# 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.

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.

## 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.

## 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 `clientConnectionLost` 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()

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 `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 in the doc/examples directory.

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.