# Writing Servers

## 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. This document is only relevant to TCP, SSL and Unix socket servers, there is a separate document for UDP.

Your protocol handling class will usually subclass `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 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 configuration.

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.

## 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

    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 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.

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 again.

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.

## 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()

    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()

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.