Twisted Documentation: Using the Twisted Application Framework

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 and/or clients.

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.

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 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 implementing 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 heirarchies 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 for an explanation of EchoFactory.

This example creates a simple heirarchy:

   application
   |
   `- echoService

More complicated heirarchies 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.

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
"""

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 - listings/application/service.tac

twistd is a program that runs Twisted applications using a .tac file. In its most simple form, it takes a single 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 component 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.

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
connectUDP

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. MultiService 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)

# Create an application as normal
application = service.Application("DNSExample")

# Connect our MultiService to the application, just like a normal service.
dnsService.setServiceParent(application)

Using the Twisted Application Framework