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Writing Clients

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 <udp> .

At the base, the place where you actually implement the protocol parsing and handling, is the Protocol class. This class will usually be descended from twisted.internet.protocol.Protocol <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.Factory <twisted.internet.protocol.Factory> (or twisted.internet.protocol.ClientFactory <twisted.internet.protocol.ClientFactory> : see below). The default factory class just instantiates the Protocol and then sets the protocol's factory attribute to point to itself (the factory). This lets the Protocol access, and possibly modify, the persistent configuration.

Protocol

As mentioned above, this and auxiliary classes and functions are where most of the code is. A Twisted protocol handles data in an asynchronous manner. This means 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 just writes whatever it reads from the connection to standard output. 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 twisted.internet.protocol.BaseProtocol.connectionMade <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 twisted.internet.protocol.Protocol.connectionLost <connectionLost> .

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.endpoints <twisted.internet.endpoints> provides the appropriate API, and in particular twisted.internet.endpoints.connectProtocol <connectProtocol> which takes a protocol instance rather than a factory.

from twisted.internet import reactor
from twisted.internet.protocol import Protocol
from twisted.internet.endpoints import TCP4ClientEndpoint, connectProtocol

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)

point = TCP4ClientEndpoint(reactor, "localhost", 1234)
d = connectProtocol(point, Greeter())
d.addCallback(gotProtocol)
reactor.run()

Regardless of the type of client endpoint, the way to set up a new connection is simply pass it to twisted.internet.endpoints.connectProtocol <connectProtocol> along with a protocol instance. This means it's easy to change the mechanism you're using to connect, without changing the rest of your program. For example, to run the greeter example over SSL, the only change required is to instantiate an twisted.internet.endpoints.SSL4ClientEndpoint <SSL4ClientEndpoint> instead of a TCP4ClientEndpoint . To take advantage of this, functions and methods which initiates a new connection should generally accept an endpoint as an argument and let the caller construct it, rather than taking arguments like 'host' and 'port' and constructing its own.

For more information on different ways you can make outgoing connections to different types of endpoints, as well as parsing strings into endpoints, see the documentation for the endpoints API <endpoints> .

You may come across code using twisted.internet.protocol.ClientCreator <ClientCreator> , an older API which is not as flexible as the endpoint API. Rather than calling connect on an endpoint, such code will look like this:

from twisted.internet.protocol import ClientCreator

...

creator = ClientCreator(reactor, Greeter)
d = creator.connectTCP("localhost", 1234)
d.addCallback(gotProtocol)
reactor.run()

In general, the endpoint API should be preferred in new code, as it lets the caller select the method of connecting.

ClientFactory

Still, there's plenty of code out there that uses lower-level APIs, and a few features (such as automatic reconnection) have not been re-implemented with endpoints yet, so in some cases they may be more convenient to use.

To use the lower-level connection APIs, you will need to call one of the reactor.connect* methods directly. For these cases, you need a twisted.internet.protocol.ClientFactory <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 in 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 twisted.internet.protocol.ClientFactory.clientConnectionFailed <clientConnectionFailed> is called when a connection could not be established, and that twisted.internet.protocol.ClientFactory.clientConnectionLost <clientConnectionLost> is called when a connection was made and then disconnected.

Reactor Client APIs

connectTCP

twisted.internet.interfaces.IReactorTCP.connectTCP <IReactorTCP.connectTCP> provides support for IPv4 and IPv6 TCP clients. The host argument it accepts can be either a hostname or an IP address literal. In the case of a hostname, the reactor will automatically resolve the name to an IP address before attempting the connection. This means that for a hostname with multiple address records, reconnection attempts may not always go to the same server (see below). It also means that there is name resolution overhead for each connection attempt. If you are creating many short-lived connections (typically around hundreds or thousands per second) then you may want to resolve the hostname to an address first and then pass the address to connectTCP instead.

Reconnection

Often, the connection of a client will be lost unintentionally due to network problems. 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()

The connector passed as the first argument is the interface between a connection and a protocol. When the connection 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 twisted.internet.protocol.ReconnectingClientFactory <ReconnectingClientFactory> instead, 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)

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/words/examples directory.

ircLogBot.py <../../words/examples/ircLogBot.py>

../../words/examples/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.

from twisted.words.protocols import irc
from twisted.internet import protocol

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):

    def __init__(self, channel, filename):
        self.channel = channel
        self.filename = filename

    def buildProtocol(self, addr):
        p = LogBot()
        p.factory = self
        return p

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.

Factories have a default implementation of buildProtocol. It does the same thing the example above does using the protocol attribute of the factory to create the protocol instance. In the example above, the factory could be rewritten to look like this:

class LogBotFactory(protocol.ClientFactory):
    protocol = LogBot

    def __init__(self, channel, filename):
        self.channel = channel
        self.filename = filename

Further Reading

The twisted.internet.protocol.Protocol <Protocol> class used throughout this document is a base implementation of twisted.internet.interfaces.IProtocol <IProtocol> used in most Twisted applications for convenience. To learn about the complete IProtocol interface, see the API documentation for twisted.internet.interfaces.IProtocol <IProtocol> .

The transport attribute used in some examples in this document provides the twisted.internet.interfaces.ITCPTransport <ITCPTransport> interface. To learn about the complete interface, see the API documentation for twisted.internet.interfaces.ITCPTransport <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 <components> document for more information on using interfaces in Twisted.