place than the implementation details of Color. This is nice, because conceivably an object could have a color but not an area, or an area but not a color. The problem, though, is that Square is not really an Area or a Color, but has an area and color. Thus, we should really be using another object oriented technique called composition, which relies on delegation rather than inheritance to break code into small reusable chunks. Let us continue with the Multiple Inheritance example, though, because it is often used in practice. What if both the Color and the Area base class defined the same method, perhaps calculate? Where would the implementation come from? The implementation that is located for Square().calculate() depends on the method resolution order, or MRO, and can change when programmers change seemingly unrelated things by refactoring classes in other parts of the system, causing obscure bugs. Our first thought might be to change the calculate CHAPTER 4. HIGH-LEVEL TWISTED 153 method name to avoid name clashes, to perhaps calculateArea and calculateColor. While explicit, this change could potentially require a large number of changes throughout a system, and is error-prone, especially when attempting to integrate two systems which you didn’t write. Let’s imagine another example. We have an electric appliance, say a hair dryer. The hair dryer is american voltage. We have two electric sockets, one of them an american 110 Volt socket, and one of them a foreign 220 Volt socket. If we plug the hair dryer into the 220 Volt socket, it is going to expect 110 Volt current and errors will result. Going back and changing the hair dryer to support both plug110Volt and plug220Volt methods would be tedious, and what if we decided we needed to plug the hair dryer into yet another type of socket? For example: class HairDryer: def plug(self, socket): if socket.voltage() == 110: print "I was plugged in properly and am operating." else: print "I was plugged in improperly and " print "now you have no hair dryer any more." class AmericanSocket: def voltage(self): return 110 class ForeignSocket: def voltage(self): return 220 Given these classes, the following operations can be performed: >>> hd = HairDryer() >>> am = AmericanSocket() >>> hd.plug(am) I was plugged in properly and am operating. >>> fs = ForeignSocket() >>> hd.plug(fs) I was plugged in improperly and now you have no hair dryer any more. We are going to attempt to solve this problem by writing an Adapter for the ForeignSocket which converts the voltage for use with an American hair dryer. An Adapter is a class which is constructed with one and only one argument, the “adaptee” or “original” object. In this example, we will show all code involved for clarity: class AdaptToAmericanSocket: def __init__(self, original): self.original = original def voltage(self): return self.original.voltage() / 2 Now, we can use it as so: >>> hd = HairDryer() >>> fs = ForeignSocket() >>> adapted = AdaptToAmericanSocket(fs) >>> hd.plug(adapted) I was plugged in properly and am operating. So, as you can see, an adapter can ’override’ the original implementation. It can also ’extend’ the interface of the original object by providing methods the original object did not have. Note that an Adapter must explicitly delegate any method calls it does not wish to modify to the original, otherwise the Adapter cannot be used in places where the original is expected. Usually this is not a problem, as an Adapter is created to conform an object to a particular interface and then discarded. CHAPTER 4. HIGH-LEVEL TWISTED 154 4.4.1 Interfaces and Components in Twisted code Adapters are a useful way of using multiple classes to factor code into discrete chunks. However, they are not very interesting without some more infrastructure. If each piece of code which wished to use an adapted object had to explicitly construct the adapter itself, the coupling between components would be too tight. We would like to achieve “loose coupling”, and this is where twisted.python.components comes in. First, we need to discuss Interfaces in more detail. As we mentioned earlier, an Interface is nothing more than a class which is used as a marker. Interfaces should be subclasses of zope.interface.Interface, and have a very odd look to python programmers not used to them: from zope.interface import Interface class IAmericanSocket(Interface): def voltage(): """Return the voltage produced by this socket object, as an integer. """ Notice how it looks just like a regular class definition, other than inheriting from Interface? However, the method definitions inside the class block do not have any method body! Since Python does not have any native language-level support for Interfaces like Java does, this is what distinguishes an Interface definition from a Class. Now that we have a defined Interface, we can talk about objects using terms like this: “The AmericanSocket class implements the IAmericanSocket interface” and “Please give me an object which adapts ForeignSocket to the IAmericanSocket interface”. We can make declarations about what interfaces a certain class implements, and we can request adapters which implement a certain interface for a specific class. Let’s look at how we declare that a class implements an interface: from zope.interface import implements class AmericanSocket: implements(IAmericanSocket) def voltage(self): return 110 So, to declare that a class implements an interface, we simply call zope.interface.implements at the class level. Now, let’s say we want to rewrite the AdaptToAmericanSocket class as a real adapter. In this case we also specify it as implementing IAmericanSocket: from zope.interface import implements class AdaptToAmericanSocket: implements(IAmericanSocket) def __init__(self, original): """ Pass the original ForeignSocket object as original """ self.original = original def voltage(self): return self.original.voltage() / 2 Notice how we placed the implements declaration on this adapter class. So far, we have not achieved anything by using components other than requiring us to type more. In order for components to be useful, we must use the com- ponent registry. Since AdaptToAmericanSocket implements IAmericanSocket and regulates the voltage of a ForeignSocket object, we can register AdaptToAmericanSocket as an IAmericanSocket adapter for the ForeignSocket class. It is easier to see how this is done in code than to describe it: CHAPTER 4. HIGH-LEVEL TWISTED 155 from zope.interface import Interface, implements from twisted.python import components class IAmericanSocket(Interface): def voltage(): """Return the voltage produced by this socket object, as an integer. """ class AmericanSocket: implements(IAmericanSocket) def voltage(self): return 110 class ForeignSocket: def voltage(self): return 220 class AdaptToAmericanSocket: implements(IAmericanSocket) def __init__(self, original): self.original = original def voltage(self): return self.original.voltage() / 2 components.registerAdapter( AdaptToAmericanSocket, ForeignSocket, IAmericanSocket) Now, if we run this script in the interactive interpreter, we can discover a little more about how to use components. The first thing we can do is discover whether an object implements an interface or not: >>> IAmericanSocket.implementedBy(AmericanSocket) True >>> IAmericanSocket.implementedBy(ForeignSocket) False >>> am = AmericanSocket() >>> fs = ForeignSocket() >>> IAmericanSocket.providedBy(am) True >>> IAmericanSocket.providedBy(fs) False As you can see, the AmericanSocket instance claims to implement IAmericanSocket, but the Foreign Socket does not. If we wanted to use the HairDryer with the AmericanSocket, we could know that it would be safe to do so by checking whether it implements IAmericanSocket. However, if we decide we want to use HairDryer with a ForeignSocket instance, we must adapt it to IAmericanSocket before doing so. We use the interface object to do this: >>> IAmericanSocket(fs) <__main__.AdaptToAmericanSocket instance at 0x1a5120> When calling an interface with an object as an argument, the interface looks in the adapter registry for an adapter which implements the interface for the given instance’s class. If it finds one, it constructs an instance of the Adapter class, passing the constructor the original instance, and returns it. Now the HairDryer can safely be used with CHAPTER 4. HIGH-LEVEL TWISTED 156 the adapted ForeignSocket. But what happens if we attempt to adapt an object which already implements IAmericanSocket? We simply get back the original instance: >>> IAmericanSocket(am) <__main__.AmericanSocket instance at 0x36bff0> So, we could write a new “smart” HairDryer which automatically looked up an adapter for the socket you tried to plug it into: class HairDryer: def plug(self, socket): adapted = IAmericanSocket(socket) assert adapted.voltage() == 110, "BOOM" print "I was plugged in properly and am operating" Now, if we create an instance of our new “smart” HairDryer and attempt to plug it in to various sockets, the HairDryer will adapt itself automatically depending on the type of socket it is plugged in to: >>> am = AmericanSocket() >>> fs = ForeignSocket() >>> hd = HairDryer() >>> hd.plug(am) I was plugged in properly and am operating >>> hd.plug(fs) I was plugged in properly and am operating Voila; the magic of components. Components and Inheritance If you inherit from a class which implements some interface, and your new subclass declares that it implements another interface, the implements will be inherited by default. For example, pb.Root is a class which implements IPBRoot. This interface indicates that an object has remotely-invokable methods and can be used as the initial object served by a new Broker instance. It has an implements setting like: from zope.interface import implements class Root(Referenceable): implements(IPBRoot) Suppose you have your own class which implements your IMyInterface interface: from zope.interface import implements, Interface class IMyInterface(Interface): pass class MyThing: implements(IMyInterface) Now if you want to make this class inherit from pb.Root, the interfaces code will automatically determine that it also implements IPBRoot: from twisted.spread import pb from zope.interface import implements, Interface class IMyInterface(Interface): pass class MyThing(pb.Root): implements(IMyInterface) CHAPTER 4. HIGH-LEVEL TWISTED 157 >>> from twisted.spread.flavors import IPBRoot >>> IPBRoot.implementedBy(MyThing) True If you want MyThing to inherit from pb.Root but not implement IPBRoot like pb.Root does, use implementOnly: from twisted.spread import pb from zope.interface import implementsOnly, Interface class IMyInterface(Interface): pass class MyThing(pb.Root): implementsOnly(IMyInterface) >>> from twisted.spread.pb import IPBRoot >>> IPBRoot.implementedBy(MyThing) False 4.5 Cred: Pluggable Authentication 4.5.1 Goals Cred is a pluggable authentication system for servers. It allows any number of network protocols to connect and authenticate to a system, and communicate to those aspects of the system which are meaningful to the specific protocol. For example, Twisted’s POP3 support passes a “username and password” set of credentials to get back a mailbox for the specified email account. IMAP does the same, but retrieves a slightly different view of the same mailbox, enabling those features specific to IMAP which are not available in other mail protocols. Cred is designed to allow both the backend implementation of the business logic - called the avatar - and the authentication database - called the credential checker - to be decided during deployment. For example, the same POP3 server should be able to authenticate against the local UNIX password database or an LDAP server without having to know anything about how or where mail is stored. To sketch out how this works - a “Realm” corresponds to an application domain and is in charge of avatars, which are network-accessible business logic objects. To connect this to an authentication database, a top-level object called a Portal stores a realm, and a number of credential checkers. Something that wishes to log in, such as a Protocol, stores a reference to the portal. Login consists of passing credentials and a request interface (e.g. POP3’s IMailbox) to the portal. The portal passes the credentials to the appropriate credential checker, which returns an avatar ID. The ID is passed to the realm, which returns the appropriate avatar. For a Portal that has a realm that creates mailbox objects and a credential checker that checks /etc/passwd, login consists of passing in a username/password and the IMailbox interface to the portal. The portal passes this to the /etc/passwd credential checker, gets back a avatar ID corresponding to an email account, passes that to the realm and gets back a mailbox object for that email account. Putting all this together, here’s how a login request will typically be processed: CHAPTER 4. HIGH-LEVEL TWISTED 158 4.5.2 Cred objects The Portal This is the the core of login, the point of integration between all the objects in the cred system. There is one concrete implementation of Portal, and no interface - it does a very simple task. A Portal associates one (1) Realm with a collection of CredentialChecker instances. (More on those later.) If you are writing a protocol that needs to authenticate against something, you will need a reference to a Portal, and to nothing else. This has only 2 methods - • login(credentials, mind, *interfaces) The docstring is quite expansive (see twisted.cred.portal), but in brief, this is what you call when you need to call in order to connect a user to the system. Typically you only pass in one interface, and the mind is None. The interfaces are the possible interfaces the returned avatar is expected to implement, in order of preference. The result is a deferred which fires a tuple of: – interface the avatar implements (which was one of the interfaces passed in the *interfaces tuple) – an object that implements that interface (an avatar) – logout, a 0-argument callable which disconnects the connection that was established by this call to login The logout method has to be called when the avatar is logged out. For POP3 this means when the protocol is disconnected or logged out, etc.. • registerChecker(checker, *credentialInterfaces) which adds a CredentialChecker to the portal. The optional list of interfaces are interfaces of credentials that the checker is able to check. CHAPTER 4. HIGH-LEVEL TWISTED 159 The CredentialChecker This is an object implementing ICredentialsChecker which resolves some credentials to an avatar ID. Whether the credentials are stored in an in-memory data structure, an Apache-style htaccess file, a UNIX password database, an SSH key database, or any other form, an implementation of ICredentialsChecker is how this data is connected to cred. A credential checker stipulates some requirements of the credentials it can check by specifying a credential- Interfaces attribute, which is a list of interfaces. Credentials passed to its requestAvatarId method must implement one of those interfaces. For the most part, these things will just check usernames and passwords and produce the username as the result, but hopefully we will be seeing some public-key, challenge-response, and certificate based credential checker mechanisms soon. A credential checker should raise an error if it cannot authenticate the user, and return twisted.cred. checkers.ANONYMOUS for anonymous access. The Credentials Oddly enough, this represents some credentials that the user presents. Usually this will just be a small static blob of data, but in some cases it will actually be an object connected to a network protocol. For example, a username/password pair is static, but a challenge/response server is an active state-machine that will require several method calls in order to determine a result. Twisted comes with a number of credentials interfaces and implementations in the twisted.cred. credentials module, such as IUsernamePassword and IUsernameHashedPassword. The Realm A realm is an interface which connects your universe of “business objects” to the authentication system. IRealm is another one-method interface: • requestAvatar(avatarId, mind, *interfaces) This method will typically be called from ’Portal.login’. The avatarId is the one returned by a CredentialChecker. Note:Note that avatarId must always be a string. In particular, do not use unicode strings. If internationalized support is needed, it is recommended to use UTF-8, and take care of decoding in the realm. The important thing to realize about this method is that if it is being called, the user has already authenticated. Therefore, if possible, the Realm should create a new user if one does not already exist whenever possible. Of course, sometimes this will be impossible without more information, and that is the case that the interfaces argument is for. Since requestAvatar should be called from a Deferred callback, it may return a Deferred or a synchronous result. The Avatar An avatar is a business logic object for a specific user. For POP3, it’s a mailbox, for a first-person-shooter it’s the object that interacts with the game, the actor as it were. Avatars are specific to an application, and each avatar represents a single “user”. The Mind As mentioned before, the mind is usually None, so you can skip this bit if you want. Masters of Perspective Broker already know this object as the ill-named “client object”. There is no “mind” class, or even interface, but it is an object which serves an important role - any notifications which are to be relayed to an authenticated client are passed through a ’mind’. In addition, it allows passing more information to the realm during login in addition to the avatar ID. The name may seem rather unusual, but considering that a Mind is representative of the entity on the “other end” of a network connection that is both receiving updates and issuing commands, I believe it is appropriate. Although many protocols will not use this, it serves an important role. It is provided as an argument both to the Portal and to the Realm, although a CredentialChecker should interact with a client program exclusively through a Credentials instance. CHAPTER 4. HIGH-LEVEL TWISTED 160 Unlike the original Perspective Broker “client object”, a Mind’s implementation is most often dictated by the protocol that is connecting rather than the Realm. A Realm which requires a particular interface to issue notifications will need to wrap the Protocol’s mind implementation with an adapter in order to get one that conforms to its expected interface - however, Perspective Broker will likely continue to use the model where the client object has a pre-specified remote interface. (If you don’t quite understand this, it’s fine. It’s hard to explain, and it’s not used in simple usages of cred, so feel free to pass None until you find yourself requiring something like this.) 4.5.3 Responsibilities Server protocol implementation The protocol implementor should define the interface the avatar should implement, and design the protocol to have a portal attached. When a user logs in using the protocol, a credential object is created, passed to the portal, and an avatar with the appropriate interface is requested. When the user logs out or the protocol is disconnected, the avatar should be logged out. The protocol designer should not hardcode how users are authenticated or the realm implemented. For example, a POP3 protocol implementation would require a portal whose realm returns avatars implementing IMailbox and whose credential checker accepts username/password credentials, but that is all. Here’s a sketch of how the code might look - note that USER and PASS are the protocol commands used to login, and the DELE command can only be used after you are logged in: from zope.interface import Interface from twisted.protocols import basic from twisted.python import log from twisted.cred import credentials, error from twisted.internet import defer class IMailbox(Interface): """Interface specification for mailbox.""" def deleteMessage(index): pass class POP3(basic.LineReceiver): # ... def __init__(self, portal): self.portal = portal def do_DELE(self, i): # uses self.mbox, which is set after login i = int(i)-1 self.mbox.deleteMessage(i) self.successResponse() def do_USER(self, user): self._userIs = user self.successResponse(’USER accepted, send PASS’) def do_PASS(self, password): if self._userIs is None: self.failResponse("USER required before PASS") return user = self._userIs self._userIs = None d = defer.maybeDeferred(self.authenticateUserPASS, user, password) d.addCallback(self._cbMailbox, user) CHAPTER 4. HIGH-LEVEL TWISTED 161 def authenticateUserPASS(self, user, password): if self.portal is not None: return self.portal.login( cred.credentials.UsernamePassword(user, password), None, IMailbox ) raise error.UnauthorizedLogin() def _cbMailbox(self, ial, user): interface, avatar, logout = ial if interface is not IMailbox: self.failResponse(’Authentication failed’) log.err("_cbMailbox() called with an interface other than IMailbox") return self.mbox = avatar self._onLogout = logout self.successResponse(’Authentication succeeded’) log.msg("Authenticated login for " + user) Application implementation The application developer can implement realms and credential checkers. For example, she might implement a realm that returns IMailbox implementing avatars, using MySQL for storage, or perhaps a credential checker that uses LDAP for authentication. In the following example, the Realm for a simple remote object service (using Twisted’s Perspective Broker protocol) is implemented: from twisted.spread import pb from twisted.cred.portal import IRealm class SimplePerspective(pb.Avatar): def perspective_echo(self, text): print ’echoing’,text return text def logout(self): print self, "logged out" class SimpleRealm: implements(IRealm) def requestAvatar(self, avatarId, mind, *interfaces): if pb.IPerspective in interfaces: avatar = SimplePerspective() return pb.IPerspective, avatar, avatar.logout else: raise NotImplementedError("no interface") Deployment Deployment involves tying together a protocol, an appropriate realm and a credential checker. For example, a POP3 server can be constructed by attaching to it a portal that wraps the MySQL-based realm and an /etc/passwd credential checker, or perhaps the LDAP credential checker if that is more useful. The following example shows how the SimpleRealm in the previous example is deployed using an in-memory credential checker: CHAPTER 4. HIGH-LEVEL TWISTED 162 from twisted.spread import pb from twisted.internet import reactor from twisted.cred.portal import Portal from twisted.cred.checkers import InMemoryUsernamePasswordDatabaseDontUse portal = Portal(SimpleRealm()) checker = InMemoryUsernamePasswordDatabaseDontUse() checker.addUser("guest", "password") portal.registerChecker(checker) reactor.listenTCP(9986, pb.PBServerFactory(portal)) reactor.run() 4.5.4 Cred plugins Authentication with cred plugins Cred offers a plugin architecture for authentication methods. The primary API for this architecture is the command- line; the plugins are meant to be specified by the end-user when deploying a TAP (twistd plugin). For more information on writing a twistd plugin and using cred plugins for your application, please refer to the Writing a twistd plugin (page 148) document. Building a cred plugin To build a plugin for cred, you should first define an authType, a short one-word string that defines your plugin to the command-line. Once you have this, the convention is to create a file named myapp plugins.py in the twisted. plugins module path. Below is an example file structure for an application that defines such a plugin: • MyApplication/ – setup.py – myapp/ ∗ init .py ∗ cred.py ∗ server.py – twisted/ ∗ plugins/ · myapp plugins.py Once you have created this structure within your application, you can create the code for your cred plugin by build- ing a factory class which implements ICheckerFactory. These factory classes should not consist of a tremendous amount of code. Most of the real application logic should reside in the cred checker itself. (For help on building those, scroll up.) The core purpose of the CheckerFactory is to translate an argstring, which is passed on the command line, into a suitable set of initialization parameters for a Checker class. In most cases this should be little more than constructing a dictionary or a tuple of arguments, then passing them along to a new checker instance. from zope.interface import implements from twisted import plugin from twisted.cred import checkers from myapp.cred import SpecialChecker class SpecialCheckerFactory(object): """ A checker factory for a specialized (fictional) API. """ CHAPTER 4. HIGH-LEVEL TWISTED 163 # The class needs to implement both of these interfaces # for the plugin system to find our factory. implements(checkers.ICheckerFactory, plugin.IPlugin) # This tells AuthOptionsMixin how to find this factory. authType = "special" # This is a one-line explanation of what arguments, if any, # your particular cred plugin requires at the command-line. argStringFormat = "A colon-separated key=value list." # This help text can be multiple lines. It will be displayed # when someone uses the "--help-auth-type special" command. authHelp = """Some help text goes here ...""" # This will be called once per command-line. def generateChecker(self, argstring=""): argdict = dict((x.split(’=’) for x in argstring.split(’:’))) return SpecialChecker(**dict) # We need to instantiate our class for the plugin to work. theSpecialCheckerFactory = SpecialCheckerFactory() For more information on how your plugin can be used in your application (and by other application developers), please see the Writing a twistd plugin (page 148) document. 4.5.5 Conclusion After reading through this tutorial, you should be able to • Understand how the cred architecture applies to your application • Integrate your application with cred’s object model • Deploy an application that uses cred for authentication • Allow your users to use command-line authentication plugins 4.6 Using the Twisted Application Framework 4.6.1 Introduction Audience The target audience of this document is a Twisted user who wants to deploy a significant amount of Twisted code in a re-usable, standard and easily configurable fashion. A Twisted user who wishes to use the Application framework needs to be familiar with developing Twisted servers (page 9) and/or clients (page 13). Goals • To introduce the Twisted Application infrastructure. • To explain how to deploy your Twisted application using .tac files and twistd • To outline the existing Twisted services. CHAPTER 4. HIGH-LEVEL TWISTED 164 4.6.2 Overview The Twisted Application infrastructure takes care of running and stopping your application. Using this infrastructure frees you from from having to write a large amount of boilerplate code by hooking your application into existing tools that manage daemonization, logging, choosing a reactor (page 140) and more. The major tool that manages Twisted applications is a command-line utility called twistd. twistd is cross platform, and is the recommended tool for running Twisted applications. The core component of the Twisted Application infrastructure is the twisted.application.service. Application object an object which represents your application. However, Application doesn’t provide anything that you’d want to manipulate directly. Instead, Application acts as a container of any “Services” (objects implement- ing IService) that your application provides. Most of your interaction with the Application infrastructure will be done through Services. By “Service”, we mean anything in your application that can be started and stopped. Typical services include web servers, FTP servers and SSH clients. Your Application object can contain many services, and can even contain structured hierarchies of Services using IServiceCollections. Here’s a simple example of constructing an Application object which represents an echo server that runs on TCP port 7001. from twisted.application import internet, service from somemodule import EchoFactory port = 7001 factory = EchoFactory() # this is the important bit application = service.Application("echo") # create the Application echoService = internet.TCPServer(port, factory) # create the service # add the service to the application echoService.setServiceParent(application) See Writing Servers (page 9) for an explanation of EchoFactory. This example creates a simple hierarchy: application | ‘- echoService More complicated hierarchies of services can be created using IServiceCollection. You will most likely want to do this to manage Services which are dependent on other Services. For example, a proxying Twisted application might want its server Service to only start up after the associated Client service. 4.6.3 Using application twistd and tac To handle start-up and configuration of your Twisted application, the Twisted Application infrastructure uses .tac files. .tac are Python files which configure an Application object and assign this object to the top-level variable “application”. The following is a simple example of a .tac file: # You can run this .tac file directly with: # twistd -ny service.tac """ This is an example .tac file which starts a webserver on port 8080 and serves files from the current working directory. The important part of this, the part that makes it a .tac file, is the final root-level section, which sets up the object called ’application’ which twistd will look for CHAPTER 4. HIGH-LEVEL TWISTED 165 """ import os from twisted.application import service, internet from twisted.web import static, server def getWebService(): """ Return a service suitable for creating an application object. This service is a simple web server that serves files on port 8080 from underneath the current working directory. """ # create a resource to serve static files fileServer = server.Site(static.File(os.getcwd())) return internet.TCPServer(8080, fileServer) # this is the core part of any tac file, the creation of the root-level # application object application = service.Application("Demo application") # attach the service to its parent application service = getWebService() service.setServiceParent(application) Source listing — service.tac twistd is a program that runs Twisted applications using a .tac file. In its most simple form, it takes a sin- gle argument -y and a tac file name. For example, you can run the above server with the command twistd -y service.tac. By default, twistd daemonizes and logs to a file called twistd.log. More usually, when debugging, you will want your application to run in the foreground and log to the command line. To run the above file like this, use the command twistd -noy service.tac For more information, see the twistd man page. Customizing twistd logging in a .tac application The logging behavior can be customized through an API accessible from .tac files. The ILogObserver compo- nent can be set on an Application in order to customize the default log observer that twistd will use. Here is an example of how to use DailyLogFile, which rotates the log once per day. from twisted.application.service import Application from twisted.python.log import ILogObserver, FileLogObserver from twisted.python.logfile import DailyLogFile application = Application("myapp") logfile = DailyLogFile("my.log", "/tmp") application.setComponent(ILogObserver, FileLogObserver(logfile).emit) invoking twistd -y my.tac will create a log file at/tmp/my.log. Services provided by Twisted Twisted provides several services that you want to know about. Each of these services (except TimerService) has a corresponding “connect” or “listen” method on the reactor, and the constructors for the services take the same arguments as the reactor methods. The “connect” methods are for clients and the “listen” methods are for servers. For example, TCPServer corresponds to reactor.listenTCP and TCPClient corresponds to reactor.connectTCP. CHAPTER 4. HIGH-LEVEL TWISTED 166 TCPServer TCPClient Services which allow you to make connections and listen for connections on TCP ports. • listenTCP • connectTCP UNIXServer UNIXClient Services which listen and make connections over UNIX sockets. • listenUNIX • connectUNIX SSLServer SSLClient Services which allow you to make SSL connections and run SSL servers. • listenSSL • connectSSL UDPServer UDPClient Services which allow you to send and receive data over UDP • listenUDP See also the UDP documentation (page 94). UNIXDatagramServer UNIXDatagramClient Services which send and receive data over UNIX datagram sockets. • listenUNIXDatagram • connectUNIXDatagram MulticastServer A server for UDP socket methods that support multicast. • listenMulticast TimerService A service to periodically call a function. Service Collection IServiceCollection objects contain IService objects. IService objects can be added to IServiceCollection by calling setServiceParent and detached by using disownServiceParent. The standard implementation of IServiceCollection is MultiService, which also implements IService. Multi- Service is useful for creating a new Service which combines two or more existing Services. For example, you could create a DNS Service as a MultiService which has a TCP and a UDP Service as children. from twisted.application import internet, service from twisted.names import server, dns, hosts port = 53 # Create a MultiService, and hook up a TCPServer and a UDPServer to it as # children. dnsService = service.MultiService() hostsResolver = hosts.Resolver(’/etc/hosts’) tcpFactory = server.DNSServerFactory([hostsResolver]) internet.TCPServer(port, tcpFactory).setServiceParent(dnsService) udpFactory = dns.DNSDatagramProtocol(tcpFactory) internet.UDPServer(port, udpFactory).setServiceParent(dnsService) CHAPTER 4. HIGH-LEVEL TWISTED 167 # Create an application as normal application = service.Application("DNSExample") # Connect our MultiService to the application, just like a normal service. dnsService.setServiceParent(application) Chapter 5 Utilities 5.1 Parsing command-lines with usage.Options 5.1.1 Introduction There is frequently a need for programs to parse a UNIX-like command line program: options preceded by - or --, sometimes followed by a parameter, followed by a list of arguments. The twisted.python.usage provides a class, Options, to facilitate such parsing. While Python has the getopt module for doing this, it provides a very low level of abstraction for options. Twisted has a higher level of abstraction, in the class twisted.python.usage.Options. It uses Python’s reflection facilities to provide an easy to use yet flexible interface to the command line. While most command line processors either force the application writer to write her own loops, or have arbitrary limitations on the command line (the most common one being not being able to have more then one instance of a specific option, thus rendering the idiom program -v -v -v impossible), Twisted allows the programmer to decide how much control she wants. The Options class is used by subclassing. Since a lot of time it will be used in the twisted.tap package, where the local conventions require the specific options parsing class to also be called Options, it is usually imported with from twisted.python import usage 5.1.2 Boolean Options For simple boolean options, define the attribute optFlags like this: class Options(usage.Options): optFlags = [["fast", "f", "Act quickly"], ["safe", "s", "Act safely"]] optFlags should be a list of 3-lists. The first element is the long name, and will be used on the command line as --fast. The second one is the short name, and will be used on the command line as -f. The last element is a description of the flag and will be used to generate the usage information text. The long name also determines the name of the key that will be set on the Options instance. Its value will be 1 if the option was seen, 0 otherwise. Here is an example for usage: class Options(usage.Options): optFlags = [ ["fast", "f", "Act quickly"], ["good", "g", "Act well"], ["cheap", "c", "Act cheaply"] ] command_line = ["-g", "--fast"] options = Options() 168 CHAPTER 5. UTILITIES 169 try: options.parseOptions(command_line) except usage.UsageError, errortext: print ’%s: %s’ % (sys.argv[0], errortext) print ’%s: Try --help for usage details.’ % (sys.argv[0]) sys.exit(1) if options[’fast’]: print "fast", if options[’good’]: print "good", if options[’cheap’]: print "cheap", print The above will print fast good. Note here that Options fully supports the mapping interface. You can access it mostly just like you can access any other dict. Options are stored as mapping items in the Options instance: parameters as ’paramname’: ’value’ and flags as ’flagname’: 1 or 0. Inheritance, Or: How I Learned to Stop Worrying and Love the Superclass Sometimes there is a need for several option processors with a unifying core. Perhaps you want all your commands to understand -q/--quiet means to be quiet, or something similar. On the face of it, this looks impossible: in Python, the subclass’s optFlags would shadow the superclass’s. However, usage.Options uses special reflection code to get all of the optFlags defined in the hierarchy. So the following: class BaseOptions(usage.Options): optFlags = [["quiet", "q", None]] class SpecificOptions(BaseOptions): optFlags = [ ["fast", "f", None], ["good", "g", None], ["cheap", "c", None] ] Is the same as: class SpecificOptions(BaseOptions): optFlags = [ ["quiet", "q", "Silence output"], ["fast", "f", "Run quickly"], ["good", "g", "Don’t validate input"], ["cheap", "c", "Use cheap resources"] ] 5.1.3 Parameters Parameters are specified using the attribute optParameters. They must be given a default. If you want to make sure you got the parameter from the command line, give a non-string default. Since the command line only has strings, this is completely reliable. Here is an example: from twisted.python import usage class Options(usage.Options): optFlags = [ CHAPTER 5. UTILITIES 170 ["fast", "f", "Run quickly"], ["good", "g", "Don’t validate input"], ["cheap", "c", "Use cheap resources"] ] optParameters = [["user", "u", None, "The user name"]] config = Options() try: config.parseOptions() # When given no argument, parses sys.argv[1:] except usage.UsageError, errortext: print ’%s: %s’ % (sys.argv[0], errortext) print ’%s: Try --help for usage details.’ % (sys.argv[0]) sys.exit(1) if config[’user’] is not None: print "Hello", config[’user’] print "So, you want it:" if config[’fast’]: print "fast", if config[’good’]: print "good", if config[’cheap’]: print "cheap", print Like optFlags, optParameters works smoothly with inheritance. 5.1.4 Option Subcommands It is useful, on occassion, to group a set of options together based on the logical “action” to which they belong. For this, the usage.Options class allows you to define a set of “subcommands”, each of which can provide its own usage.Options instance to handle its particular options. Here is an example for an Options class that might parse options like those the cvs program takes from twisted.python import usage class ImportOptions(usage.Options): optParameters = [ [’module’, ’m’, None, None], [’vendor’, ’v’, None, None], [’release’, ’r’, None] ] class CheckoutOptions(usage.Options): optParameters = [[’module’, ’m’, None, None], [’tag’, ’r’, None, None]] class Options(usage.Options): subCommands = [[’import’, None, ImportOptions, "Do an Import"], [’checkout’, None, CheckoutOptions, "Do a Checkout"]] optParameters = [ [’compression’, ’z’, 0, ’Use compression’], [’repository’, ’r’, None, ’Specify an alternate repository’] ] config = Options(); config.parseOptions() if config.subCommand == ’import’: doImport(config.subOptions) CHAPTER 5. UTILITIES 171 elif config.subCommand == ’checkout’: doCheckout(config.subOptions) The subCommands attribute of Options directs the parser to the two other Options subclasses when the strings "import" or "checkout" are present on the command line. All options after the given command string are passed to the specified Options subclass for further parsing. Only one subcommand may be specified at a time. After parsing has completed, the Options instance has two new attributes - subCommand and subOptions - which hold the command string and the Options instance used to parse the remaining options. 5.1.5 Generic Code For Options Sometimes, just setting an attribute on the basis of the options is not flexible enough. In those cases, Twisted does not even attempt to provide abstractions such as “counts” or “lists”, but rathers lets you call your own method, which will be called whenever the option is encountered. Here is an example of counting verbosity from twisted.python import usage class Options(usage.Options): def __init__(self): usage.Options.__init__(self) self[’verbosity’] = 0 # default def opt_verbose(self): self[’verbosity’] = self[’verbosity’]+1 def opt_quiet(self): self[’verbosity’] = self[’verbosity’]-1 opt_v = opt_verbose opt_q = opt_quiet Command lines that look like command -v -v -v -v will increase verbosity to 4, while command -q -q -q will decrease verbosity to -3. The usage.Options class knows that these are parameter-less options, since the methods do not receive an argument. Here is an example for a method with a parameter: from twisted.python import usage class Options(usage.Options): def __init__(self): usage.Options.__init__(self) self[’symbols’] = [] def opt_define(self, symbol): self[’symbols’].append(symbol) opt_D = opt_define This example is useful for the common idiom of having command -DFOO -DBAR to define symbols. 5.1.6 Parsing Arguments usage.Options does not stop helping when the last parameter is gone. All the other arguments are sent into a function which should deal with them. Here is an example for a cmp like command. CHAPTER 5. UTILITIES 172 from twisted.python import usage class Options(usage.Options): optParameters = [["max_differences", "d", 1, None]] def parseArgs(self, origin, changed): self[’origin’] = origin self[’changed’] = changed The command should look like command origin changed. If you want to have a variable number of left-over arguments, just use def parseArgs(self, *args):. This is useful for commands like the UNIX cat(1). 5.1.7 Post Processing Sometimes, you want to perform post processing of options to patch up inconsistencies, and the like. Here is an example: from twisted.python import usage class Options(usage.Options): optFlags = [ ["fast", "f", "Run quickly"], ["good", "g", "Don’t validate input"], ["cheap", "c", "Use cheap resources"] ] def postOptions(self): if self[’fast’] and self[’good’] and self[’cheap’]: raise usage.UsageError, "can’t have it all, brother" 5.1.8 Type enforcement By default, all options are handled as strings. You may want to enforce the type of your option in some specific case, the classic example being port number. Any callable can be specified in the fifth row of optParameters and will be called with the string value passed in parameter. from twisted.python import usage class Options(usage.Options): optParameters = [["shiny_integer", "s", 1, None, int]] optParameters = [["dummy_float", "d", 3.14159, None, float]] Note that default values are not coerced, so you should either declare it with the good type (as above) or handle it when you use your options. The coerce function may have a coerceDoc attribute, the content of which will be printed after the documentation of the option. It’s particularly useful for reusing the function at multiple places. def oneTwoThree(val): val = int(val) if val not in range(1, 4): raise ValueError("Not in range") return val oneTwoThree.coerceDoc = "Must be 1, 2 or 3." from twisted.python import usage CHAPTER 5. UTILITIES 173 class Options(usage.Options): optParameters = [["one_choice", "o", 1, None, oneTwoThree]] This example code will print the following help when added to your program: $ python myprogram.py --help Usage: myprogram [options] Options: -o, --one_choice= [default: 0]. Must be 1, 2 or 3. 5.2 Logging with twisted.python.log 5.2.1 Basic usage Twisted provides a simple and flexible logging system in the twisted.python.log module. It has three com- monly used functions: msg Logs a new message. For example: from twisted.python import log log.msg(’Hello, world.’) err Writes a failure to the log, including traceback information (if any). You can pass it a Failure or Exception instance, or nothing. If you pass something else, it will be converted to a string with repr and logged. If you pass nothing, it will construct a Failure from the currently active exception, which makes it convenient to use in an except clause: try: x=1/0 except: log.err() # will log the ZeroDivisionError startLogging Starts logging to a given file-like object. For example: log.startLogging(open(’/var/log/foo.log’, ’w’)) or: log.startLogging(sys.stdout) By default, startLogging will also redirect anything written to sys.stdout and sys.stderr to the log. You can disable this by passing setStdout=False to startLogging. Before startLogging is called, log messages will be discarded and errors will be written to stderr. Logging and twistd If you are using twistd to run your daemon, it will take care of calling startLogging for you, and will also rotate log files. See twistd and tac (page 164) and the twistd man page for details of using twistd. Log files The twisted.python.logfile module provides some standard classes suitable for use with startLogging, such as DailyLogFile, which will rotate the log to a new file once per day. CHAPTER 5. UTILITIES 174 Using the Python logging module If your application uses the logging module or you want to use its ease of configuration but don’t want to lose twisted- produced messages, the observer PythonLoggingObserver should be useful to you You just start it like any other observers: observer = log.PythonLoggingObserver() observer.start() And then you’ll just have to configure logging to do what you want: logging documentation1. This method allows you to customize the log level received by the logging module using the logLevel keyword: log.msg("This is important!", logLevel=logging.CRITICAL) log.msg("Don’t mind", logLevel=logging.DEBUG) Unless logLevel is provided, logging.INFO is used for log.msg and logging.ERROR is used for log.err. One special care should be made when you use special configuration of the python logging module: some handlers (e.g. SMTP, HTTP) uses network so can block inside the reactor loop. Nothing in the bridge is done to prevent that. 5.2.2 Writing log observers Log observers are the basis of the Twisted logging system. An example of a log observer in Twisted is the FileLog Observer used by startLogging that writes events to a log file. A log observer is just a callable that accepts a dictionary as its only argument. You can then register it to receive all log events (in addition to any other observers): twisted.python.log.addObserver(yourCallable) The dictionary will have at least two items: message The message (a list, usually of strings) for this log event, as passed to log.msg or the message in the failure passed to log.err. isError This is a boolean that will be true if this event came from a call to log.err. If this is set, there may be a failure item in the dictionary as will, with a Failure object in it. Other items the built in logging functionality may add include: printed This message was captured from sys.stdout, i.e. this message came from a print statement. If is Error is also true, it came from sys.stderr. You can pass additional items to the event dictionary by passing keyword arguments to log.msg and log.err. The standard log observers will ignore dictionary items they don’t use. Important notes: • Never block in a log observer, as it may run in main Twisted thread. This means you can’t use socket or syslog Python-logging backends. • The observer needs to be thread safe if you anticipate using threads in your program. 5.2.3 Customizing twistd logging The behavior of the logging that twistd does can be customized by setting the ILogObserver component on the application object. See the Application document (page 163) for more information. 1http://docs.python.org/lib/module-logging.html CHAPTER 5. UTILITIES 175 5.3 DirDBM: Directory-based Storage 5.3.1 dirdbm.DirDBM twisted.persisted.dirdbm.DirDBM is a DBM-like storage system. That is, it stores mappings between keys and values, like a Python dictionary, except that it stores the values in files in a directory - each entry is a different file. The keys must always be strings, as are the values. Other than that, DirDBM objects act just like Python dictionaries. DirDBM is useful for cases when you want to store small amounts of data in an organized fashion, without having to deal with the complexity of a RDBMS or other sophisticated database. It is simple, easy to use, cross-platform, and doesn’t require any external C libraries, unlike Python’s built-in DBM modules. >>> from twisted.persisted import dirdbm >>> d = dirdbm.DirDBM("/tmp/dir") >>> d["librarian"] = "ook" >>> d["librarian"] ’ook’ >>> d.keys() [’librarian’] >>> del d["librarian"] >>> d.items() [] 5.3.2 dirdbm.Shelf Sometimes it is neccessary to persist more complicated objects than strings. With some care, dirdbm.Shelf can transparently persist them. Shelf works exactly like DirDBM, except that the values (but not the keys) can be arbitrary picklable objects. However, notice that mutating an object after it has been stored in the Shelf has no effect on the Shelf. When mutating objects, it is neccessary to explictly store them back in the Shelf afterwards: >>> from twisted.persisted import dirdbm >>> d = dirdbm.Shelf("/tmp/dir2") >>> d["key"] = [1, 2] >>> d["key"] [1, 2] >>> l = d["key"] >>> l.append(3) >>> d["key"] [1, 2] >>> d["key"] = l >>> d["key"] [1, 2, 3] 5.4 Using telnet to manipulate a twisted server To start things off, we’re going to create a simple server that just gives you remote access to a Python interpreter. We will use a telnet client to access this server. Run twistd telnet -p 4040 -u admin -w admin at your shell prompt. The Application has a telnet server that you specified to be on port 4040, and it will start listening for connections on this port. Try connecting with your favorite telnet utility to 127.0.0.1 port 4040. $ telnet localhost 4040 Trying 127.0.0.1... Connected to localhost. Escape character is ’ˆ]’. twisted.manhole.telnet.ShellFactory Twisted 1.1.0 username: admin CHAPTER 5. UTILITIES 176 password: admin >>> Now, you should see a Python prompt – >>>. You can type any valid Python code here. Let’s try looking around. >>> dir() [’__builtins__’] Ok, not much. let’s play a little more: >>> import __main__ >>> dir(__main__) [’__builtins__’, ’__doc__’, ’__name__’, ’os’, ’run’, ’string’, ’sys’] >>> service >>> service._port >>> service.parent The service object is the service used to serve the telnet shell, and that it is listening on port 4040 with something called a ShellFactory. Its parent is a twisted.application.service.MultiService, a collection of services. We can keep getting the parent attribute of services until we hit the root of all services. As you can see, this is quite useful - we can introspect a running process, see the internal objects, and even change their attributes. The telnet server can of course be used from straight Python code; you can see how to do this by reading the code for twisted.tap.telnet. A final note - if you want access to be more secure, you can even have the telnet server use SSL. Assuming you have the appropriate certificate and private key files, you can twistd telnet -p ssl:443:privateKey=mykey.pem:certKey=cert.pem -u admin -w admin. See twisted. application.strports for more examples of options for listening on a port. 5.5 Writing tests for Twisted code using Trial 5.5.1 Trial basics Trial is Twisted’s testing framework. It provides a library for writing test cases and utility functions for working with the Twisted environment in your tests, and a command-line utility for running your tests. Trial is built on the Python standard library’s unittest module. To run all the Twisted tests, do: $ trial twisted Refer to the Trial man page for other command-line options. 5.5.2 Trial directories You might notice a new trial temp folder in the current working directory after Trial completes the tests. This folder is the working directory for the Trial process. It can be used by unit tests and allows them to write whatever data they like to disk, and not worry about polluting the current working directory. Folders named trial temp- are created if two instances of Trial are run in parallel from the same directory, so as to avoid giving two different test-runs the same temporary directory. The twisted.python.lockfile utility is used to lock the trial temp directories. On Linux, this results in symlinks to pids. On Windows, directories are created with a single file with a pid as the contents. These lock files will be cleaned up if Trial exits normally and otherwise they will be left behind. They should be cleaned up the next time Trial tries to use the directory they lock, but it’s also safe to delete them manually if desired. CHAPTER 5. UTILITIES 177 5.5.3 Twisted-specific quirks: reactor, Deferreds, callLater The standard Python unittest framework, from which Trial is derived, is ideal for testing code with a fairly linear flow of control. Twisted is an asynchronous networking framework which provides a clean, sensible way to establish functions that are run in response to events (like timers and incoming data), which creates a highly non-linear flow of control. Trial has a few extensions which help to test this kind of code. This section provides some hints on how to use these extensions and how to best structure your tests. Leave the Reactor as you found it Trial runs the entire test suite (over four thousand tests) in a single process, with a single reactor. Therefore it is important that your test leave the reactor in the same state as it found it. Leftover timers may expire during somebody else’s unsuspecting test. Leftover connection attempts may complete (and fail) during a later test. These lead to intermittent failures that wander from test to test and are very time-consuming to track down. If your test leaves event sources in the reactor, Trial will fail the test. The tearDown method is a good place to put cleanup code: it is always run regardless of whether your test passes or fails (like a bare except clause in a try-except construct). Exceptions in tearDown are flagged as errors and flunk the test. TestCase.add Cleanup is another useful tool for cleaning up. With it, you can register callables to clean up resources as the test allocates them. Generally, code should be written so that only resources allocated in the tests need to be cleaned up in the tests. Resources which are allocated internally by the implementation should be cleaned up by the implementation. If your code uses Deferreds or depends on the reactor running, you can return a Deferred from your test method, setUp, or tearDown and Trial will do the right thing. That is, it will run the reactor for you until the Deferred has triggered and its callbacks have been run. Don’t use reactor.run(), reactor.stop(), reactor.crash() or reactor.iterate() in your tests. Calls to reactor.callLater create IDelayedCalls. These need to be run or cancelled during a test, otherwise they will outlive the test. This would be bad, because they could interfere with a later test, causing confusing failures in unrelated tests! For this reason, Trial checks the reactor to make sure there are no leftover IDelayed Calls in the reactor after a test, and will fail the test if there are. The cleanest and simplest way to make sure this all works is to return a Deferred from your test. Similarly, sockets created during a test should be closed by the end of the test. This applies to both listening ports and client connections. So, calls to reactor.listenTCP (and listenUNIX, and so on) return IListening Ports, and these should be cleaned up before a test ends by calling their stopListening method. Calls to reactor.connectTCP return IConnectors, which should be cleaned up by calling their disconnect method. Trial will warn about unclosed sockets. The golden rule is: If your tests call a function which returns a Deferred, your test should return a Deferred. Using Timers to Detect Failing Tests It is common for tests to establish some kind of fail-safe timeout that will terminate the test in case something unex- pected has happened and none of the normal test-failure paths are followed. This timeout puts an upper bound on the time that a test can consume, and prevents the entire test suite from stalling because of a single test. This is especially important for the Twisted test suite, because it is run automatically by the buildbot whenever changes are committed to the Subversion repository. The way to do this in Trial is to set the .timeout attribute on your unit test method. Set the attribute to the number of seconds you wish to elapse before the test raises a timeout error. Trial has a default timeout which will be applied even if the timeout attribute is not set. The Trial default timeout is usually sufficient and should be overridden only in unusual cases. Interacting with warnings in tests Trial includes specific support for interacting with Python’s warnings module. This support allows warning-emitting code to be written test-driven, just as any other code would be. It also improves the way in which warnings reporting when a test suite is running. TestCase.assertWarns and TestCase.flushWarnings allow tests to be written which make asser- tions about what warnings have been emitted during a particular test method. flushWarnings is the new method and has a simpler and more flexible API and should be preferred when writing new code. In order to test a warning with flushWarnings, write a test which first invokes the code which will emit a warning and then calls flush Warnings and makes assertions about the result. For example: CHAPTER 5. UTILITIES 178 def test_warning(self): warnings.warn("foo is bad") self.assertEqual(len(self.flushWarnings()), 1) Warnings emitted in tests which are not flushed will be included by the default reporter in its output after the result of the test. If Python’s warnings filter system (see the -W command line option to Python2) is configured to treat a warning as an error, then unflushed warnings will causes tests to fail and will be included in the summary section of the default reporter. Note that unlike usual operation, when warnings.warn is called as part of a test method, it will not raise an exception when warnings have been configured as errors. However, if called outside of a test method (for example, at module scope in a test module or a module imported by a test module) then it will raise an exception. 2http://docs.python.org/using/cmdline.html#cmdoption-W Chapter 6 Twisted RDBMS support 6.1 twisted.enterprise.adbapi: Twisted RDBMS support 6.1.1 Abstract Twisted is an asynchronous networking framework, but most database API implementations unfortunately have block- ing interfaces – for this reason, twisted.enterprise.adbapi was created. It is a non-blocking interface to the standardized DB-API 2.0 API, which allows you to access a number of different RDBMSes. 6.1.2 What you should already know • Python :-) • How to write a simple Twisted Server (see this tutorial (page 9) to learn how) • Familiarity with using database interfaces (see the documentation for DBAPI 2.01 or this article2 by Andrew Kuchling) 6.1.3 Quick Overview Twisted is an asynchronous framework. This means standard database modules cannot be used directly, as they typically work something like: # Create connection... db = dbmodule.connect(’mydb’, ’andrew’, ’password’) # ...which blocks for an unknown amount of time # Create a cursor cursor = db.cursor() # Do a query... resultset = cursor.query(’SELECT * FROM table WHERE ...’) # ...which could take a long time, perhaps even minutes. Those delays are unacceptable when using an asynchronous framework such as Twisted. For this reason, twisted provides twisted.enterprise.adbapi, an asynchronous wrapper for any DB-API 2.03-compliant module. enterprise.adbapi will do blocking database operations in seperate threads, which trigger callbacks in the originating thread when they complete. In the meantime, the original thread can continue doing normal work, like servicing other requests. 1http://www.python.org/dev/peps/pep-0249/ 2http://www.amk.ca/python/writing/DB-API.html 3http://www.python.org/topics/database/DatabaseAPI-2.0.html 179 CHAPTER 6. TWISTED RDBMS SUPPORT 180 6.1.4 How do I use adbapi? Rather than creating a database connection directly, use the adbapi.ConnectionPool class to manage a connec- tions for you. This allows enterprise.adbapi to use multiple connections, one per thread. This is easy: # Using the "dbmodule" from the previous example, create a ConnectionPool from twisted.enterprise import adbapi dbpool = adbapi.ConnectionPool("dbmodule", ’mydb’, ’andrew’, ’password’) Things to note about doing this: • There is no need to import dbmodule directly. You just pass the name to adbapi.ConnectionPool’s constructor. • The parameters you would pass to dbmodule.connect are passed as extra arguments to adbapi.Connection Pool’s constructor. Keyword parameters work as well. Now we can do a database query: # equivalent of cursor.execute(statement), return cursor.fetchall(): def getAge(user): return dbpool.runQuery("SELECT age FROM users WHERE name = ?", user) def printResult(l): if l: print l[0][0], "years old" else: print "No such user" getAge("joe").addCallback(printResult) This is straightforward, except perhaps for the return value of getAge. It returns a twisted.internet. defer.Deferred, which allows arbitrary callbacks to be called upon completion (or upon failure). More docu- mentation on Deferred is available here (page 104). In addition to runQuery, there is also runOperation, and runInteraction that gets called with a callable (e.g. a function). The function will be called in the thread with a twisted.enterprise.adbapi. Transaction, which basically mimics a DB-API cursor. In all cases a database transaction will be commited after your database usage is finished, unless an exception is raised in which case it will be rolled back. def _getAge(txn, user): # this will run in a thread, we can use blocking calls txn.execute("SELECT * FROM foo") # ... other cursor commands called on txn ... txn.execute("SELECT age FROM users WHERE name = ?", user) result = txn.fetchall() if result: return result[0][0] else: return None def getAge(user): return dbpool.runInteraction(_getAge, user) def printResult(age): if age != None: print age, "years old" else: print "No such user" getAge("joe").addCallback(printResult) CHAPTER 6. TWISTED RDBMS SUPPORT 181 Also worth noting is that these examples assumes that dbmodule uses the “qmarks” paramstyle (see the DB-API specification). If your dbmodule uses a different paramstyle (e.g. pyformat) then use that. Twisted doesn’t attempt to offer any sort of magic paramater munging – runQuery(query, params, ...) maps directly onto cursor. execute(query, params, ...). 6.1.5 Examples of various database adapters Notice that the first argument is the module name you would usually import and get connect(...) from, and that following arguments are whatever arguments you’d call connect(...) with. from twisted.enterprise import adbapi # Gadfly cp = adbapi.ConnectionPool("gadfly", "test", "/tmp/gadflyDB") # PostgreSQL PyPgSQL cp = adbapi.ConnectionPool("pyPgSQL.PgSQL", database="test") # MySQL cp = adbapi.ConnectionPool("MySQLdb", db="test") 6.1.6 And that’s it! That’s all you need to know to use a database from within Twisted. You probably should read the adbapi module’s documentation to get an idea of the other functions it has, but hopefully this document presents the core ideas. 6.2 Twisted Enterprise Row Objects Note: Due to lack of maintenance, twisted.enterprise.row and twisted.enterprise. reflector have been deprecated since Twisted 8.0. This documentation is maintained only for users with an existing codebase. The twisted.enterprise.row module is a method of interfacing simple python objects with rows in rela- tional database tables. It has two components: the RowObject class which developers sub-class for each relational table that their code interacts with, and the Reflector which is responsible for updates, inserts, queries and deletes against the database. The row module is intended for applications such as on-line games, and websites that require a back-end database interface. It is not a full functioned object-relational mapper for python - it deals best with simple data types structured in ways that can be easily represented in a relational database. It is well suited to building a python interface to an existing relational database, and slightly less suited to added database persistance to an existing python application. If row does not fit your model, you will be best off using the low-level database API (page 179) directly, or writing your own object/relational layer on top of it. 6.2.1 Class Definitions To interface to relational database tables, the developer must create a class derived from the twisted. enterprise.row.RowObject class for each table. These derived classes must define a number of class at- tributes which contains information about the database table that class corresponds to. The required class attributes are: • rowColumns - list of the column names and types in the table with the correct case • rowKeyColumns - list of key columns in form: [(columnName, typeName)] • rowTableName - the name of the database table There are also two optional class attributes that can be specified: CHAPTER 6. TWISTED RDBMS SUPPORT 182 • rowForeignKeys - list of foreign keys to other database tables in the form: [(tableName, [(child ColumnName, childColumnType), ...], [(parentColumnName, parentColumnType), ...], containerMethodName, autoLoad] • rowFactoryMethod - a method that creates instances of this class For example: class RoomRow(row.RowObject): rowColumns = [("roomId", "int"), ("town_id", "int"), ("name", "varchar"), ("owner", "varchar"), ("posx", "int"), ("posy", "int"), ("width", "int"), ("height", "int")] rowKeyColumns = [("roomId", "int4")] rowTableName = "testrooms" rowFactoryMethod = [testRoomFactory] The items in the rowColumns list will become data members of classes of this type when they are created by the Reflector. 6.2.2 Initialization The initialization phase builds the SQL for the database interactions. It uses the system catalogs of the database to do this, but requires some basic information to get started. The class attributes of the classes derived from RowClass are used for this. Those classes are passed to a Reflector when it is created. There are currently two available reflectors in Twisted Enterprise, the SQL Reflector for relational databases which uses the python DB API, and the XML Reflector which uses a file system containing XML files. The XML reflector is currently extremely slow. An example class list for the RoomRow class we specified above using the SQLReflector: from twisted.enterprise.sqlreflector import SQLReflector dbpool = adbapi.ConnectionPool("pyPgSQL.PgSQL") reflector = SQLReflector( dbpool, [RoomRow] ) 6.2.3 Creating Row Objects There are two methods of creating RowObjects - loading from the database, and creating a new instance ready to be inserted. To load rows from the database and create RowObject instances for each of the rows, use the loadObjectsFrom method of the Reflector. This takes a tableName, an optional “user data” parameter, and an optional “where clause”. The where clause may be omitted which will retrieve all the rows from the table. For example: def gotRooms(rooms): for room in rooms: print "Got room:", room.id d = reflector.loadObjectsFrom("testrooms", whereClause=[("id", reflector.EQUAL, 5)]) d.addCallback(gotRooms) For more advanced RowObject construction, loadObjectsFrom may use a factoryMethod that was specified as a class attribute for the RowClass derived class. This method will be called for each of the rows with the class object, the userData parameter, and a dictionary of data from the database keyed by column name. This factory method should return a fully populated RowObject instance and may be used to do pre-processing, lookups, and data transformations before exposing the data to user code. An example factory method: CHAPTER 6. TWISTED RDBMS SUPPORT 183 def testRoomFactory(roomClass, userData, kw): newRoom = roomClass(userData) newRoom.__dict__.update(kw) return newRoom The last method of creating a row object is for new instances that do not already exist in the database table. In this case, create a new instance and assign its primary key attributes and all of its member data attributes, then pass it to the insertRow method of the Reflector. For example: newRoom = RoomRow() newRoom.assignKeyAttr("roomI", 11) newRoom.town_id = 20 newRoom.name = ’newRoom1’ newRoom.owner = ’fred’ newRoom.posx = 100 newRoom.posy = 100 newRoom.width = 15 newRoom.height = 20 reflector.insertRow(newRoom).addCallback(onInsert) This will insert a new row into the database table for this new RowObject instance. Note that the assignKey Attr method must be used to set primary key attributes - regular attribute assignment of a primary key attribute of a rowObject will raise an exception. This prevents the database identity of RowObject from being changed by mistake. 6.2.4 Relationships Between Tables Specifying a foreign key for a RowClass creates a relationship between database tables. When loadObjectsFrom is called for a table, it will automatically load all the children rows for the rows from the specified table. The child rows will be put into a list member variable of the rowObject instance with the name childRows or if a containerMethod is specified for the foreign key relationship, that method will be called on the parent row object for each row that is being added to it as a child. The autoLoad member of the foreign key definition is a flag that specifies whether child rows should be auto-loaded for that relationship when a parent row is loaded. 6.2.5 Duplicate Row Objects If a reflector tries to load an instance of a rowObject that is already loaded, it will return a reference to the existing rowObject rather than creating a new instance. The reflector maintains a cache of weak references to all loaded row objects by their unique keys for this purpose. 6.2.6 Updating Row Objects RowObjects have a dirty member attribute that is set to 1 when any of the member attributes of the instance that map to database columns are changed. This dirty flag can be used to tell when RowObjects need to be updated back to the database. In addition, the setDirty method can be overridden to provide more complex automated handling such as dirty lists (be sure to call the base class setDirty though!). When it is determined that a RowObject instance is dirty and need to have its state updated into the database, pass that object to the updateRow method of the Reflector. For example: reflector.updateRow(room).addCallback(onUpdated) For more complex behavior, the reflector can generate the SQL for the update but not perform the update. This can be useful for batching up multiple updates into single requests. For example: updateSQL = reflector.updateRowSQL(room) 6.2.7 Deleting Row Objects To delete a row from a database pass the RowObject instance for that row to the Reflector deleteRow method. Deleting the python Rowobject instance does not automatically delete the row from the database. For example: reflector.deleteRow(room) Chapter 7 Perspective Broker 7.1 Overview of Twisted Spread Perspective Broker (affectionately known as “PB”) is an asynchronous, symmetric1 network protocol for secure, re- mote method calls and transferring of objects. PB is “translucent, not transparent”, meaning that it is very visible and obvious to see the difference between local method calls and potentially remote method calls, but remote method calls are still extremely convenient to make, and it is easy to emulate them to have objects which work both locally and remotely. PB supports user-defined serialized data in return values, which can be either copied each time the value is returned, or “cached”: only copied once and updated by notifications. PB gets its name from the fact that access to objects is through a “perspective”. This means that when you are responding to a remote method call, you can establish who is making the call. 7.1.1 Rationale No other currently existing protocols have all the properties of PB at the same time. The particularly interesting combination of attributes, though, is that PB is flexible and lightweight, allowing for rapid development, while still powerful enough to do two-way method calls and user-defined data types. It is important to have these attributes in order to allow for a protocol which is extensible. One of the facets of this flexibility is that PB can integrate an arbitrary number of services could be aggregated over a single connection, as well as publish and call new methods on existing objects without restarting the server or client. 7.2 Introduction to Perspective Broker 7.2.1 Introduction Suppose you find yourself in control of both ends of the wire: you have two programs that need to talk to each other, and you get to use any protocol you want. If you can think of your problem in terms of objects that need to make method calls on each other, then chances are good that you can use twisted’s Perspective Broker protocol rather than trying to shoehorn your needs into something like HTTP, or implementing yet another RPC mechanism2. The Perspective Broker system (abbreviated “PB”, spawning numerous sandwich-related puns) is based upon a few central concepts: • serialization: taking fairly arbitrary objects and types, turning them into a chunk of bytes, sending them over a wire, then reconstituting them on the other end. By keeping careful track of object ids, the serialized objects can contain references to other objects and the remote copy will still be useful. • remote method calls: doing something to a local object and causing a method to get run on a distant one. The local object is called a RemoteReference, and you “do something” by running its .callRemote method. 1There is a negotiation phase for banana with particular roles for listener and initiator, so it’s not completely symmetric, but after the connection is fully established, the protocol is completely symmetrical. 2Most of Twisted is like this. Hell, most of unix is like this: if you think it would be useful, someone else has probably thought that way in the past, and acted on it, and you can take advantage of the tool they created to solve the same problem you’re facing now. 184 CHAPTER 7. PERSPECTIVE BROKER 185 This document will contain several examples that will (hopefully) appear redundant and verbose once you’ve figured out what’s going on. To begin with, much of the code will just be labelled “magic”: don’t worry about how these parts work yet. It will be explained more fully later. 7.2.2 Object Roadmap To start with, here are the major classes, interfaces, and functions involved in PB, with links to the file where they are defined (all of which are under twisted/, of course). Don’t worry about understanding what they all do yet: it’s easier to figure them out through their interaction than explaining them one at a time. • Factory : internet/protocol.py • PBServerFactory : spread/pb.py • Broker : spread/pb.py Other classes that are involved at some point: • RemoteReference : spread/pb.py • pb.Root : spread/pb.py, actually defined as twisted.spread.flavors.Root in spread/flavors.py • pb.Referenceable : spread/pb.py, actually defined as twisted.spread.flavors. Referenceable in spread/flavors.py Classes and interfaces that get involved when you start to care about authorization and security: • Portal : cred/portal.py • IRealm : cred/portal.py • IPerspective : spread/pb.py, which you will usually be interacting with via pb.Avatar (a basic imple- mentor of the interface). Subclassing and Implementing Technically you can subclass anything you want, but technically you could also write a whole new framework, which would just waste a lot of time. Knowing which classes are useful to subclass or which interfaces to implement is one of the bits of knowledge that’s crucial to using PB (and all of Twisted) successfully. Here are some hints to get started: • pb.Root, pb.Referenceable: you’ll subclass these to make remotely-referenceable objects (i.e., objects which you can call methods on remotely) using PB. You don’t need to change any of the existing behavior, just inherit all of it and add the remotely-accessible methods that you want to export. • pb.Avatar: You’ll be subclassing this when you get into PB programming with authorization. This is an implementor of IPerspective. • ICredentialsChecker: Implement this if you want to authenticate your users against some sort of data store: i.e., an LDAP database, an RDBMS, etc. There are already a few implementations of this for various back-ends in twisted.cred.checkers. 7.2.3 Things you can Call Remotely At this writing, there are three “flavors” of objects that can be accessed remotely through RemoteReference objects. Each of these flavors has a rule for how the callRemote message is transformed into a local method call on the server. In order to use one of these “flavors”, subclass them and name your published methods with the appropriate prefix. CHAPTER 7. PERSPECTIVE BROKER 186 • twisted.spread.pb.IPerspective implementors This is the first interface we deal with. It is a “perspective” onto your PB application. Perspectives are slightly special because they are usually the first object that a given user can access in your application (after they log on). A user should only receive a reference to their own perspective. PB works hard to verify, as best it can, that any method that can be called on a perspective directly is being called on behalf of the user who is represented by that perspective. (Services with unusual requirements for “on behalf of”, such as simulations with the ability to posess another player’s avatar, are accomplished by providing indirected access to another user’s perspective.) Perspectives are not usually serialized as remote references, so do not return an IPerspective-implementor di- rectly. The way most people will want to implement IPerspective is by subclassing pb.Avatar. Remotely accessible methods on pb.Avatar instances are named with the perspective prefix. • twisted.spread.pb.Referenceable Referenceable objects are the simplest kind of PB object. You can call methods on them and return them from methods to provide access to other objects’ methods. However, when a method is called on a Referenceable, it’s not possible to tell who called it. Remotely accessible methods on Referenceables are named with the remote prefix. • twisted.spread.pb.Viewable Viewable objects are remotely referenceable objects which have the additional requirement that it must be possi- ble to tell who is calling them. The argument list to a Viewable’s remote methods is modified in order to include the Perspective representing the calling user. Remotely accessible methods on Viewables are named with the view prefix. 7.2.4 Things you can Copy Remotely In addition to returning objects that you can call remote methods on, you can return structured copies of local objects. There are 2 basic flavors that allow for copying objects remotely. Again, you can use these by subclassing them. In order to specify what state you want to have copied when these are serialized, you can either use the Python default getstate or specialized method calls for that flavor. • twisted.spread.pb.Copyable This is the simpler kind of object that can be copied. Every time this object is returned from a method or passed as an argument, it is serialized and unserialized. Copyable provides a method you can override, getStateToCopyFor(perspective), which allows you to decide what an object will look like for the perspective who is requesting it. The perspective argument will be the perspective which is either passing an argument or returning a result an instance of your Copyable class. For security reasons, in order to allow a particular Copyable class to actually be copied, you must declare a RemoteCopy handler for that Copyable subclass. The easiest way to do this is to declare both in the same module, like so: from twisted.spread import flavors class Foo(flavors.Copyable): pass class RemoteFoo(flavors.RemoteCopy): pass flavors.setUnjellyableForClass(Foo, RemoteFoo) In this case, each time a Foo is copied between peers, a RemoteFoo will be instantiated and populated with the Foo’s state. If you do not do this, PB will complain that there have been security violations, and it may close the connection. CHAPTER 7. PERSPECTIVE BROKER 187 • twisted.spread.pb.Cacheable Let me preface this with a warning: Cacheable may be hard to understand. The motivation for it may be unclear if you don’t have some experience with real-world applications that use remote method calling of some kind. Once you understand why you need it, what it does will likely seem simple and obvious, but if you get confused by this, forget about it and come back later. It’s possible to use PB without understanding Cacheable at all. Cacheable is a flavor which is designed to be copied only when necessary, and updated on the fly as changes are made to it. When passed as an argument or a return value, if a Cacheable exists on the side of the connection it is being copied to, it will be referred to by ID and not copied. Cacheable is designed to minimize errors involved in replicating an object between multiple servers, espe- cially those related to having stale information. In order to do this, Cacheable automatically registers observers and queries state atomically, together. You can override the method getStateToCacheAndObserve For(self, perspective, observer) in order to specify how your observers will be stored and up- dated. Similar to getStateToCopyFor, getStateToCacheAndObserveFor gets passed a perspective. It also gets passed an observer, which is a remote reference to a “secret” fourth referenceable flavor: Remote Cache. A RemoteCache is simply the object that represents your Cacheable on the other side of the connection. It is registered using the same method as RemoteCopy, above. RemoteCache is different, however, in that it will be referenced by its peer. It acts as a Referenceable, where all methods prefixed with observe will be callable remotely. It is recommended that your object maintain a list (note: library support for this is forthcoming!) of observers, and update them using callRemote when the Cacheable changes in a way that should be noticeable to its clients. Finally, when all references to a Cacheable from a given perspective are lost, stopped Observing(perspective, observer) will be called on the Cacheable, with the same perspec- tive/observer pair that getStateToCacheAndObserveFor was originally called with. Any cleanup re- mote calls can be made there, as well as removing the observer object from any lists which it was previously in. Any further calls to this observer object will be invalid. 7.3 Using Perspective Broker 7.3.1 Basic Example The first example to look at is a complete (although somewhat trivial) application. It uses PBServerFactory() on the server side, and PBClientFactory() on the client side. from twisted.spread import pb from twisted.internet import reactor class Echoer(pb.Root): def remote_echo(self, st): print ’echoing:’, st return st if __name__ == ’__main__’: reactor.listenTCP(8789, pb.PBServerFactory(Echoer())) reactor.run() Source listing — pbsimple.py from twisted.spread import pb from twisted.internet import reactor from twisted.python import util factory = pb.PBClientFactory() CHAPTER 7. PERSPECTIVE BROKER 188 reactor.connectTCP("localhost", 8789, factory) d = factory.getRootObject() d.addCallback(lambda object: object.callRemote("echo", "hello network")) d.addCallback(lambda echo: ’server echoed: ’+echo) d.addErrback(lambda reason: ’error: ’+str(reason.value)) d.addCallback(util.println) d.addCallback(lambda _: reactor.stop()) reactor.run() Source listing — pbsimpleclient.py First we look at the server. This defines an Echoer class (derived from pb.Root), with a method called remote echo(). pb.Root objects (because of their inheritance of pb.Referenceable, described later) can define methods with names of the form remote *; a client which obtains a remote reference to that pb.Root object will be able to invoke those methods. The pb.Root-ish object is given to a pb.PBServerFactory(). This is a Factory object like any other: the Protocol objects it creates for new connections know how to speak the PB protocol. The object you give to pb.PBServerFactory() becomes the “root object”, which simply makes it available for the client to retrieve. The client may only request references to the objects you want to provide it: this helps you implement your security model. Because it is so common to export just a single object (and because a remote * method on that one can return a reference to any other object you might want to give out), the simplest example is one where the PBServer Factory is given the root object, and the client retrieves it. The client side uses pb.PBClientFactory to make a connection to a given port. This is a two-step pro- cess involving opening a TCP connection to a given host and port and requesting the root object using .getRoot Object(). Because .getRootObject() has to wait until a network connection has been made and exchange some data, it may take a while, so it returns a Deferred, to which the gotObject() callback is attached. (See the documentation on Deferring Execution (page 104) for a complete explanation of Deferreds). If and when the connection succeeds and a reference to the remote root object is obtained, this callback is run. The first argument passed to the callback is a remote reference to the distant root object. (you can give other arguments to the callback too, see the other parameters for .addCallback() and .addCallbacks()). The callback does: object.callRemote("echo", "hello network") which causes the server’s .remote echo() method to be invoked. (running .callRemote("boom") would cause .remote boom() to be run, etc). Again because of the delay involved, callRemote() returns a Deferred. Assuming the remote method was run without causing an exception (including an attempt to invoke an unknown method), the callback attached to that Deferred will be invoked with any objects that were returned by the remote method call. In this example, the server’s Echoer object has a method invoked, exactly as if some code on the server side had done: echoer_object.remote_echo("hello network") and from the definition of remote echo() we see that this just returns the same string it was given: “hello network”. From the client’s point of view, the remote call gets another Deferred object instead of that string. call Remote() always returns a Deferred. This is why PB is described as a system for “translucent” remote method calls instead of “transparent” ones: you cannot pretend that the remote object is really local. Trying to do so (as some other RPC mechanisms do, coughCORBAcough) breaks down when faced with the asynchronous nature of the network. Using Deferreds turns out to be a very clean way to deal with the whole thing. The remote reference object (the one given to getRootObject()’s success callback) is an instance the Remote Reference class. This means you can use it to invoke methods on the remote object that it refers to. Only instances of RemoteReference are eligible for .callRemote(). The RemoteReference object is the one that lives on the remote side (the client, in this case), not the local side (where the actual object is defined). In our example, the local object is that Echoer() instance, which inherits from pb.Root, which inherits from pb.Referenceable. It is that Referenceable class that makes the object eligible to be available for remote CHAPTER 7. PERSPECTIVE BROKER 189 method calls3. If you have an object that is Referenceable, then any client that manages to get a reference to it can invoke any remote * methods they please. Note: The only thing they can do is invoke those methods. In particular, they cannot access attributes. From a security point of view, you control what they can do by limiting what the remote * methods can do. Also note: the other classes like Referenceable allow access to other methods, in particular perspective * and view * may be accessed. Don’t write local-only methods with these names, because then remote callers will be able to do more than you intended. Also also note: the other classes like pb.Copyable do allow access to attributes, but you control which ones they can see. You don’t have to be a pb.Root to be remotely callable, but you do have to be pb.Referenceable. (Objects that inherit from pb.Referenceable but not from pb.Root can be remotely called, but only pb.Root-ish objects can be given to the PBServerFactory.) 7.3.2 Complete Example Here is an example client and server which uses pb.Referenceable as a root object and as the result of a remotely exposed method. In each context, methods can be invoked on the exposed Referenceable instance. In this example, the initial root object has a method that returns a reference to the second object. #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb class Two(pb.Referenceable): def remote_three(self, arg): print "Two.three was given", arg class One(pb.Root): def remote_getTwo(self): two = Two() print "returning a Two called", two return two from twisted.internet import reactor reactor.listenTCP(8800, pb.PBServerFactory(One())) reactor.run() Source listing — pb1server.py #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor def main(): factory = pb.PBClientFactory() 3There are a few other classes that can bestow this ability, but pb.Referenceable is the easiest to understand; see ’flavors’ below for details on the others. CHAPTER 7. PERSPECTIVE BROKER 190 reactor.connectTCP("localhost", 8800, factory) def1 = factory.getRootObject() def1.addCallbacks(got_obj1, err_obj1) reactor.run() def err_obj1(reason): print "error getting first object", reason reactor.stop() def got_obj1(obj1): print "got first object:", obj1 print "asking it to getTwo" def2 = obj1.callRemote("getTwo") def2.addCallbacks(got_obj2) def got_obj2(obj2): print "got second object:", obj2 print "telling it to do three(12)" obj2.callRemote("three", 12) main() Source listing — pb1client.py pb.PBClientFactory.getRootObject will handle all the details of waiting for the creation of a con- nection. It returns a Deferred, which will have its callback called when the reactor connects to the remote server and pb.PBClientFactory gets the root, and have its errback called when the object-connection fails for any reason, whether it was host lookup failure, connection refusal, or some server-side error. The root object has a method called remote getTwo, which returns the Two() instance. On the client end, the callback gets a RemoteReference to that instance. The client can then invoke two’s .remote three() method. RemoteReference objects have one method which is their purpose for being: callRemote. This method allows you to call a remote method on the object being referred to by the Reference. RemoteReference.call Remote, like pb.PBClientFactory.getRootObject, returns a Deferred. When a response to the method- call being sent arrives, the Deferred’s callback or errback will be made, depending on whether an error occurred in processing the method call. You can use this technique to provide access to arbitrary sets of objects. Just remember that any object that might get passed “over the wire” must inherit from Referenceable (or one of the other flavors). If you try to pass a non- Referenceable object (say, by returning one from a remote * method), you’ll get an InsecureJelly exception4. 7.3.3 References can come back to you If your server gives a reference to a client, and then that client gives the reference back to the server, the server will wind up with the same object it gave out originally. The serialization layer watches for returning reference identifiers and turns them into actual objects. You need to stay aware of where the object lives: if it is on your side, you do actual method calls. If it is on the other side, you do .callRemote()5. #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb 4This can be overridden, by subclassing one of the Serializable flavors and defining custom serialization code for your class. See Passing Complex Types (page 198) for details. 5The binary nature of this local vs. remote scheme works because you cannot give RemoteReferences to a third party. If you could, then your object A could go to B, B could give it to C, C might give it back to you, and you would be hard pressed to tell if the object lived in C’s memory space, in B’s, or if it was really your own object, tarnished and sullied after being handed down like a really ugly picture that your great aunt owned and which nobody wants but which nobody can bear to throw out. Ok, not really like that, but you get the idea. CHAPTER 7. PERSPECTIVE BROKER 191 from twisted.internet import reactor class Two(pb.Referenceable): def remote_print(self, arg): print "two.print was given", arg class One(pb.Root): def __init__(self, two): #pb.Root.__init__(self) # pb.Root doesn’t implement __init__ self.two = two def remote_getTwo(self): print "One.getTwo(), returning my two called", two return two def remote_checkTwo(self, newtwo): print "One.checkTwo(): comparing my two", self.two print "One.checkTwo(): against your two", newtwo if two == newtwo: print "One.checkTwo(): our twos are the same" two = Two() root_obj = One(two) reactor.listenTCP(8800, pb.PBServerFactory(root_obj)) reactor.run() Source listing — pb2server.py #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor def main(): foo = Foo() factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) factory.getRootObject().addCallback(foo.step1) reactor.run() # keeping globals around is starting to get ugly, so we use a simple class # instead. Instead of hooking one function to the next, we hook one method # to the next. class Foo: def __init__(self): self.oneRef = None def step1(self, obj): print "got one object:", obj self.oneRef = obj print "asking it to getTwo" self.oneRef.callRemote("getTwo").addCallback(self.step2) CHAPTER 7. PERSPECTIVE BROKER 192 def step2(self, two): print "got two object:", two print "giving it back to one" print "one is", self.oneRef self.oneRef.callRemote("checkTwo", two) main() Source listing — pb2client.py The server gives a Two() instance to the client, who then returns the reference back to the server. The server compares the “two” given with the “two” received and shows that they are the same, and that both are real objects instead of remote references. A few other techniques are demonstrated in pb2client.py. One is that the callbacks are are added with .add Callback instead of .addCallbacks. As you can tell from the Deferred (page 104) documentation, .add Callback is a simplified form which only adds a success callback. The other is that to keep track of state from one callback to the next (the remote reference to the main One() object), we create a simple class, store the reference in an instance thereof, and point the callbacks at a sequence of bound methods. This is a convenient way to encapsulate a state machine. Each response kicks off the next method, and any data that needs to be carried from one state to the next can simply be saved as an attribute of the object. Remember that the client can give you back any remote reference you’ve given them. Don’t base your zillion- dollar stock-trading clearinghouse server on the idea that you trust the client to give you back the right reference. The security model inherent in PB means that they can only give you back a reference that you’ve given them for the current connection (not one you’ve given to someone else instead, nor one you gave them last time before the TCP session went down, nor one you haven’t yet given to the client), but just like with URLs and HTTP cookies, the particular reference they give you is entirely under their control. 7.3.4 References to client-side objects Anything that’s Referenceable can get passed across the wire, in either direction. The “client” can give a reference to the “server”, and then the server can use .callRemote() to invoke methods on the client end. This fuzzes the distinction between “client” and “server”: the only real difference is who initiates the original TCP connection; after that it’s all symmetric. #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor class One(pb.Root): def remote_takeTwo(self, two): print "received a Two called", two print "telling it to print(12)" two.callRemote("print", 12) reactor.listenTCP(8800, pb.PBServerFactory(One())) reactor.run() Source listing — pb3server.py #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. CHAPTER 7. PERSPECTIVE BROKER 193 # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor class Two(pb.Referenceable): def remote_print(self, arg): print "Two.print() called with", arg def main(): two = Two() factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) def1 = factory.getRootObject() def1.addCallback(got_obj, two) # hands our ’two’ to the callback reactor.run() def got_obj(obj, two): print "got One:", obj print "giving it our two" obj.callRemote("takeTwo", two) main() Source listing — pb3client.py In this example, the client gives a reference to its own object to the server. The server then invokes a remote method on the client-side object. 7.3.5 Raising Remote Exceptions Everything so far has covered what happens when things go right. What about when they go wrong? The Python Way is to raise an exception of some sort. The Twisted Way is the same. The only special thing you do is to define your Exception subclass by deriving it from pb.Error. When any remotely-invokable method (like remote * or perspective *) raises a pb.Error-derived exception, a serialized form of that Exception object will be sent back over the wire6. The other side (which did callRemote) will have the “errback” callback run with a Failure object that contains a copy of the exception object. This Failure object can be queried to retrieve the error message and a stack traceback. Failure is a special class, defined in twisted/python/failure.py, created to make it easier to handle asynchronous exceptions. Just as exception handlers can be nested, errback functions can be chained. If one errback can’t handle the particular type of failure, it can be “passed along” to a errback handler further down the chain. For simple purposes, think of the Failure as just a container for remotely-thrown Exception objects. To extract the string that was put into the exception, use its .getErrorMessage() method. To get the type of the exception (as a string), look at its .type attribute. The stack traceback is available too. The intent is to let the errback function get just as much information about the exception as Python’s normal try: clauses do, even though the exception occurred in somebody else’s memory space at some unknown time in the past. #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor 6To be precise, the Failure will be sent if any exception is raised, not just pb.Error-derived ones. But the server will print ugly error messages if you raise ones that aren’t derived from pb.Error. CHAPTER 7. PERSPECTIVE BROKER 194 class MyError(pb.Error): """This is an Expected Exception. Something bad happened.""" pass class MyError2(Exception): """This is an Unexpected Exception. Something really bad happened.""" pass class One(pb.Root): def remote_broken(self): msg = "fall down go boom" print "raising a MyError exception with data ’%s’" % msg raise MyError(msg) def remote_broken2(self): msg = "hadda owie" print "raising a MyError2 exception with data ’%s’" % msg raise MyError2(msg) def main(): reactor.listenTCP(8800, pb.PBServerFactory(One())) reactor.run() if __name__ == ’__main__’: main() Source listing — exc server.py #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor def main(): factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) d = factory.getRootObject() d.addCallbacks(got_obj) reactor.run() def got_obj(obj): # change "broken" into "broken2" to demonstrate an unhandled exception d2 = obj.callRemote("broken") d2.addCallback(working) d2.addErrback(broken) def working(): print "erm, it wasn’t *supposed* to work.." def broken(reason): print "got remote Exception" # reason should be a Failure (or subclass) holding the MyError exception print " .__class__ =", reason.__class__ print " .getErrorMessage() =", reason.getErrorMessage() CHAPTER 7. PERSPECTIVE BROKER 195 print " .type =", reason.type reactor.stop() main() Source listing — exc client.py % ./exc_client.py got remote Exception .__class__ = twisted.spread.pb.CopiedFailure .getErrorMessage() = fall down go boom .type = __main__.MyError Main loop terminated. Oh, and what happens if you raise some other kind of exception? Something that isn’t subclassed from pb. Error? Well, those are called “unexpected exceptions”, which make Twisted think that something has really gone wrong. These will raise an exception on the server side. This won’t break the connection (the exception is trapped, just like most exceptions that occur in response to network traffic), but it will print out an unsightly stack trace on the server’s stderr with a message that says “Peer Will Receive PB Traceback”, just as if the exception had happened outside a remotely-invokable method. (This message will go the current log target, if log.startLogging was used to redirect it). The client will get the same Failure object in either case, but subclassing your exception from pb. Error is the way to tell Twisted that you expect this sort of exception, and that it is ok to just let the client handle it instead of also asking the server to complain. Look at exc client.py and change it to invoke broken2() instead of broken() to see the change in the server’s behavior. If you don’t add an errback function to the Deferred, then a remote exception will still send a Failure object back over, but it will get lodged in the Deferred with nowhere to go. When that Deferred finally goes out of scope, the side that did callRemote will emit a message about an “Unhandled error in Deferred”, along with an ugly stack trace. It can’t raise an exception at that point (after all, the callRemote that triggered the problem is long gone), but it will emit a traceback. So be a good programmer and always add errback handlers, even if they are just calls to log.err. 7.3.6 Try/Except blocks and Failure.trap To implement the equivalent of the Python try/except blocks (which can trap particular kinds of exceptions and pass others “up” to higher-level try/except blocks), you can use the .trap() method in conjunction with multiple errback handlers on the Deferred. Re-raising an exception in an errback handler serves to pass that new exception to the next handler in the chain. The trap method is given a list of exceptions to look for, and will re-raise anything that isn’t on the list. Instead of passing unhandled exceptions “up” to an enclosing try block, this has the effect of passing the exception “off” to later errback handlers on the same Deferred. The trap calls are used in chained errbacks to test for each kind of exception in sequence. #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.internet import reactor from twisted.spread import pb class MyException(pb.Error): pass class One(pb.Root): def remote_fooMethod(self, arg): if arg == "panic!": raise MyException return "response" CHAPTER 7. PERSPECTIVE BROKER 196 def remote_shutdown(self): reactor.stop() reactor.listenTCP(8800, pb.PBServerFactory(One())) reactor.run() Source listing — trap server.py #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb, jelly from twisted.python import log from twisted.internet import reactor class MyException(pb.Error): pass class MyOtherException(pb.Error): pass class ScaryObject: # not safe for serialization pass def worksLike(obj): # the callback/errback sequence in class One works just like an # asynchronous version of the following: try: response = obj.callMethod(name, arg) except pb.DeadReferenceError: print " stale reference: the client disconnected or crashed" except jelly.InsecureJelly: print " InsecureJelly: you tried to send something unsafe to them" except (MyException, MyOtherException): print " remote raised a MyException" # or MyOtherException except: print " something else happened" else: print " method successful, response:", response class One: def worked(self, response): print " method successful, response:", response def check_InsecureJelly(self, failure): failure.trap(jelly.InsecureJelly) print " InsecureJelly: you tried to send something unsafe to them" return None def check_MyException(self, failure): which = failure.trap(MyException, MyOtherException) if which == MyException: print " remote raised a MyException" else: print " remote raised a MyOtherException" return None def catch_everythingElse(self, failure): print " something else happened" CHAPTER 7. PERSPECTIVE BROKER 197 log.err(failure) return None def doCall(self, explanation, arg): print explanation try: deferred = self.remote.callRemote("fooMethod", arg) deferred.addCallback(self.worked) deferred.addErrback(self.check_InsecureJelly) deferred.addErrback(self.check_MyException) deferred.addErrback(self.catch_everythingElse) except pb.DeadReferenceError: print " stale reference: the client disconnected or crashed" def callOne(self): self.doCall("callOne: call with safe object", "safe string") def callTwo(self): self.doCall("callTwo: call with dangerous object", ScaryObject()) def callThree(self): self.doCall("callThree: call that raises remote exception", "panic!") def callShutdown(self): print "telling them to shut down" self.remote.callRemote("shutdown") def callFour(self): self.doCall("callFour: call on stale reference", "dummy") def got_obj(self, obj): self.remote = obj reactor.callLater(1, self.callOne) reactor.callLater(2, self.callTwo) reactor.callLater(3, self.callThree) reactor.callLater(4, self.callShutdown) reactor.callLater(5, self.callFour) reactor.callLater(6, reactor.stop) factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) deferred = factory.getRootObject() deferred.addCallback(One().got_obj) reactor.run() Source listing — trap client.py % ./trap_client.py callOne: call with safe object method successful, response: response callTwo: call with dangerous object InsecureJelly: you tried to send something unsafe to them callThree: call that raises remote exception remote raised a MyException telling them to shut down callFour: call on stale reference stale reference: the client disconnected or crashed % In this example, callTwo tries to send an instance of a locally-defined class through callRemote. The default security model implemented by pb.Jelly on the remote end will not allow unknown classes to be unserialized (i.e. CHAPTER 7. PERSPECTIVE BROKER 198 taken off the wire as a stream of bytes and turned back into an object: a living, breathing instance of some class): one reason is that it does not know which local class ought to be used to create an instance that corresponds to the remote object7. The receiving end of the connection gets to decide what to accept and what to reject. It indicates its disapproval by raising a pb.InsecureJelly exception. Because it occurs at the remote end, the exception is returned to the caller asynchronously, so an errback handler for the associated Deferred is run. That errback receives a Failure which wraps the InsecureJelly. Remember that trap re-raises exceptions that it wasn’t asked to look for. You can only check for one set of exceptions per errback handler: all others must be checked in a subsequent handler. check MyException shows how multiple kinds of exceptions can be checked in a single errback: give a list of exception types to trap, and it will return the matching member. In this case, the kinds of exceptions we are checking for (MyException and My OtherException) may be raised by the remote end: they inherit from pb.Error. The handler can return None to terminate processing of the errback chain (to be precise, it switches to the callback that follows the errback; if there is no callback then processing terminates). It is a good idea to put an errback that will catch everything (no trap tests, no possible chance of raising more exceptions, always returns None) at the end of the chain. Just as with regular try: except: handlers, you need to think carefully about ways in which your errback handlers could themselves raise exceptions. The extra importance in an asynchronous environment is that an exception that falls off the end of the Deferred will not be signalled until that Deferred goes out of scope, and at that point may only cause a log message (which could even be thrown away if log.startLogging is not used to point it at stdout or a log file). In contrast, a synchronous exception that is not handled by any other except: block will very visibly terminate the program immediately with a noisy stack trace. callFour shows another kind of exception that can occur while using callRemote: pb.DeadReference Error. This one occurs when the remote end has disconnected or crashed, leaving the local side with a stale reference. This kind of exception happens to be reported right away (XXX: is this guaranteed? probably not), so must be caught in a traditional synchronous try: except pb.DeadReferenceError block. Yet another kind that can occur is a pb.PBConnectionLost exception. This occurs (asynchronously) if the connection was lost while you were waiting for a callRemote call to complete. When the line goes dead, all pending requests are terminated with this exception. Note that you have no way of knowing whether the request made it to the other end or not, nor how far along in processing it they had managed before the connection was lost. XXX: explain transaction semantics, find a decent reference. 7.4 PB Copyable: Passing Complex Types 7.4.1 Overview This chapter focuses on how to use PB to pass complex types (specifically class instances) to and from a remote process. The first section is on simply copying the contents of an object to a remote process (pb.Copyable). The second covers how to copy those contents once, then update them later when they change (Cacheable). 7.4.2 Motivation From the previous chapter (page 187), you’ve seen how to pass basic types to a remote process, by using them in the arguments or return values of a callRemote function. However, if you’ve experimented with it, you may have discovered problems when trying to pass anything more complicated than a primitive int/list/dict/string type, or another pb.Referenceable object. At some point you want to pass entire objects between processes, instead of having to reduce them down to dictionaries on one end and then re-instantiating them on the other. 7.4.3 Passing Objects The most obvious and straightforward way to send an object to a remote process is with something like the following code. It also happens that this code doesn’t work, as will be explained below. 7The naive approach of simply doing import SomeClass to match a remote caller who claims to have an object of type “Some- Class” could have nasty consequences for some modules that do significant operations in their init methods (think telnetlib. Telnet(host=’localhost’, port=’chargen’), or even more powerful classes that you have available in your server program). Al- lowing a remote entity to create arbitrary classes in your namespace is nearly equivalent to allowing them to run arbitrary code. The pb.InsecureJelly exception arises because the class being sent over the wire has not been registered with the serialization layer (known as jelly). The easiest way to make it possible to copy entire class instances over the wire is to have them inherit from pb.Copyable, and then to use setUnjellyableForClass(remoteClass, localClass) on the receiving side. See Passing Complex Types (page 198) for an example. CHAPTER 7. PERSPECTIVE BROKER 199 class LilyPond: def __init__(self, frogs): self.frogs = frogs pond = LilyPond(12) ref.callRemote("sendPond", pond) If you try to run this, you might hope that a suitable remote end which implements the remote sendPond method would see that method get invoked with an instance from the LilyPond class. But instead, you’ll encounter the dreaded InsecureJelly exception. This is Twisted’s way of telling you that you’ve violated a security restric- tion, and that the receiving end refuses to accept your object. Security Options What’s the big deal? What’s wrong with just copying a class into another process’ namespace? Reversing the question might make it easier to see the issue: what is the problem with accepting a stranger’s request to create an arbitrary object in your local namespace? The real question is how much power you are granting them: what actions can they convince you to take on the basis of the bytes they are sending you over that remote connection. Objects generally represent more power than basic types like strings and dictionaries because they also contain (or reference) code, which can modify other data structures when executed. Once previously-trusted data is subverted, the rest of the program is compromised. The built-in Python “batteries included” classes are relatively tame, but you still wouldn’t want to let a foreign program use them to create arbitrary objects in your namespace or on your computer. Imagine a protocol that involved sending a file-like object with a read() method that was supposed to used later to retrieve a document. Then imagine what if that object were created with os.fdopen("˜/.gnupg/secring.gpg"). Or an instance of telnetlib.Telnet("localhost", "chargen"). Classes you’ve written for your own program are likely to have far more power. They may run code during init , or even have special meaning simply because of their existence. A program might have User objects to represent user accounts, and have a rule that says all User objects in the system are referenced when authorizing a login session. (In this system, User. init would probably add the object to a global list of known users). The simple act of creating an object would give access to somebody. If you could be tricked into creating a bad object, an unauthorized user would get access. So object creation needs to be part of a system’s security design. The dotted line between “trusted inside” and “untrusted outside” needs to describe what may be done in response to outside events. One of those events is the receipt of an object through a PB remote procedure call, which is a request to create an object in your “inside” namespace. The question is what to do in response to it. For this reason, you must explicitly specific what remote classes will be accepted, and how their local representatives are to be created. What class to use? Another basic question to answer before we can do anything useful with an incoming serialized object is: what class should we create? The simplistic answer is to create the “same kind” that was serialized on the sender’s end of the wire, but this is not as easy or as straightforward as you might think. Remember that the request is coming from a different program, using a potentially different set of class libraries. In fact, since PB has also been implemented in Java, Emacs-Lisp, and other languages, there’s no guarantee that the sender is even running Python! All we know on the receiving end is a list of two things which describe the instance they are trying to send us: the name of the class, and a representation of the contents of the object. PB lets you specify the mapping from remote class names to local classes with the setUnjellyableFor Class function8. This function takes a remote/sender class reference (either the fully-qualified name as used by the sending end, or a class object from which the name can be extracted), and a local/recipient class (used to create the local representation for incoming serialized objects). Whenever the remote end sends an object, the class name that 8 Note that, in this context, “unjelly” is a verb with the opposite meaning of “jelly”. The verb “to jelly” means to serialize an object or data structure into a sequence of bytes (or other primitive transmittable/storable representation), while “to unjelly” means to unserialize the bytestream into a live object in the receiver’s memory space. “Unjellyable” is a noun, (not an adjective), referring to the the class that serves as a destination or recipient of the unjellying process. “A is unjellyable into B” means that a serialized representation A (of some remote object) can be unserialized into a local object of type B. It is these objects “B” that are the “Unjellyable” second argument of the setUnjellyableForClass function. In particular, “unjellyable” does not mean “cannot be jellied”. Unpersistable means “not persistable”, but “unjelly”, “unserialize”, and “unpickle” mean to reverse the operations of “jellying”, “serializing”, and “pickling”. CHAPTER 7. PERSPECTIVE BROKER 200 they transmit is looked up in the table controlled by this function. If a matching class is found, it is used to create the local object. If not, you get the InsecureJelly exception. In general you expect both ends to share the same codebase: either you control the program that is running on both ends of the wire, or both programs share some kind of common language that is implemented in code which exists on both ends. You wouldn’t expect them to send you an object of the MyFooziWhatZit class unless you also had a definition for that class. So it is reasonable for the Jelly layer to reject all incoming classes except the ones that you have explicitly marked with setUnjellyableForClass. But keep in mind that the sender’s idea of a User object might differ from the recipient’s, either through namespace collisions between unrelated packages, version skew between nodes that haven’t been updated at the same rate, or a malicious intruder trying to cause your code to fail in some interesting or potentially vulnerable way. 7.4.4 pb.Copyable Ok, enough of this theory. How do you send a fully-fledged object from one side to the other? #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb, jelly from twisted.python import log from twisted.internet import reactor class LilyPond: def setStuff(self, color, numFrogs): self.color = color self.numFrogs = numFrogs def countFrogs(self): print "%d frogs" % self.numFrogs class CopyPond(LilyPond, pb.Copyable): pass class Sender: def __init__(self, pond): self.pond = pond def got_obj(self, remote): self.remote = remote d = remote.callRemote("takePond", self.pond) d.addCallback(self.ok).addErrback(self.notOk) def ok(self, response): print "pond arrived", response reactor.stop() def notOk(self, failure): print "error during takePond:" if failure.type == jelly.InsecureJelly: print " InsecureJelly" else: print failure reactor.stop() return None def main(): from copy_sender import CopyPond # so it’s not __main__.CopyPond pond = CopyPond() CHAPTER 7. PERSPECTIVE BROKER 201 pond.setStuff("green", 7) pond.countFrogs() # class name: print ".".join([pond.__class__.__module__, pond.__class__.__name__]) sender = Sender(pond) factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) deferred = factory.getRootObject() deferred.addCallback(sender.got_obj) reactor.run() if __name__ == ’__main__’: main() Source listing — copy sender.py # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. """ PB copy receiver example. This is a Twisted Application Configuration (tac) file. Run with e.g. twistd -ny copy_receiver.tac See the twistd(1) man page or http://twistedmatrix.com/documents/current/howto/application for details. """ import sys if __name__ == ’__main__’: print __doc__ sys.exit(1) from twisted.application import service, internet from twisted.internet import reactor from twisted.spread import pb from copy_sender import LilyPond, CopyPond from twisted.python import log #log.startLogging(sys.stdout) class ReceiverPond(pb.RemoteCopy, LilyPond): pass pb.setUnjellyableForClass(CopyPond, ReceiverPond) class Receiver(pb.Root): def remote_takePond(self, pond): print " got pond:", pond pond.countFrogs() return "safe and sound" # positive acknowledgement def remote_shutdown(self): reactor.stop() application = service.Application("copy_receiver") CHAPTER 7. PERSPECTIVE BROKER 202 internet.TCPServer(8800, pb.PBServerFactory(Receiver())).setServiceParent( service.IServiceCollection(application)) Source listing — copy receiver.tac The sending side has a class called LilyPond. To make this eligble for transport through callRemote (either as an argument, a return value, or something referenced by either of those [like a dictionary value]), it must inherit from one of the four Serializable classes. In this section, we focus on Copyable. The copyable subclass of LilyPond is called CopyPond. We create an instance of it and send it through callRemote as an argument to the receiver’s remote takePond method. The Jelly layer will serialize (“jelly”) that object as an instance with a class name of “copy sender.CopyPond” and some chunk of data that represents the object’s state. pond. class . module and pond. class . name are used to derive the class name string. The object’s getState ToCopy method is used to get the state: this is provided by pb.Copyable, and the default just retrieves self. dict . This works just like the optional getstate method used by pickle. The pair of name and state are sent over the wire to the receiver. The receiving end defines a local class named ReceiverPond to represent incoming LilyPond instances. This class derives from the sender’s LilyPond class (with a fully-qualified name of copy sender.LilyPond), which specifies how we expect it to behave. We trust that this is the same LilyPond class as the sender used. (At the very least, we hope ours will be able to accept a state created by theirs). It also inherits from pb.RemoteCopy, which is a requirement for all classes that act in this local-representative role (those which are given to the second argument of setUnjellyableForClass). RemoteCopy provides the methods that tell the Jelly layer how to create the local object from the incoming serialized state. Then setUnjellyableForClass is used to register the two classes. This has two effects: instances of the remote class (the first argument) will be allowed in through the security layer, and instances of the local class (the second argument) will be used to contain the state that is transmitted when the sender serializes the remote object. When the receiver unserializes (“unjellies”) the object, it will create an instance of the local ReceiverPond class, and hand the transmitted state (usually in the form of a dictionary) to that object’s setCopyableState method. This acts just like the setstate method that pickle uses when unserializing an object. getState ToCopy/setCopyableState are distinct from getstate / setstate to allow objects to be persisted (across time) differently than they are transmitted (across [memory]space). When this is run, it produces the following output: [-] twisted.spread.pb.PBServerFactory starting on 8800 [-] Starting factory [Broker,0,127.0.0.1] got pond: <__builtin__.ReceiverPond instance at 0x406ec5ec> [Broker,0,127.0.0.1] 7 frogs % ./copy_sender.py 7 frogs copy_sender.CopyPond pond arrived safe and sound Main loop terminated. % Controlling the Copied State By overriding getStateToCopy and setCopyableState, you can control how the object is transmitted over the wire. For example, you might want perform some data-reduction: pre-compute some results instead of sending all the raw data over the wire. Or you could replace references to a local object on the sender’s side with markers before sending, then upon receipt replace those markers with references to a receiver-side proxy that could perform the same operations against a local cache of data. Another good use for getStateToCopy is to implement “local-only” attributes: data that is only accessible by the local process, not to any remote users. For example, a .password attribute could be removed from the object state before sending to a remote system. Combined with the fact that Copyable objects return unchanged from a round trip, this could be used to build a challenge-response system (in fact PB does this with pb.Referenceable objects to implement authorization as described here (page 210)). CHAPTER 7. PERSPECTIVE BROKER 203 Whatever getStateToCopy returns from the sending object will be serialized and sent over the wire; set CopyableState gets whatever comes over the wire and is responsible for setting up the state of the object it lives in. #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb class FrogPond: def __init__(self, numFrogs, numToads): self.numFrogs = numFrogs self.numToads = numToads def count(self): return self.numFrogs + self.numToads class SenderPond(FrogPond, pb.Copyable): def getStateToCopy(self): d = self.__dict__.copy() d[’frogsAndToads’] = d[’numFrogs’] + d[’numToads’] del d[’numFrogs’] del d[’numToads’] return d class ReceiverPond(pb.RemoteCopy): def setCopyableState(self, state): self.__dict__ = state def count(self): return self.frogsAndToads pb.setUnjellyableForClass(SenderPond, ReceiverPond) Source listing — copy2 classes.py #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb, jelly from twisted.python import log from twisted.internet import reactor from copy2_classes import SenderPond class Sender: def __init__(self, pond): self.pond = pond def got_obj(self, obj): d = obj.callRemote("takePond", self.pond) d.addCallback(self.ok).addErrback(self.notOk) def ok(self, response): print "pond arrived", response reactor.stop() CHAPTER 7. PERSPECTIVE BROKER 204 def notOk(self, failure): print "error during takePond:" if failure.type == jelly.InsecureJelly: print " InsecureJelly" else: print failure reactor.stop() return None def main(): pond = SenderPond(3, 4) print "count %d" % pond.count() sender = Sender(pond) factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) deferred = factory.getRootObject() deferred.addCallback(sender.got_obj) reactor.run() if __name__ == ’__main__’: main() Source listing — copy2 sender.py #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.application import service, internet from twisted.internet import reactor from twisted.spread import pb import copy2_classes # needed to get ReceiverPond registered with Jelly class Receiver(pb.Root): def remote_takePond(self, pond): print " got pond:", pond print " count %d" % pond.count() return "safe and sound" # positive acknowledgement def remote_shutdown(self): reactor.stop() application = service.Application("copy_receiver") internet.TCPServer(8800, pb.PBServerFactory(Receiver())).setServiceParent( service.IServiceCollection(application)) Source listing — copy2 receiver.py In this example, the classes are defined in a separate source file, which also sets up the binding between them. The SenderPond and ReceiverPond are unrelated save for this binding: they happen to implement the same methods, but use different internal instance variables to accomplish them. The recipient of the object doesn’t even have to import the class definition into their namespace. It is sufficient that they import the class definition (and thus execute the setUnjellyableForClass statement). The Jelly layer remembers the class definition until a matching object is received. The sender of the object needs the definition, of course, to create the object in the first place. CHAPTER 7. PERSPECTIVE BROKER 205 When run, the copy2 example emits the following: % twistd -n -y copy2_receiver.py [-] twisted.spread.pb.PBServerFactory starting on 8800 [-] Starting factory [Broker,0,127.0.0.1] got pond: [Broker,0,127.0.0.1] count 7 % ./copy2_sender.py count 7 pond arrived safe and sound Main loop terminated. % Things To Watch Out For • The first argument to setUnjellyableForClass must refer to the class as known by the sender. The sender has no way of knowing about how your local import statements are set up, and Python’s flexible namespace semantics allow you to access the same class through a variety of different names. You must match whatever the sender does. Having both ends import the class from a separate file, using a canonical module name (no “sibiling imports”), is a good way to get this right, especially when both the sending and the receiving classes are defined together, with the setUnjellyableForClass immediately following them. • The class that is sent must inherit from pb.Copyable. The class that is registered to receive it must inherit from pb.RemoteCopy9. • The same class can be used to send and receive. Just have it inherit from both pb.Copyable and pb.Remote Copy. This will also make it possible to send the same class symmetrically back and forth over the wire. But don’t get confused about when it is coming (and using setCopyableState) versus when it is going (using getStateToCopy). • InsecureJelly exceptions are raised by the receiving end. They will be delivered asynchronously to an errback handler. If you do not add one to the Deferred returned by callRemote, then you will never receive notification of the problem. • The class that is derived from pb.RemoteCopy will be created using a constructor init method that takes no arguments. All setup must be performed in the setCopyableState method. As the docstring on RemoteCopy says, don’t implement a constructor that requires arguments in a subclass of RemoteCopy. More Information • pb.Copyable is mostly implemented in twisted.spread.flavors, and the docstrings there are the best source of additional information. • Copyable is also used in twisted.web.distrib to deliver HTTP requests to other programs for render- ing, allowing subtrees of URL space to be delegated to multiple programs (on multiple machines). • twisted.manhole.explorer also uses Copyable to distribute debugging information from the program under test to the debugging tool. 7.4.5 pb.Cacheable Sometimes the object you want to send to the remote process is big and slow. “big” means it takes a lot of data (storage, network bandwidth, processing) to represent its state. “slow” means that state doesn’t change very frequently. It may be more efficient to send the full state only once, the first time it is needed, then afterwards only send the differences or changes in state whenever it is modified. The pb.Cacheable class provides a framework to implement this. 9pb.RemoteCopy is actually defined as pb.RemoteCopy, but pb.RemoteCopy is the preferred way to access it CHAPTER 7. PERSPECTIVE BROKER 206 pb.Cacheable is derived from pb.Copyable, so it is based upon the idea of an object’s state being captured on the sending side, and then turned into a new object on the receiving side. This is extended to have an object “publishing” on the sending side (derived from pb.Cacheable), matched with one “observing” on the receiving side (derived from pb.RemoteCache). To effectively use pb.Cacheable, you need to isolate changes to your object into accessor functions (specifi- cally “setter” functions). Your object needs to get control every single time some attribute is changed10. You derive your sender-side class from pb.Cacheable, and you add two methods: getStateToCacheAnd ObserveFor and stoppedObserving. The first is called when a remote caching reference is first created, and retrieves the data with which the cache is first filled. It also provides an object called the “observer”11 that points at that receiver-side cache. Every time the state of the object is changed, you give a message to the observer, informing them of the change. The other method, stoppedObserving, is called when the remote cache goes away, so that you can stop sending updates. On the receiver end, you make your cache class inherit from pb.RemoteCache, and implement the set CopyableState as you would for a pb.RemoteCopy object. In addition, you must implement methods to receive the updates sent to the observer by the pb.Cacheable: these methods should have names that start with observe , and match the callRemote invocations from the sender side just as the usual remote * and perspective * methods match normal callRemote calls. The first time a reference to the pb.Cacheable object is sent to any particular recipient, a sender-side Observer will be created for it, and the getStateToCacheAndObserveFor method will be called to get the current state and register the Observer. The state which that returns is sent to the remote end and turned into a local representation using setCopyableState just like pb.RemoteCopy, described above (in fact it inherits from that class). After that, your “setter” functions on the sender side should call callRemote on the Observer, which causes observe * methods to run on the receiver, which are then supposed to update the receiver-local (cached) state. When the receiver stops following the cached object and the last reference goes away, the pb.RemoteCache object can be freed. Just before it dies, it tells the sender side it no longer cares about the original object. When that reference count goes to zero, the Observer goes away and the pb.Cacheable object can stop announcing every change that takes place. The stoppedObserving method is used to tell the pb.Cacheable that the Observer has gone away. With the pb.Cacheable and pb.RemoteCache classes in place, bound together by a call to pb.set UnjellyableForClass, all that remains is to pass a reference to your pb.Cacheable over the wire to the remote end. The corresponding pb.RemoteCache object will automatically be created, and the matching methods will be used to keep the receiver-side slave object in sync with the sender-side master object. Example Here is a complete example, in which the MasterDuckPond is controlled by the sending side, and the SlaveDuck Pond is a cache that tracks changes to the master: #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb class MasterDuckPond(pb.Cacheable): def __init__(self, ducks): self.observers = [] self.ducks = ducks def count(self): print "I have [%d] ducks" % len(self.ducks) def addDuck(self, duck): 10of course you could be clever and add a hook to setattr , along with magical change-announcing subclasses of the usual builtin types, to detect changes that result from normal “=” set operations. The semi-magical “property attributes” that were introduced in Python-2.2 could be useful too. The result might be hard to maintain or extend, though. 11 this is actually a RemoteCacheObserver, but it isn’t very useful to subclass or modify, so simply treat it as a little demon that sits in your pb.Cacheable class and helps you distribute change notifications. The only useful thing to do with it is to run its callRemote method, which acts just like a normal pb.Referenceable’s method of the same name. CHAPTER 7. PERSPECTIVE BROKER 207 self.ducks.append(duck) for o in self.observers: o.callRemote(’addDuck’, duck) def removeDuck(self, duck): self.ducks.remove(duck) for o in self.observers: o.callRemote(’removeDuck’, duck) def getStateToCacheAndObserveFor(self, perspective, observer): self.observers.append(observer) # you should ignore pb.Cacheable-specific state, like self.observers return self.ducks # in this case, just a list of ducks def stoppedObserving(self, perspective, observer): self.observers.remove(observer) class SlaveDuckPond(pb.RemoteCache): # This is a cache of a remote MasterDuckPond def count(self): return len(self.cacheducks) def getDucks(self): return self.cacheducks def setCopyableState(self, state): print " cache - sitting, er, setting ducks" self.cacheducks = state def observe_addDuck(self, newDuck): print " cache - addDuck" self.cacheducks.append(newDuck) def observe_removeDuck(self, deadDuck): print " cache - removeDuck" self.cacheducks.remove(deadDuck) pb.setUnjellyableForClass(MasterDuckPond, SlaveDuckPond) Source listing — cache classes.py #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb, jelly from twisted.python import log from twisted.internet import reactor from cache_classes import MasterDuckPond class Sender: def __init__(self, pond): self.pond = pond def phase1(self, remote): self.remote = remote d = remote.callRemote("takePond", self.pond) d.addCallback(self.phase2).addErrback(log.err) def phase2(self, response): self.pond.addDuck("ugly duckling") self.pond.count() reactor.callLater(1, self.phase3) def phase3(self): d = self.remote.callRemote("checkDucks") CHAPTER 7. PERSPECTIVE BROKER 208 d.addCallback(self.phase4).addErrback(log.err) def phase4(self, dummy): self.pond.removeDuck("one duck") self.pond.count() self.remote.callRemote("checkDucks") d = self.remote.callRemote("ignorePond") d.addCallback(self.phase5) def phase5(self, dummy): d = self.remote.callRemote("shutdown") d.addCallback(self.phase6) def phase6(self, dummy): reactor.stop() def main(): master = MasterDuckPond(["one duck", "two duck"]) master.count() sender = Sender(master) factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) deferred = factory.getRootObject() deferred.addCallback(sender.phase1) reactor.run() if __name__ == ’__main__’: main() Source listing — cache sender.py #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.application import service, internet from twisted.internet import reactor from twisted.spread import pb import cache_classes class Receiver(pb.Root): def remote_takePond(self, pond): self.pond = pond print "got pond:", pond # a DuckPondCache self.remote_checkDucks() def remote_checkDucks(self): print "[%d] ducks: " % self.pond.count(), self.pond.getDucks() def remote_ignorePond(self): # stop watching the pond print "dropping pond" # gc causes __del__ causes ’decache’ msg causes stoppedObserving self.pond = None def remote_shutdown(self): reactor.stop() application = service.Application("copy_receiver") internet.TCPServer(8800, pb.PBServerFactory(Receiver())).setServiceParent( service.IServiceCollection(application)) CHAPTER 7. PERSPECTIVE BROKER 209 Source listing — cache receiver.py When run, this example emits the following: % twistd -n -y cache_receiver.py [-] twisted.spread.pb.PBServerFactory starting on 8800 [-] Starting factory [Broker,0,127.0.0.1] cache - sitting, er, setting ducks [Broker,0,127.0.0.1] got pond: [Broker,0,127.0.0.1] [2] ducks: [’one duck’, ’two duck’] [Broker,0,127.0.0.1] cache - addDuck [Broker,0,127.0.0.1] [3] ducks: [’one duck’, ’two duck’, ’ugly duckling’] [Broker,0,127.0.0.1] cache - removeDuck [Broker,0,127.0.0.1] [2] ducks: [’two duck’, ’ugly duckling’] [Broker,0,127.0.0.1] dropping pond % % ./cache_sender.py I have [2] ducks I have [3] ducks I have [2] ducks Main loop terminated. % Points to notice: • There is one Observer for each remote program that holds an active reference. Multiple references inside the same program don’t matter: the serialization layer notices the duplicates and does the appropriate reference counting12. • Multiple Observers need to be kept in a list, and all of them need to be updated when something changes. By sending the initial state at the same time as you add the observer to the list, in a single atomic action that cannot be interrupted by a state change, you insure that you can send the same status update to all the observers. • The observer.callRemote calls can still fail. If the remote side has disconnected very recently and stoppedObserving has not yet been called, you may get a DeadReferenceError. It is a good idea to add an errback to those callRemotes to throw away such an error. This is a useful idiom: observer.callRemote(’foo’, arg).addErrback(lambda f: None) • getStateToCacheAndObserverFor must return some object that represents the current state of the ob- ject. This may simply be the object’s dict attribute. It is a good idea to remove the pb.Cacheable- specific members of it before sending it to the remote end. The list of Observers, in particular, should be left out, to avoid dizzying recursive Cacheable references. The mind boggles as to the potential consequences of leaving in such an item. •A perspective argument is available to getStateToCacheAndObserveFor, as well as stopped Observing. I think the purpose of this is to allow viewer-specific changes to the way the cache is updated. If all remote viewers are supposed to see the same data, it can be ignored. More Information • The best source for information comes from the docstrings in twisted.spread.flavors, where pb. Cacheable is implemented. • twisted.manhole.explorer uses Cacheable, and does some fairly interesting things with it. • The spread.publish module also uses Cacheable, and might be a source of further information. 12this applies to multiple references through the same Broker. If you’ve managed to make multiple TCP connections to the same program, you deserve whatever you get. CHAPTER 7. PERSPECTIVE BROKER 210 7.5 Authentication with Perspective Broker 7.5.1 Overview The examples shown in Using Perspective Broker (page 187) demonstrate how to do basic remote method calls, but provided no facilities for authentication. In this context, authentication is about who gets which remote references, and how to restrict access to the “right” set of people or programs. As soon as you have a program which offers services to multiple users, where those users should not be allowed to interfere with each other, you need to think about authentication. Many services use the idea of an “account”, and rely upon fact that each user has access to only one account. Twisted uses a system called cred (page 157) to handle authentication issues, and Perspective Broker has code to make it easy to implement the most common use cases. 7.5.2 Compartmentalizing Services Imagine how you would write a chat server using PB. The first step might be a ChatServer object which had a bunch of pb.RemoteReferences that point at user clients. Pretend that those clients offered a remote print method which lets the server print a message on the user’s console. In that case, the server might look something like this: class ChatServer(pb.Referenceable): def __init__(self): self.groups = {} # indexed by name self.users = {} # indexed by name def remote_joinGroup(self, username, groupname): if not self.groups.has_key(groupname): self.groups[groupname] = [] self.groups[groupname].append(self.users[username]) def remote_sendMessage(self, from_username, groupname, message): group = self.groups[groupname] if group: # send the message to all members of the group for user in group: user.callRemote("print", "<%s> says: %s" % (from_username, message)) For now, assume that all clients have somehow acquired a pb.RemoteReference to this ChatServer object, perhaps using pb.Root and getRootObject as described in the previous chapter (page 187). In this scheme, when a user sends a message to the group, their client runs something like the following: remotegroup.callRemote("sendMessage", "alice", "Hi, my name is alice.") Incorrect Arguments You’ve probably seen the first problem: users can trivially spoof each other. We depend upon the user to pass a correct value in their “username” argument, and have no way to tell if they’re lying or not. There is nothing to prevent Alice from modifying her client to do: remotegroup.callRemote("sendMessage", "bob", "i like pork") much to the horror of Bob’s vegetarian friends.13 (In general, learn to get suspicious if you see any argument of a remotely-invokable method described as “must be X”) 13Apparently Alice is one of those weirdos who has nothing better to do than to try and impersonate Bob. She will lie to her chat client, send incorrect objects to remote methods, even rewrite her local client code entirely to accomplish this juvenile prank. Given this adversarial relationship, one must wonder why she and Bob seem to spend so much time together: their adventures are clearly documented by the cryptographic literature. CHAPTER 7. PERSPECTIVE BROKER 211 The best way to fix this is to keep track of the user’s name locally, rather than asking them to send it to the server with each message. The best place to keep state is in an object, so this suggests we need a per-user object. Rather than choosing an obvious name14, let’s call this the User class. class User(pb.Referenceable): def __init__(self, username, server, clientref): self.name = username self.server = server self.remote = clientref def remote_joinGroup(self, groupname): self.server.joinGroup(groupname, self) def remote_sendMessage(self, groupname, message): self.server.sendMessage(self.name, groupname, message) def send(self, message): self.remote.callRemote("print", message) class ChatServer: def __init__(self): self.groups = {} # indexed by name def joinGroup(self, groupname, user): if not self.groups.has_key(groupname): self.groups[groupname] = [] self.groups[groupname].append(user) def sendMessage(self, from_username, groupname, message): group = self.groups[groupname] if group: # send the message to all members of the group for user in group: user.send("<%s> says: %s" % (from_username, message)) Again, assume that each remote client gets access to a single User object, which is created with the proper username. Note how the ChatServer object has no remote access: it isn’t even pb.Referenceable anymore. This means that all access to it must be mediated through other objects, with code that is under your control. As long as Alice only has access to her own User object, she can no longer spoof Bob. The only way for her to invoke ChatServer.sendMessage is to call her User object’s remote sendMessage method, and that method uses its own state to provide the from username argument. It doesn’t give her any way to change that state. This restriction is important. The User object is able to maintain its own integrity because there is a wall between the object and the client: the client cannot inspect or modify internal state, like the .name attribute. The only way through this wall is via remote method invocations, and the only control Alice has over those invocations is when they get invoked and what arguments they are given. Note: No object can maintain its integrity against local threats: by design, Python offers no mechanism for class instances to hide their attributes, and once an intruder has a copy of self. dict , they can do everything the original object was able to do. Unforgeable References Now suppose you wanted to implement group parameters, for example a mode in which nobody was allowed to talk about mattresses because some users were sensitive and calming them down after someone said “mattress” is a hassle that were best avoided altogether. Again, per-group state implies a per-group object. We’ll go out on a limb and call this the Group object: class User(pb.Referenceable): def __init__(self, username, server, clientref): 14the obvious name is clearly ServerSidePerUserObjectWhichNobodyElseHasAccessTo, but because python makes everything else so easy to read, it only seems fair to make your audience work for something CHAPTER 7. PERSPECTIVE BROKER 212 self.name = username self.server = server self.remote = clientref def remote_joinGroup(self, groupname, allowMattress=True): return self.server.joinGroup(groupname, self) def send(self, message): self.remote.callRemote("print", message) class Group(pb.Referenceable): def __init__(self, groupname, allowMattress): self.name = groupname self.allowMattress = allowMattress self.users = [] def remote_send(self, from_user, message): if not self.allowMattress and message.find("mattress") != -1: raise ValueError, "Don’t say that word" for user in self.users: user.send("<%s> says: %s" % (from_user.name, message)) def addUser(self, user): self.users.append(user) class ChatServer: def __init__(self): self.groups = {} # indexed by name def joinGroup(self, groupname, user, allowMattress): if not self.groups.has_key(groupname): self.groups[groupname] = Group(groupname, allowMattress) self.groups[groupname].addUser(user) return self.groups[groupname] This example takes advantage of the fact that pb.Referenceable objects sent over a wire can be returned to you, and they will be turned into references to the same object that you originally sent. The client cannot modify the object in any way: all they can do is point at it and invoke its remote * methods. Thus, you can be sure that the .name attribute remains the same as you left it. In this case, the client code would look something like this: class ClientThing(pb.Referenceable): def remote_print(self, message): print message def join(self): d = self.remoteUser.callRemote("joinGroup", "#twisted", allowMattress=False) d.addCallback(self.gotGroup) def gotGroup(self, group): group.callRemote("send", self.remoteUser, "hi everybody") The User object is sent from the server side, and is turned into a pb.RemoteReference when it arrives at the client. The client sends it back to Group.remote send, and PB turns it back into a reference to the original User when it gets there. Group.remote send can then use its .name attribute as the sender of the message. Note: Third party references (there aren’t any) This technique also relies upon the fact that the pb.Referenceable reference can only come from someone who holds a corresponding pb.RemoteReference. The design of the serialization mechanism (implemented in twisted.spread.jelly: pb, jelly, spread.. get it? Look for “banana”, too. What other networking framework can claim API names based on sandwich ingredients?) makes it impossible for a client to obtain a reference that they weren’t explicitly given. References passed over the wire are given id numbers and recorded in a per-connection dictionary. If you didn’t give them the reference, the id number won’t be in the dict, and no amount of guessing by a malicious client will give CHAPTER 7. PERSPECTIVE BROKER 213 them anything else. The dict goes away when the connection is dropped, further limiting the scope of those references. Futhermore, it is not possible for Bob to send his User reference to Alice (perhaps over some other PB channel just between the two of them). Outside the context of Bob’s connection to the server, that reference is just a meaningless number. To prevent confusion, PB will tell you if you try to give it away: when you try to hand a pb.RemoteReference to a third party, you’ll get an exception (implemented with an assert in pb.py:364 RemoteReference.jellyFor). This helps the security model somewhat: only the client you gave the reference to can cause any damage with it. Of course, the client might be a brainless zombie, simply doing anything some third party wants. When it’s not proxying callRemote invocations, it’s probably terrorizing the living and searching out human brains for sustenance. In short, if you don’t trust them, don’t give them that reference. And remember that everything you’ve ever given them over that connection can come back to you. If expect the client to invoke your method with some object A that you sent to them earlier, and instead they send you object B (that you also sent to them earlier), and you don’t check it somehow, then you’ve just opened up a security hole (we’ll see an example of this shortly). It may be better to keep such objects in a dictionary on the server side, and have the client send you an index string instead. Doing it that way makes it obvious that they can send you anything they want, and improves the chances that you’ll remember to implement the right checks. (This is exactly what PB is doing underneath, with a per-connection dictionary of Referenceable objects, indexed by a number). And, of course, you have to make sure you don’t accidentally hand out a reference to the wrong object. But again, note the vulnerability. If Alice holds a RemoteReference to any object on the server side that has a .name attribute, she can use that name as a spoofed “from” parameter. As a simple example, what if her client code looked like: class ClientThing(pb.Referenceable): def join(self): d = self.remoteUser.callRemote("joinGroup", "#twisted") d.addCallback(self.gotGroup) def gotGroup(self, group): group.callRemote("send", from_user=group, "hi everybody") This would let her send a message that appeared to come from “#twisted” rather than “Alice”. If she joined a group that happened to be named “bob” (perhaps it is the “How To Be Bob” channel, populated by Alice and countless others, a place where they can share stories about their best impersonating-Bob moments), then she would be able to emit a message that looked like “ says: hi there”, and she has accomplished her lifelong goal. Argument Typechecking There are two techniques to close this hole. The first is to have your remotely-invokable methods do type-checking on their arguments: if Group.remote send asserted isinstance(from user, User) then Alice couldn’t use non-User objects to do her spoofing, and hopefully the rest of the system is designed well enough to prevent her from obtaining access to somebody else’s User object. Objects as Capabilities The second technique is to avoid having the client send you the objects altogether. If they don’t send you anything, there is nothing to verify. In this case, you would have to have a per-user-per-group object, in which the remote send method would only take a single message argument. The UserGroup object is created with references to the only User and Group objects that it will ever use, so no lookups are needed: class UserGroup(pb.Referenceable): def __init__(self, user, group): self.user = user self.group = group def remote_send(self, message): self.group.send(self.user.name, message) class Group: CHAPTER 7. PERSPECTIVE BROKER 214 def __init__(self, groupname, allowMattress): self.name = groupname self.allowMattress = allowMattress self.users = [] def send(self, from_user, message): if not self.allowMattress and message.find("mattress") != -1: raise ValueError, "Don’t say that word" for user in self.users: user.send("<%s> says: %s" % (from_user.name, message)) def addUser(self, user): self.users.append(user) The only message-sending method Alice has left is UserGroup.remote send, and it only accepts a message: there are no remaining ways to influence the “from” name. In this model, each remotely-accessible object represents a very small set of capabilities. Security is achieved by only granting a minimal set of abilities to each remote user. PB provides a shortcut which makes this technique easier to use. The Viewable class will be discussed below (page 223). 7.5.3 Avatars and Perspectives In Twisted’s cred (page 157) system, an “Avatar” is an object that lives on the “server” side (defined here as the side farthest from the human who is trying to get something done) which lets the remote user get something done. The avatar isn’t really a particular class, it’s more like a description of a role that some object plays, as in “the Foo object here is acting as the user’s avatar for this particular service”. Generally, the remote user has some way of getting their avatar to run some code. The avatar object may enforce some security checks, and provide additional data, then call other methods which get things done. The two pieces in the cred puzzle (for any protocol, not just PB) are: “what serves as the Avatar?”, and “how does the user get access to it?”. For PB, the first question is easy. The Avatar is a remotely-accessible object which can run code: this is a perfect description of pb.Referenceable and its subclasses. We shall defer the second question until the next section. In the example above, you can think of the ChatServer and Group objects as a service. The User object is the user’s server-side representative: everything the user is capable of doing is done by running one of its methods. Anything that the server wants to do to the user (change their group membership, change their name, delete their pet cat, whatever) is done by manipulating the User object. There are multiple User objects living in peace and harmony around the ChatServer. Each has a different point of view on the services provided by the ChatServer and the Groups: each may belong to different groups, some might have more permissions than others (like the ability to create groups). These different points of view are called “Perspectives”. This is the origin of the term “Perspective” in “Perspective Broker”: PB provides and controls (i.e. “brokers”) access to Perspectives. Once upon a time, these local-representative objects were actually called pb.Perspective. But this has changed with the advent of the rewritten cred system, and now the more generic term for a local representative object is an Avatar. But you will still see reference to “Perspective” in the code, the docs, and the module names15. Just remember that perspectives and avatars are basically the same thing. Despite all we’ve been telling you (page 157) about how Avatars are more of a concept than an actual class, the base class from which you can create your server-side avatar-ish objects is, in fact, named pb.Avatar16. These objects behave very much like pb.Referenceable. The only difference is that instead of offering “remote FOO” methods, they offer “perspective FOO” methods. The other way in which pb.Avatar differs from pb.Referenceable is that the avatar objects are designed to be the first thing retrieved by a cred-using remote client. Just as PBClientFactory.getRootObject gives 15We could just go ahead and rename Perspective Broker to be Avatar Broker, but 1) that would cause massive compatibility problems, and 2) “AB” doesn’t fit into the whole sandwich-themed naming scheme nearly as well as “PB” does. If we changed it to AB, we’d probably have to change Banana to be CD (CoderDecoder), and Jelly to be EF (EncapsulatorFragmentor). twisted.spread would then have to be renamed twisted.alphabetsoup, and then the whole food-pun thing would start all over again. 16The avatar-ish class is named pb.Avatar because pb.Perspective was already taken, by the (now obsolete) oldcred perspective- ish class. It is a pity, but it simply wasn’t possible both replace pb.Perspective in-place and maintain a reasonable level of backwards- compatibility. CHAPTER 7. PERSPECTIVE BROKER 215 the client access to a pb.Root object (which can then provide access to all kinds of other objects), PBClient Factory.login gives client access to a pb.Avatar object (which can return other references). So, the first half of using cred in your PB application is to create an Avatar object which implements perspective methods and is careful to do useful things for the remote user while remaining vigilant against being tricked with unexpected argument values. It must also be careful to never give access to objects that the user should not have access to, whether by returning them directly, returning objects which contain them, or returning objects which can be asked (remotely) to provide them. The second half is how the user gets a pb.RemoteReference to your Avatar. As explained elsewhere (page 157), Avatars are obtained from a Realm. The Realm doesn’t deal with authentication at all (usernames, pass- words, public keys, challenge-response systems, retinal scanners, real-time DNA sequencers, etc). It simply takes an “avatarID” (which is effectively a username) and returns an Avatar object. The Portal and its Checkers deal with authenticating the user: by the time they are done, the remote user has proved their right to access the avatarID that is given to the Realm, so the Realm can return a remotely-controllable object that has whatever powers you wish to grant to this particular user. For PB, the realm is expected to return a pb.Avatar (or anything which implements pb.IPerspective, really, but there’s no reason to not return a pb.Avatar subclass). This object will be given to the client just like a pb.Root would be without cred, and the user can get access to other objects through it (if you let them). The basic idea is that there is a separate IPerspective-implementing object (i.e. the Avatar subclass) (i.e. the “perspective”) for each user, and only the authorized user gets a remote reference to that object. You can store whatever permissions or capabilities the user possesses in that object, and then use them when the user invokes a remote method. You give the user access to the perspective object instead of the objects that do the real work. 7.5.4 Perspective Examples Here is a brief example of using a pb.Avatar. Most of the support code is magic for now: we’ll explain it later. One Client #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from zope.interface import implements from twisted.spread import pb from twisted.cred import checkers, portal from twisted.internet import reactor class MyPerspective(pb.Avatar): def __init__(self, name): self.name = name def perspective_foo(self, arg): print "I am", self.name, "perspective_foo(",arg,") called on", self class MyRealm: implements(portal.IRealm) def requestAvatar(self, avatarId, mind, *interfaces): if pb.IPerspective not in interfaces: raise NotImplementedError return pb.IPerspective, MyPerspective(avatarId), lambda:None p = portal.Portal(MyRealm()) p.registerChecker( checkers.InMemoryUsernamePasswordDatabaseDontUse(user1="pass1")) reactor.listenTCP(8800, pb.PBServerFactory(p)) reactor.run() CHAPTER 7. PERSPECTIVE BROKER 216 Source listing — pb5server.py #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor from twisted.cred import credentials def main(): factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) def1 = factory.login(credentials.UsernamePassword("user1", "pass1")) def1.addCallback(connected) reactor.run() def connected(perspective): print "got perspective ref:", perspective print "asking it to foo(12)" perspective.callRemote("foo", 12) main() Source listing — pb5client.py Ok, so that wasn’t really very exciting. It doesn’t accomplish much more than the first PB example, and used a lot more code to do it. Let’s try it again with two users this time. Note: When the client runs login to request the Perspective, they can provide it with an optional client argument (which must be a pb.Referenceable object). If they do, then a reference to that object will be handed to the realm’s requestAvatar in the mind argument. The server-side Perspective can use it to invoke remote methods on something in the client, so that the client doesn’t always have to drive the interaction. In a chat server, the client object would be the one to which “display text” messages were sent. In a board game server, this would provide a way to tell the clients that someone has made a move, so they can update their game boards. Two Clients #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from zope.interface import implements from twisted.spread import pb from twisted.cred import checkers, portal from twisted.internet import reactor class MyPerspective(pb.Avatar): def __init__(self, name): self.name = name def perspective_foo(self, arg): CHAPTER 7. PERSPECTIVE BROKER 217 print "I am", self.name, "perspective_foo(",arg,") called on", self class MyRealm: implements(portal.IRealm) def requestAvatar(self, avatarId, mind, *interfaces): if pb.IPerspective not in interfaces: raise NotImplementedError return pb.IPerspective, MyPerspective(avatarId), lambda:None p = portal.Portal(MyRealm()) c = checkers.InMemoryUsernamePasswordDatabaseDontUse(user1="pass1", user2="pass2") p.registerChecker(c) reactor.listenTCP(8800, pb.PBServerFactory(p)) reactor.run() Source listing — pb6server.py #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor from twisted.cred import credentials def main(): factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) def1 = factory.login(credentials.UsernamePassword("user1", "pass1")) def1.addCallback(connected) reactor.run() def connected(perspective): print "got perspective1 ref:", perspective print "asking it to foo(13)" perspective.callRemote("foo", 13) main() Source listing — pb6client1.py #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor from twisted.spread import pb from twisted.internet import reactor from twisted.cred import credentials CHAPTER 7. PERSPECTIVE BROKER 218 def main(): factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) def1 = factory.login(credentials.UsernamePassword("user2", "pass2")) def1.addCallback(connected) reactor.run() def connected(perspective): print "got perspective2 ref:", perspective print "asking it to foo(14)" perspective.callRemote("foo", 14) main() Source listing — pb6client2.py While pb6server.py is running, try starting pb6client1, then pb6client2. Compare the argument passed by the .callRemote() in each client. You can see how each client gets connected to a different Perspective. How that example worked Let’s walk through the previous example and see what was going on. First, we created a subclass called MyPerspective which is our server-side Avatar. It implements a perspective foo method that is exposed to the remote client. Second, we created a realm (an object which implements IRealm, and therefore implements requestAvatar). This realm manufactures MyPerspective objects. It makes as many as we want, and names each one with the avatarID (a username) that comes out of the checkers. This MyRealm object returns two other objects as well, which we will describe later. Third, we created a portal to hold this realm. The portal’s job is to dispatch incoming clients to the credential checkers, and then to request Avatars for any which survive the authentication process. Fourth, we made a simple checker (an object which implements IChecker) to hold valid user/password pairs. The checker gets registered with the portal, so it knows who to ask when new clients connect. We use a checker named InMemoryUsernamePasswordDatabaseDontUse, which suggests that 1: all the username/password pairs are kept in memory instead of being saved to a database or something, and 2: you shouldn’t use it. The admonition against using it is because there are better schemes: keeping everything in memory will not work when you have thousands or millions of users to keep track of, the passwords will be stored in the .tap file when the application shuts down (possibly a security risk), and finally it is a nuisance to add or remove users after the checker is constructed. Fifth, we create a pb.PBServerFactory to listen on a TCP port. This factory knows how to connect the remote client to the Portal, so incoming connections will be handed to the authentication process. Other protocols (non-PB) would do something similar: the factory that creates Protocol objects will give those objects access to the Portal so authentication can take place. On the client side, a pb.PBClientFactory is created (as before (page 187)) and attached to a TCP connection. When the connection completes, the factory will be asked to produce a Protocol, and it will create a PB object. Unlike the previous chapter, where we used .getRootObject, here we use factory.login to initiate the cred authentication process. We provide a credentials object, which is the client-side agent for doing our half of the authentication process. This process may involve several messages: challenges, responses, encrypted passwords, secure hashes, etc. We give our credentials object everything it will need to respond correctly (in this case, a username and password, but you could write a credential that used public-key encryption or even fancier techniques). login returns a Deferred which, when it fires, will return a pb.RemoteReference to the remote avatar. We can then do callRemote to invoke a perspective foo method on that Avatar. Anonymous Clients #!/usr/bin/env python # Copyright (c) 2007-2009 Twisted Matrix Laboratories. # See LICENSE for details. CHAPTER 7. PERSPECTIVE BROKER 219 """ Implement the realm for and run on port 8800 a PB service which allows both anonymous and username/password based access. Successful username/password-based login requests given an instance of MyPerspective with a name which matches the username with which they authenticated. Success anonymous login requests are given an instance of MyPerspective with the name "Anonymous". """ from sys import stdout from zope.interface import implements from twisted.python.log import startLogging from twisted.cred.checkers import ANONYMOUS, AllowAnonymousAccess from twisted.cred.checkers import InMemoryUsernamePasswordDatabaseDontUse from twisted.cred.portal import IRealm, Portal from twisted.internet import reactor from twisted.spread.pb import Avatar, IPerspective, PBServerFactory class MyPerspective(Avatar): """ Trivial avatar exposing a single remote method for demonstrative purposes. All successful login attempts in this example will result in an avatar which is an instance of this class. @type name: C{str} @ivar name: The username which was used during login or C{"Anonymous"} if the login was anonymous (a real service might want to avoid the collision this introduces between anonoymous users and authenticated users named "Anonymous"). """ def __init__(self, name): self.name = name def perspective_foo(self, arg): """ Print a simple message which gives the argument this method was called with and this avatar’s name. """ print "I am %s. perspective_foo(%s) called on %s." % ( self.name, arg, self) class MyRealm(object): """ Trivial realm which supports anonymous and named users by creating avatars which are instances of MyPerspective for either. """ implements(IRealm) def requestAvatar(self, avatarId, mind, *interfaces): CHAPTER 7. PERSPECTIVE BROKER 220 if IPerspective not in interfaces: raise NotImplementedError("MyRealm only handles IPerspective") if avatarId is ANONYMOUS: avatarId = "Anonymous" return IPerspective, MyPerspective(avatarId), lambda: None def main(): """ Create a PB server using MyRealm and run it on port 8800. """ startLogging(stdout) p = Portal(MyRealm()) # Here the username/password checker is registered. c1 = InMemoryUsernamePasswordDatabaseDontUse(user1="pass1", user2="pass2") p.registerChecker(c1) # Here the anonymous checker is registered. c2 = AllowAnonymousAccess() p.registerChecker(c2) reactor.listenTCP(8800, PBServerFactory(p)) reactor.run() if __name__ == ’__main__’: main() Source listing — pbAnonServer.py #!/usr/bin/env python # Copyright (c) 2007-2009 Twisted Matrix Laboratories. # See LICENSE for details. """ Client which will talk to the server run by pbAnonServer.py, logging in either anonymously or with username/password credentials. """ from sys import stdout from twisted.python.log import err, startLogging from twisted.cred.credentials import Anonymous, UsernamePassword from twisted.internet import reactor from twisted.internet.defer import gatherResults from twisted.spread.pb import PBClientFactory def error(why, msg): """ Catch-all errback which simply logs the failure. This isn’t expected to be invoked in the normal case for this example. CHAPTER 7. PERSPECTIVE BROKER 221 """ err(why, msg) def connected(perspective): """ Login callback which invokes the remote "foo" method on the perspective which the server returned. """ print "got perspective1 ref:", perspective print "asking it to foo(13)" return perspective.callRemote("foo", 13) def finished(ignored): """ Callback invoked when both logins and method calls have finished to shut down the reactor so the example exits. """ reactor.stop() def main(): """ Connect to a PB server running on port 8800 on localhost and log in to it, both anonymously and using a username/password it will recognize. """ startLogging(stdout) factory = PBClientFactory() reactor.connectTCP("localhost", 8800, factory) anonymousLogin = factory.login(Anonymous()) anonymousLogin.addCallback(connected) anonymousLogin.addErrback(error, "Anonymous login failed") usernameLogin = factory.login(UsernamePassword("user1", "pass1")) usernameLogin.addCallback(connected) usernameLogin.addErrback(error, "Username/password login failed") bothDeferreds = gatherResults([anonymousLogin, usernameLogin]) bothDeferreds.addCallback(finished) reactor.run() if __name__ == ’__main__’: main() Source listing — pbAnonClient.py pbAnonServer.py implements a server based on pb6server.py, extending it to permit anonymous logins in ad- dition to authenticated logins. A AllowAnonymousAccess checker and a InMemoryUsernamePassword DatabaseDontUse checker are registered and the client’s choice of credentials object determines which is used to authenticate the login. In either case, the realm will be called on to create an avatar for the login. AllowAnonymous Access always produces an avatarId of ANONYMOUS. On the client side, the only change is the use of an instance of Anonymous when calling PBClientFactory. login. CHAPTER 7. PERSPECTIVE BROKER 222 7.5.5 Using Avatars Avatar Interfaces The first element of the 3-tuple returned by requestAvatar indicates which Interface this Avatar implements. For PB avatars, it will always be pb.IPerspective, because that’s the only interface these avatars implement. This element is present because requestAvatar is actually presented with a list of possible Interfaces. The question being posed to the Realm is: “do you have an avatar for (avatarID) that can implement one of the following set of Interfaces?”. Some portals and checkers might give a list of Interfaces and the Realm could pick; the PB code only knows how to do one, so we cannot take advantage of this feature. Logging Out The third element of the 3-tuple is a zero-argument callable, which will be invoked by the protocol when the connection has been lost. We can use this to notify the Avatar when the client has lost its connection. This will be described in more detail below. Making Avatars In the example above, we create Avatars upon request, during requestAvatar. Depending upon the service, these Avatars might already exist before the connection is received, and might outlive the connection. The Avatars might also accept multiple connections. Another possibility is that the Avatars might exist ahead of time, but in a different form (frozen in a pickle and/or saved in a database). In this case, requestAvatar may need to perform a database lookup and then do something with the result before it can provide an avatar. In this case, it would probably return a Deferred so it could provide the real Avatar later, once the lookup had completed. Here are some possible implementations of MyRealm.requestAvatar: # pre-existing, static avatars def requestAvatar(self, avatarID, mind, *interfaces): assert pb.IPerspective in interfaces avatar = self.avatars[avatarID] return pb.IPerspective, avatar, lambda:None # database lookup and unpickling def requestAvatar(self, avatarID, mind, *interfaces): assert pb.IPerspective in interfaces d = self.database.fetchAvatar(avatarID) d.addCallback(self.doUnpickle) return pb.IPerspective, d, lambda:None def doUnpickle(self, pickled): avatar = pickle.loads(pickled) return avatar # everybody shares the same Avatar def requestAvatar(self, avatarID, mind, *interfaces): assert pb.IPerspective in interfaces return pb.IPerspective, self.theOneAvatar, lambda:None # anonymous users share one Avatar, named users each get their own def requestAvatar(self, avatarID, mind, *interfaces): assert pb.IPerspective in interfaces if avatarID == checkers.ANONYMOUS: return pb.IPerspective, self.anonAvatar, lambda:None else: return pb.IPerspective, self.avatars[avatarID], lambda:None # anonymous users get independent (but temporary) Avatars # named users get their own persistent one CHAPTER 7. PERSPECTIVE BROKER 223 def requestAvatar(self, avatarID, mind, *interfaces): assert pb.IPerspective in interfaces if avatarID == checkers.ANONYMOUS: return pb.IPerspective, MyAvatar(), lambda:None else: return pb.IPerspective, self.avatars[avatarID], lambda:None The last example, note that the new MyAvatar instance is not saved anywhere: it will vanish when the connection is dropped. By contrast, the avatars that live in the self.avatars dictionary will probably get persisted into the .tap file along with the Realm, the Portal, and anything else that is referenced by the top-level Application object. This is an easy way to manage saved user profiles. Connecting and Disconnecting It may be useful for your Avatars to be told when remote clients gain (and lose) access to them. For example, and Avatar might be updated by something in the server, and if there are clients attached, it should update them (through the “mind” argument which lets the Avatar do callRemote on the client). One common idiom which accomplishes this is to have the Realm tell the avatar that a remote client has just attached. The Realm can also ask the protocol to let it know when the connection goes away, so it can then inform the Avatar that the client has detached. The third member of the requestAvatar return tuple is a callable which will be invoked when the connection is lost. class MyPerspective(pb.Avatar): def __init__(self): self.clients = [] def attached(self, mind): self.clients.append(mind) print "attached to", mind def detached(self, mind): self.clients.remove(mind) print "detached from", mind def update(self, message): for c in self.clients: c.callRemote("update", message) class MyRealm: def requestAvatar(self, avatarID, mind, *interfaces): assert pb.IPerspective in interfaces avatar = self.avatars[avatarID] avatar.attached(mind) return pb.IPerspective, avatar, lambda a=avatar:a.detached(mind) Viewable Once you have IPerspective objects (i.e. the Avatar) to represent users, the Viewable class can come into play. This class behaves a lot like Referenceable: it turns into a RemoteReference when sent over the wire, and certain methods can be invoked by the holder of that reference. However, the methods that can be called have names that start with view instead of remote , and those methods are always called with an extra perspective argument that points to the Avatar through which the reference was sent: class Foo(pb.Viewable): def view_doFoo(self, perspective, arg1, arg2): pass This is useful if you want to let multiple clients share a reference to the same object. The view methods can use the “perspective” argument to figure out which client is calling them. This gives them a way to do additional permission checks, do per-user accounting, etc. This is the shortcut which makes per-user-per-group capability objects much easier to use. Instead of creating such per-(user,group) objects, you just have per-group objects which inherit from pb.Viewable, and give the user CHAPTER 7. PERSPECTIVE BROKER 224 references to them. The local pb.Avatar object will automatically show up as the “perspective” argument in the view * method calls, give you a chance to involve the Avatar in the process. Chat Server with Avatars Combining all the above techniques, here is an example chat server which uses a fixed set of identities (say, for the three members of your bridge club, who hang out in “#NeedAFourth” hoping that someone will discover your server, guess somebody’s password, break in, join the group, and also be available for a game next saturday afternoon). #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from zope.interface import implements from twisted.cred import portal, checkers from twisted.spread import pb from twisted.internet import reactor class ChatServer: def __init__(self): self.groups = {} # indexed by name def joinGroup(self, groupname, user, allowMattress): if not self.groups.has_key(groupname): self.groups[groupname] = Group(groupname, allowMattress) self.groups[groupname].addUser(user) return self.groups[groupname] class ChatRealm: implements(portal.IRealm) def requestAvatar(self, avatarID, mind, *interfaces): assert pb.IPerspective in interfaces avatar = User(avatarID) avatar.server = self.server avatar.attached(mind) return pb.IPerspective, avatar, lambda a=avatar:a.detached(mind) class User(pb.Avatar): def __init__(self, name): self.name = name def attached(self, mind): self.remote = mind def detached(self, mind): self.remote = None def perspective_joinGroup(self, groupname, allowMattress=True): return self.server.joinGroup(groupname, self, allowMattress) def send(self, message): self.remote.callRemote("print", message) class Group(pb.Viewable): def __init__(self, groupname, allowMattress): self.name = groupname self.allowMattress = allowMattress self.users = [] def addUser(self, user): self.users.append(user) CHAPTER 7. PERSPECTIVE BROKER 225 def view_send(self, from_user, message): if not self.allowMattress and message.find("mattress") != -1: raise ValueError, "Don’t say that word" for user in self.users: user.send("<%s> says: %s" % (from_user.name, message)) realm = ChatRealm() realm.server = ChatServer() checker = checkers.InMemoryUsernamePasswordDatabaseDontUse() checker.addUser("alice", "1234") checker.addUser("bob", "secret") checker.addUser("carol", "fido") p = portal.Portal(realm, [checker]) reactor.listenTCP(8800, pb.PBServerFactory(p)) reactor.run() Source listing — chatserver.py Notice that the client uses perspective joinGroup to both join a group and retrieve a RemoteReference to the Group object. However, the reference they get is actually to a special intermediate object called a pb.View Point. When they do group.callRemote("send", "message"), their avatar is inserted into the argument list that Group.view send actually sees. This lets the group get their username out of the Avatar without giving the client an opportunity to spoof someone else. The client side code that joins a group and sends a message would look like this: #!/usr/bin/env python # Copyright (c) 2009 Twisted Matrix Laboratories. # See LICENSE for details. from twisted.spread import pb from twisted.internet import reactor from twisted.cred import credentials class Client(pb.Referenceable): def remote_print(self, message): print message def connect(self): factory = pb.PBClientFactory() reactor.connectTCP("localhost", 8800, factory) def1 = factory.login(credentials.UsernamePassword("alice", "1234"), client=self) def1.addCallback(self.connected) reactor.run() def connected(self, perspective): print "connected, joining group #lookingForFourth" # this perspective is a reference to our User object d = perspective.callRemote("joinGroup", "#lookingForFourth") d.addCallback(self.gotGroup) def gotGroup(self, group): print "joined group, now sending a message to all members" # ’group’ is a reference to the Group object (through a ViewPoint) CHAPTER 7. PERSPECTIVE BROKER 226 d = group.callRemote("send", "You can call me Al.") d.addCallback(self.shutdown) def shutdown(self, result): reactor.stop() Client().connect() Source listing — chatclient.py Chapter 8 Manual Pages 8.1 MANHOLE.1 8.1.1 NAME manhole - Connect to a Twisted Manhole service 8.1.2 SYNOPSIS manhole 8.1.3 DESCRIPTION manhole is a GTK interface to Twisted Manhole services. You can execute python code as if at an interactive Python console inside a running Twisted process with this. 8.1.4 AUTHOR Written by Chris Armstrong, copied from Moshe Zadka’s “faucet” manpage. 8.1.5 REPORTING BUGS To report a bug, visit http://twistedmatrix.com/bugs/ 8.1.6 COPYRIGHT Copyright 2000-2008 Twisted Matrix Laboratories. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 227 CHAPTER 8. MANUAL PAGES 228 8.2 TAP2DEB.1 8.2.1 NAME tap2deb - create Debian packages which wrap .tap files 8.2.2 SYNOPSIS tap2deb [options] 8.2.3 DESCRIPTION Create a ready to upload Debian package in “.build” -u,–unsigned do not sign the Debian package -t,–tapfile Build the application around the given .tap (default twistd.tap) -y,–type The configuration has the given type . Allowable types are tap, source, xml and python. The first three types are mktap(1) output formats, while the last one is a manual building of application (see twistd(1), the -y option). -p,–protocol The name of the protocol this will be used to serve. This is intended as a part of the description. Default is the name of the tapfile, minus any extensions. -d,–debfile The name of the debian package. Default is ’twisted-’+protocol. -V,–set-version The version of the Debian package. The default is 1.0 -e,–description The one-line description. Default is uninteresting. -l,–long description A multi-line description. Default is explanation about this being an auto- matic package created from tap2deb. -m,–maintainer The maintainer, as “Name Lastname ”. This will go in the meta- files, as well as be used as the id to sign the package. –version Output version information and exit. 8.2.4 AUTHOR Written by Moshe Zadka, based on twistd’s help messages 8.2.5 REPORTING BUGS To report a bug, visit http://twistedmatrix.com/bugs/ 8.2.6 COPYRIGHT Copyright 2000-2008 Twisted Matrix Laboratories. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 8.2.7 SEE ALSO mktap(1) CHAPTER 8. MANUAL PAGES 229 8.3 TAP2RPM.1 8.3.1 NAME tap2rpm - create RPM packages which wrap .tap files 8.3.2 SYNOPSIS tap2rpm [options] 8.3.3 DESCRIPTION Create a set of RPM/SRPM packages in the current directory -u,–unsigned do not sign the RPM package -t,–tapfile Build the application around the given .tap (default twistd.tap) -y,–type The configuration has the given type . Allowable types are tap, source, xml and python. The first three types are mktap(1) output formats, while the last one is a manual building of application (see twistd(1), the -y option). -p,–protocol The name of the protocol this will be used to serve. This is intended as a part of the description. Default is the name of the tapfile, minus any extensions. -d,–rpmfile The name of the RPM package. Default is ’twisted-’+protocol. -V,–set-version The version of the RPM package. The default is 1.0 -e,–description The one-line description. Default is uninteresting. -l,–long description A multi-line description. Default is explanation about this being an auto- matic package created from tap2rpm. -m,–maintainer The maintainer, as “Name Lastname ”. This will go in the meta- files, as well as be used as the id to sign the package. –version Output version information and exit. 8.3.4 AUTHOR tap2rpm was written by Sean Reifschneider based on tap2deb by Moshe Zadka. This man page is heavily based on the tap2deb man page by Moshe Zadka. 8.3.5 REPORTING BUGS To report a bug, visit http://twistedmatrix.com/bugs/ 8.3.6 COPYRIGHT Copyright 2000-2008 Twisted Matrix Laboratories. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 8.3.7 SEE ALSO mktap(1) CHAPTER 8. MANUAL PAGES 230 8.4 TAPCONVERT.1 8.4.1 NAME tapconvert - convert Twisted configurations from one format to another 8.4.2 SYNOPSIS tapconvert -i input -o output [-f input-type] [-t output-type] [-d] [-e] tapconvert –help 8.4.3 DESCRIPTION The –help prints out a usage message to standard output. –in,-i The name of the input configuration. –out,-o The name of the output configuration. –typein,-f The type of the input file. Can be either ’guess’, ’python’, ’pickle’, ’xml’, or ’source’. Default is ’guess’. –typeout,-t The type of the output file. Can be either ’pickle’, ’xml’, or ’source’. Default is ’source’. –decrypt,-d Decrypt the specified tap/aos/xml input file. –encrypt,-e Encrypt output file before writing. –version Output version information and exit. 8.4.4 AUTHOR Written by Moshe Zadka, based on tapconvert’s help messages 8.4.5 REPORTING BUGS To report a bug, visit http://twistedmatrix.com/bugs/ 8.4.6 COPYRIGHT Copyright 2000-2008 Twisted Matrix Laboratories. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. CHAPTER 8. MANUAL PAGES 231 8.5 TRIAL.1 8.5.1 NAME trial - run unit tests 8.5.2 SYNOPSIS trial [options] [[file—package—module—TestCase—testmethod]...] trial –help 8.5.3 DESCRIPTION trial loads and executes a suite of unit tests, obtained from modules, packages and files listed on the command line. trial will take either filenames or fully qualified Python names as arguments. Thus ’trial myproject/foo.py’, ’trial myproject.foo’ and ’trial myproject.foo.SomeTestCase.test method’ are all valid ways to invoke trial. -b,–debug Run the tests in the Python debugger. Also does post-mortem debugging on exceptions. Will load ’.pdbrc’ from current directory if it exists. -B,–debug-stacktraces Report Deferred creation and callback stack traces –coverage Generate coverage information in trial temp/coverage/. Requires Python 2.3 or higher. –disablegc Disable the garbage collector. I don’t know why this is in trial. -x,–extra Add an extra argument. (This is a hack necessary for interfacing with emacs’s ‘gud’.) -e,–rterrors Print tracebacks to standard output as soon as they occur –force-gc Run gc.collect() before and after each test case. This can be used to isolate errors that occur when objects get collected. This option would be the default, except it makes tests run about ten times slower. -h,–help Print a usage message to standard output, then exit. –help-reporters Print a list of valid reporters to standard output, then exit. –help-reactors List the names of possibly available reactors. -l,–logfile Direct the log to a different file. The default file is ’test.log’. is relative to trial temp. -n,–dry-run Go through all the tests and make them pass without running. -N,–no-recurse By default, trial recurses through packages to find every module inside every subpackage. Unless, that is, you specify this option. –nopm Don’t automatically jump into debugger for post-mortem analysis of exceptions. Only usable in conjunction with –debug. –profile Run tests under the Python profiler. -r,–reactor Choose which reactor to use. See –help-reactors for a list. –recursionlimit Set Python’s recursion limit. See sys.setrecursionlimit() –reporter Select the reporter to use for Trial’s output. Use the –help-reporters option to see a list of valid reporters. –spew Print an insanely verbose log of everything that happens. Useful when debugging freezes or locks in complex code. –tbformat Format to display tracebacks with. Acceptable values are ’default’, ’brief’ and ’verbose’. ’brief’ produces tracebacks that play nicely with Emacs’ GUD. CHAPTER 8. MANUAL PAGES 232 –temp-directory WARNING: Do not use this options unless you know what you are doing. By default, trial creates a directory called trial temp under the current working directory. When trial runs, it first deletes this directory, then creates it, then changes into the directory to run the tests. The log file and any coverage files are stored here. Use this option if you wish to have trial run in a directory other than trial temp. Be warned, trial will delete the directory before re-creating it. –testmodule <filename> Ask trial to look into <filename> and run any tests specified using the Emacs-style buffer variable ’test-case-name’. –unclean-warnings As of Twisted 8.0, trial will report an error if the reactor is left unclean at the end of the test. This option is provided to assist in migrating from Twisted 2.5 to Twisted 8.0 and later. Enabling this option will turn the errors into warnings. -u,–until-failure Keep looping the tests until one of them raises an error or a failure. This is particularly useful for reproducing intermittent failures. –version Prints the Twisted version number and exit. –without-module Simulate the lack of the specified comma-separated list of modules. This makes it look like the modules are not present in the system, causing tests to check the behavior for that configuration. -z,–random [] Run the tests in random order using the specified seed. 8.5.4 AUTHOR Written by Jonathan M. Lange 8.5.5 REPORTING BUGS To report a bug, visit http://twistedmatrix.com/bugs/ 8.5.6 COPYRIGHT Copyright 2003-2008 Twisted Matrix Laboratories This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. CHAPTER 8. MANUAL PAGES 233 8.6 TWISTD.1 8.6.1 NAME twistd - run Twisted applications (TACs, TAPs) 8.6.2 SYNOPSIS twistd [options] 8.6.3 DESCRIPTION Read a twisted.application.service.Application out of a file and run it. 8.6.4 OPTIONS -n,–nodaemon Don’t daemonize (stay in foreground). -q,–quiet No-op for backwards compatibility. -p,–profile Run the application under the profiler, dumping results to the specified file. –profiler Specify the profiler to use. Defaults to the ’hotshot’ profiler. –savestats Save the Stats object rather than the text output of the profiler. -b,–debug Run the application in the Python Debugger (implies –nodaemon option). Sending a SIGINT or SIGUSR2 signal to the process will drop it into the debugger. -e,–encrypted <file> The specified tap/aos file is encrypted. –euid Set only effective user-id rather than real user-id. This option has no effect unless the server is running as root, in which case it means not to shed all privileges after binding ports, retaining the option to regain privileges in cases such as spawning processes. Use with caution. -o,–no save Do not save shutdown state. –originalname Behave as though the specified Application has no process name set, and run with the standard process name (the Python binary in most cases). -l,–logfile Log to a specified file, - for stdout (default: twistd.log). The log file will be rotated on SI- GUSR1. –pidfile Save pid in specified file (default: twistd.pid). –chroot Chroot to a supplied directory before running (default: don’t chroot). Chrooting is done before changing the current directory. -d,–rundir Change to a supplied directory before running (default: .). -u,–uid The uid to run as (default: don’t change). -g,–gid The gid to run as (default: don’t change). –umask The (octal) file creation mask to apply. (default: 0077 for daemons, no change otherwise). -r,–reactor Choose which reactor to use. See –help-reactors for a list of possibilities. –help-reactors List the names of possibly available reactors. –spew Write an extremely verbose log of everything that happens. Useful for debugging freezes or locks in complex code. -f,–file Read the given .tap file (default: twistd.tap). CHAPTER 8. MANUAL PAGES 234 -s,–source Load an Application from the given .tas (AOT Python source) file. -y,–python Use the variable “application” from the given Python file. This option overrides -f. This option implies –no save. -g,–plugin Read config.tac from a plugin package, as with -y. –syslog Log to syslog instead of a file. -u,–uid The uid to run as. -g,–gid The gid to run as. –version Print version information and exit. –prefix Use the specified prefix when logging to logfile. Default is “twisted”. Note that if twistd is run as root, the working directory is not searched for Python modules. 8.6.5 SIGNALS A running twistd accepts SIGINT for a clean shutdown and SIGUSR1 to rotate log files. 8.6.6 AUTHOR Written by Moshe Zadka, based on twistd’s help messages. 8.6.7 REPORTING BUGS To report a bug, visit http://twistedmatrix.com/trac/wiki/TwistedDevelopment#DevelopmentProcess 8.6.8 COPYRIGHT Copyright 2001-2010 Twisted Matrix Laboratories. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. Chapter 9 Appendix 9.1 Twisted Glossary adaptee An object that has been adapted, also called “original”. See Adapter (this page). Adapter An object whose sole purpose is to implement an Interface for another object. See Interfaces and Adapters (page 151). Application A twisted.application.service.Application. There are HOWTOs on creating and manipulating (page 145) them as a system-administrator, as well as using (page 163) them in your code. Avatar (from Twisted Cred (this page)) business logic for specific user. For example, in PB(page 236) these are perspectives, in pop3 these are mailboxes, and so on. Banana The low-level data marshalling layer of Twisted Spread (page 237). See twisted.spread.banana. Broker A twisted.spread.pb.Broker, the object request broker for Twisted Spread (page 237). cache A way to store data in readily accessible place for later reuse. Caching data is often done because the data is expensive to produce or access. Caching data risks being stale, or out of sync with the original data. component A special kind of (persistent) Adapter that works with a twisted.python.components. Componentized. See also Interfaces and Adapters (page 151). Componentized A Componentized object is a collection of information, separated into domain-specific or role- specific instances, that all stick together and refer to each other. Each object is an Adapter, which, in the context of Componentized, we call “components”. See also Interfaces and Adapters (page 151). conch Twisted’s SSH implementation. Connector Object used to interface between client connections and protocols, usually used with a twisted. internet.protocol.ClientFactory to give you control over how a client connection reconnects. See twisted.internet.interfaces.IConnector and Writing Clients (page 13). Consumer An object that consumes data from a Producer (page 236). See twisted.internet.interfaces. IConsumer. Cred Twisted’s authentication API, twisted.cred. See Introduction to Twisted Cred (page 157) and Twisted Cred usage (page 210). credentials A username/password, public key, or some other information used for authentication. credential checker Where authentication actually happens. See ICredentialChecker. CVSToys A nifty set of tools for CVS, available at http://twistedmatrix.com/users/acapnotic/wares/code/CVSToys/. Daemon A background process that does a job or handles client requests. Daemon is a Unix term; service is the NT equivalent. 235 CHAPTER 9. APPENDIX 236 Deferred A instance of twisted.internet.defer.Deferred, an abstraction for handling chains of call- backs and error handlers (“errbacks”). See the Deferring Execution (page 104) HOWTO. Enterprise Twisted’s RDBMS support. It contains twisted.enterprise.adbapi for asynchronous access to any standard DB-API 2.0 module, and twisted.enterprise.row, a “Relational Object Wrapper (page 237)”. See Introduction to Twisted Enterprise (page 179) and Twisted Enterprise Row Objects (page 181) for more details. errback A callback attached to a Deferred (this page) with .addErrback to handle errors. Factory In general, an object that constructs other objects. In Twisted, a Factory usually refers to a twisted. internet.protocol.Factory, which constructs Protocol (this page) instances for incoming or outgoing connections. See Writing Servers (page 9) and Writing Clients (page 13). Failure Basically, an asynchronous exception that contains traceback information; these are used for passing errors through asynchronous callbacks. im Abbreviation of “(Twisted) Instance Messenger (this page)”. Instance Messenger Instance Messenger is a multi-protocol chat program that comes with Twisted. It can communi- cate via TOC with the AOL servers, via IRC, as well as via PB(this page) with Twisted Words (page 237). See twisted.im. Interface A class that defines and documents methods that a class conforming to that interface needs to have. A collection of core twisted.internet interfaces can be found in twisted.internet.interfaces. See also Interfaces and Adapters (page 151). Jelly The serialization layer for Twisted Spread (page 237), although it can be used seperately from Twisted Spread as well. It is similar in purpose to Python’s standard pickle module, but is more network-friendly, and depends on a separate marshaller (Banana (page 235), in most cases). See twisted.spread.jelly. Lore Lore1 is Twisted’s documentation system. The source format is a subset of XHTML, and output formats include HTML and LaTeX. Manhole A debugging/administration interface to a Twisted application. Microdom A partial DOM implementation using SUX(page 237). It is simple and pythonic, rather than strictly standards-compliant. See twisted.web.microdom. Names Twisted’s DNS server, found in twisted.names. Nevow The successor to Woven (page 237); available from Divmod2. PB Abbreviation of “Perspective Broker (this page)”. Perspective Broker The high-level object layer of Twisted Spread (page 237), implementing semantics for method calling and object copying, caching, and referencing. See twisted.spread.pb. Portal Glues credential checkers (page 235) and realm (page 237)s together. Producer An object that generates data a chunk at a time, usually to be processed by a Consumer (page 235). See twisted.internet.interfaces.IProducer. Protocol In general each network connection has its own Protocol instance to manage connection-specific state. There is a collection of standard protocol implementations in twisted.protocols. See also Writing Servers (page 9) and Writing Clients (page 13). PSU There is no PSU. Reactor The core event-loop of a Twisted application. See Reactor Basics (page 93). Reality See “Twisted Reality (page 237)” 1http://twistedmatrix.com/trac/wiki/TwistedLore/ 2http://divmod.org/trac/ CHAPTER 9. APPENDIX 237 realm (in Twisted Cred (page 235)) stores avatars (page 235) and perhaps general business logic. See IRealm. Resource A twisted.web.resource.Resource, which are served by Twisted Web. Resources can be as simple as a static file on disk, or they can have dynamically generated content. ROWRelational Object Wrapper, an object-oriented interface to a relational database. See Twisted Enterprise Row Objects (page 181). Service A twisted.application.service.Service. See Application howto (page 163) for a description of how they relate to Applications (page 235). Spread Twisted Spread3 is Twisted’s remote-object suite. It consists of three layers: Perspective Broker (page 236), Jelly (page 236) and Banana. (page 235) See Writing Applications with Perspective Broker (page 184). SUXSmall Uncomplicated XML, Twisted’s simple XML parser written in pure Python. See twisted. protocols.sux. TACATwisted Application Configuration is a Python source file, generally with the .tac extension, which defines configuration to make an application runnable using twistd. TAPTwisted Application Pickle (deprecated), or simply just a Twisted APplication. A serialised application that was created with mktap and runnable by twistd. See Using the Utilities (page 145). Trial twisted.trial, Twisted’s unit-testing framework, modelled after pyunit4. See also Writing tests for Twisted code (page 176). Twisted Matrix Laboratories The team behind Twisted. http://twistedmatrix.com/. Twisted Reality In days of old, the Twisted Reality multiplayer text-based interactive-fiction system was the main focus of Twisted Matrix Labs; Twisted, the general networking framework, grew out of Reality’s need for better network functionality. Twisted Reality has been superseded by the Imaginary5 project. usage The twisted.python.usage module, a replacement for the standard getopt module for parsing command-lines which is much easier to work with. See Parsing command-lines (page 168). Words Twisted Words is a multi-protocol chat server that uses the Perspective Broker (page 236) protocol as its native communication style. See twisted.words. Woven Web Object Visualization Environment. A templating system previously, but no longer, included with Twisted. Woven has largely been superceded by Divmod Nevow6. 9.2 Banana Protocol Specifications 9.2.1 Introduction Banana is an efficient, extendable protocol for sending and receiving s-expressions. A s-expression in this context is a list composed of byte strings, integers, large integers, floats and/or s-expressions. 9.2.2 Banana Encodings The banana protocol is a stream of data composed of elements. Each element has the following general structure - first, the length of element encoded in base-128, least signficant bit first. For example length 4674 will be sent as 0x42 0x24. For certain element types the length will be omitted (e.g. float) or have a different meaning (it is the actual value of integer elements). Following the length is a delimiter byte, which tells us what kind of element this is. Depending on the element type, there will then follow the number of bytes specified in the length. The byte’s high-bit will always be set, so that we can differentiate between it and the length (since the length bytes use 128-base, their high bit will never be set). 3http://twistedmatrix.com/products/spread 4http://pyunit.sourceforge.net/ 5http://www.divmod.org/trac/wiki/DivmodImaginary 6http://divmod.org/trac/wiki/DivmodNevow CHAPTER 9. APPENDIX 238 9.2.3 Element Types Given a series of bytes that gave us length N, these are the different delimiter bytes: List – 0x80 The following bytes are a list of N elements. Lists may be nested, and a child list counts as only one element to its parent (regardless of how many elements the child list contains). Integer – 0x81 The value of this element is the positive integer N. Following bytes are not part of this element. Integers can have values of 0 <= N <= 2147483647. String – 0x82 The following N bytes are a string element. Negative Integer – 0x83 The value of this element is the integer N * -1, i.e. -N. Following bytes are not part of this element. Negative integers can have values of 0 >= -N >= -2147483648. Float - 0x84 The next 8 bytes are the float encoded in IEEE 754 floating-point “double format” bit layout. No length bytes should have been defined. Large Integer – 0x85 The value of this element is the positive large integer N. Following bytes are not part of this element. Large integers have no size limitation. Large Negative Integer – 0x86 The value of this element is the negative large integer -N. Following bytes are not part of this element. Large integers have no size limitation. Large integers are intended for arbitary length integers. Regular integers types (positive and negative) are limited to 32-bit values. Examples Here are some examples of elements and their encodings - the type bytes are marked in bold: 1 0x01 0x81 -1 0x01 0x83 1.5 0x84 0x3f 0xf8 0x00 0x00 0x00 0x00 0x00 0x00 "hello" 0x05 0x82 0x68 0x65 0x6c 0x6c 0x6f [] 0x00 0x80 [1, 23] 0x02 0x80 0x01 0x81 0x17 0x81 123456789123456789 0x15 0x3e 0x41 0x66 0x3a 0x69 0x26 0x5b 0x01 0x85 [1, ["hello"]] 0x02 0x80 0x01 0x81 0x01 0x80 0x05 0x82 0x68 0x65 0x6c 0x6c 0x6f 9.2.4 Profiles The Banana protocol is extendable. Therefore, it supports the concept of profiles. Profiles allow developers to extend the banana protocol, adding new element types, while still keeping backwards compatability with implementations that don’t support the extensions. The profile used in each session is determined at the handshake stage (see below.) A profile is specified by a unique string. This specification defines two profiles - "none" and "pb". The "none" profile is the standard profile that should be supported by all Banana implementations. Additional profiles may be added in the future. The ”none” Profile The "none" profile is identical to the delimiter types listed above. It is highly recommended that all Banana clients and servers support the "none" profile. CHAPTER 9. APPENDIX 239 The ”pb” Profile The "pb" profile is intended for use with the Perspective Broker protocol, that runs on top of Banana. Basically, it converts commonly used PB strings into shorter versions, thus minimizing bandwidth usage. It starts with a single byte, which tells us to which string element to convert it, and ends with the delimiter byte, 0x87, which should not be prefixed by a length. 0x01 ’None’ 0x02 ’class’ 0x03 ’dereference’ 0x04 ’reference’ 0x05 ’dictionary’ 0x06 ’function’ 0x07 ’instance’ 0x08 ’list’ 0x09 ’module’ 0x0a ’persistent’ 0x0b ’tuple’ 0x0c ’unpersistable’ 0x0d ’copy’ 0x0e ’cache’ 0x0f ’cached’ 0x10 ’remote’ 0x11 ’local’ 0x12 ’lcache’ 0x13 ’version’ 0x14 ’login’ 0x15 ’password’ 0x16 ’challenge’ 0x17 ’logged in’ 0x18 ’not logged in’ 0x19 ’cachemessage’ 0x1a ’message’ 0x1b ’answer’ 0x1c ’error’ 0x1d ’decref’ 0x1e ’decache’ 0x1f ’uncache’ CHAPTER 9. APPENDIX 240 9.2.5 Protocol Handshake and Behaviour The initiating side of the connection will be referred to as “client”, and the other side as “server”. Upon connection, the server will send the client a list of string elements, signifying the profiles it supports. It is recommended that "none" be included in this list. The client then sends the server a string from this list, telling the server which profile it wants to use. At this point the whole session will use this profile. Once a profile has been established, the two sides may start exchanging elements. There is no limitation on order or dependencies of messages. Any such limitation (e.g. “server can only send an element to client in response to a request from client”) is application specific. Upon receiving illegal messages, failed handshakes, etc., a Banana client or server should close its connection.
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