Writing Servers


This document explains how you can use Twisted to implement network protocol parsing and handling for TCP servers (the same code can be reused for SSL and Unix socket servers). There is a separate document covering 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 is instantiated per-connection, on demand, 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. The buildProtocol method of the Factory is used to create a Protocol for each new connection.

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 the endpoints documentation for more information, or IReactorTCP.listenTCP and the other IReactor*.listen* APIs for the lower level APIs that endpoints are based on.

This document will explain each step of the way.


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. The protocol responds to events as they arrive from the network and the events arrive as calls to methods on the protocol.

Here is a simple example:

from twisted.internet.protocol import Protocol

class Echo(Protocol):

    def dataReceived(self, 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")

This protocol responds to the initial connection with a well known quote, and then terminates the connection.

The connectionMade event is usually where setup 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 __init__(self, factory):
        self.factory = factory

    def connectionMade(self):
        self.factory.numProtocols = self.factory.numProtocols + 1
            "Welcome! There are currently %d open connections.\n" %

    def connectionLost(self, reason):
        self.factory.numProtocols = self.factory.numProtocols - 1

    def dataReceived(self, data):

Here connectionMade and connectionLost cooperate to keep a count of the active protocols in a shared object, the factory. The factory must be passed to Echo.__init__ when creating a new instance. The factory is used to share state that exists beyond the lifetime of any given connection. You will see why this object is called a “factory” in the next section.

loseConnection() and abortConnection()

In the code above, loseConnection is called immediately after writing to the transport. The loseConnection call will close the connection only when all the data has been written by Twisted out to the operating system, so it is safe to use in this case without worrying about transport writes being lost. If a producer is being used with the transport, loseConnection will only close the connection once the producer is unregistered.

In some cases, waiting until all the data is written out is not what we want. Due to network failures, or bugs or maliciousness in the other side of the connection, data written to the transport may not be deliverable, and so even though loseConnection was called the connection will not be lost. In these cases, abortConnection can be used: it closes the connection immediately, regardless of buffered data that is still unwritten in the transport, or producers that are still registered. Note that abortConnection is only available in Twisted 11.1 and newer.

Using the Protocol

In this section, you will learn how to run a server which uses your Protocol.

Here is code that will run the QOTD server discussed earlier:

from twisted.internet.protocol import Factory
from twisted.internet.endpoints import TCP4ServerEndpoint
from twisted.internet import reactor

class QOTDFactory(Factory):
    def buildProtocol(self, addr):
        return QOTD()

# 8007 is the port you want to run under. Choose something >1024
endpoint = TCP4ServerEndpoint(reactor, 8007)

In this example, I create a protocol Factory. I want to tell this factory that its job is to build QOTD protocol instances, so I set its buildProtocol method to return instances of the QOTD class. Then, I want to listen on a TCP port, so I make a TCP4ServerEndpoint to identify the port that I want to bind to, and then pass the factory I just created to its listen method.

endpoint.listen() tells the reactor to handle connections to the endpoint’s address using a particular protocol, but the reactor needs to be running in order for it to do anything. reactor.run() starts the reactor and then waits forever for connections to arrive on the port you’ve specified. You can stop the reactor by hitting Control-C in a terminal or calling reactor.stop().

For more information on different ways you can listen for incoming connections, see the documentation for the endpoints API. For more information on using the reactor, see the reactor overview.

Helper Protocols

Many protocols build upon similar lower-level abstractions.

For example, many popular internet protocols are line-based, containing text data terminated by line breaks (commonly CR-LF), rather than containing straight raw data. 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. It also provides a method, sendLine, that writes data to the transport along with the delimiter the class uses to split lines (by default, \r\n).

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 line in self.answers:

Note that the delimiter is not part of the line.

Several other helpers exist, such as a netstring based protocol and prefixed-message-length protocols.

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.

  • 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.


Simpler Protocol Creation

For a factory which simply instantiates instances of a specific protocol class, there is a simpler way to implement the factory. The default implementation of the buildProtocol method calls the protocol attribute of the factory to create a Protocol instance, and then sets an attribute on it called factory which points to the factory itself. This lets every Protocol access, and possibly modify, the persistent configuration. Here is an example that uses these features instead of overriding buildProtocol:

from twisted.internet.protocol import Factory, Protocol
from twisted.internet.endpoints import TCP4ServerEndpoint
from twisted.internet import reactor

class QOTD(Protocol):

    def connectionMade(self):
        # self.factory was set by the factory's default buildProtocol:
        self.transport.write(self.factory.quote + '\r\n')

class QOTDFactory(Factory):

    # This will be used by the default buildProtocol to create new protocols:
    protocol = QOTD

    def __init__(self, quote=None):
        self.quote = quote or 'An apple a day keeps the doctor away'

endpoint = TCP4ServerEndpoint(reactor, 8007)
endpoint.listen(QOTDFactory("configurable quote"))

If all you need is a simple factory that builds a protocol without any additional behavior, Twisted 13.1 added Factory.forProtocol, an even simpler approach.

Factory Startup and Shutdown

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):

Putting it All Together

As a final example, here’s a simple chat server that allows users to choose a username and then communicate with other users. It demonstrates the use of shared state in the factory, a state machine for each individual protocol, and communication between different protocols.


from twisted.internet import reactor
from twisted.internet.protocol import Factory
from twisted.protocols.basic import LineReceiver

class Chat(LineReceiver):
    def __init__(self, users):
        self.users = users
        self.name = None
        self.state = "GETNAME"

    def connectionMade(self):
        self.sendLine(b"What's your name?")

    def connectionLost(self, reason):
        if self.name in self.users:
            del self.users[self.name]

    def lineReceived(self, line):
        if self.state == "GETNAME":

    def handle_GETNAME(self, name):
        if name in self.users:
            self.sendLine(b"Name taken, please choose another.")
        self.sendLine(f"Welcome, {name.decode('utf-8')}!".encode("utf-8"))
        self.name = name
        self.users[name] = self
        self.state = "CHAT"

    def handle_CHAT(self, message):
        message = f"<{self.name.decode('utf-8')}> {message.decode('utf-8')}".encode(
        for name, protocol in self.users.items():
            if protocol != self:

class ChatFactory(Factory):
    def __init__(self):
        self.users = {}  # maps user names to Chat instances

    def buildProtocol(self, addr):
        return Chat(self.users)

reactor.listenTCP(8123, ChatFactory())

The only API you might not be familiar with is listenTCP. listenTCP is the method which connects a Factory to the network. This is the lower-level API that endpoints wraps for you.

Here’s a sample transcript of a chat session (emphasised text is entered by the user):

 $ telnet 8123
 Connected to
 Escape character is '^]'.
 What's your name?
 Name taken, please choose another.
 Welcome, bob!
 <alice> hi bob
 twisted makes writing servers so easy!
 <alice> I couldn't agree more
 <carrol> yeah, it's great