Writing a twistd Plugin

This document describes adding subcommands to the twistd command, as a way to facilitate the deployment of your applications.

The target audience of this document are those that have developed a Twisted application which needs a command line-based deployment mechanism.

There are a few prerequisites to understanding this document:

• A basic understanding of the Twisted Plugin System (i.e., the twisted.plugin module) is necessary, however, step-by-step instructions will be given. Reading The Twisted Plugin System is recommended, in particular the “Extending an Existing Program” section.

• The Application infrastructure is used in twistd plugins; in particular, you should know how to expose your program’s functionality as a Service.

• In order to parse command line arguments, the twistd plugin mechanism relies on twisted.python.usage , which is documented in Using usage.Options .

Goals

After reading this document, the reader should be able to expose their Service-using application as a subcommand of twistd , taking into consideration whatever was passed on the command line.

Alternatives to twistd Plugins

The major alternative to the twistd plugin mechanism is the .tac file, which is a simple script to be used with the twistd -y/--python parameter. The twistd plugin mechanism exists to offer a more extensible command-line-driven interface to your application. For more information on .tac files, see the document Using the Twisted Application Framework .

Creating the Plugin

The following directory structure is assumed of your project:

• MyProject - Top level directory

  • myproject - Python package

    • __init__.py

During development of your project, Twisted plugins can be loaded from a special directory in your project, assuming your top level directory ends up in sys.path. Create a directory named twisted containing a directory named plugins , and add a file named myproject_plugin.py to it. This file will contain your plugin. Note that you must not add any __init__.py files to this directory structure, and the plugin file should not be named myproject.py (because that would conflict with your project’s module name).

In this file, define an object which provides the interfaces twisted.plugin.IPlugin and twisted.application.service.IServiceMaker .

The tapname attribute of your IServiceMaker provider will be used as the subcommand name in a command like twistd [subcommand] [args...] , and the options attribute (which should be a usage.Options subclass) will be used to parse the given args.

from zope.interface import implementer

from twisted.python import usage
from twisted.plugin import IPlugin
from twisted.application.service import IServiceMaker
from twisted.application import internet

from myproject import MyFactory


class Options(usage.Options):
    optParameters = [["port", "p", 1235, "The port number to listen on."]]


@implementer(IServiceMaker, IPlugin)
class MyServiceMaker(object):
    tapname = "myproject"
    description = "Run this! It'll make your dog happy."
    options = Options

    def makeService(self, options):
        """
        Construct a TCPServer from a factory defined in myproject.
        """
        return internet.TCPServer(int(options["port"]), MyFactory())


# Now construct an object which *provides* the relevant interfaces
# The name of this variable is irrelevant, as long as there is *some*
# name bound to a provider of IPlugin and IServiceMaker.
serviceMaker = MyServiceMaker()

Now running twistd --help should print myproject in the list of available subcommands, followed by the description that we specified in the plugin. twistd -n myproject would, assuming we defined a MyFactory factory inside myproject , start a listening server on port 1235 with that factory.

Using cred with your TAP

Twisted ships with a robust authentication framework to use with your application. If your server needs authentication functionality, and you haven’t read about twisted.cred yet, read up on it first.

If you are building a twistd plugin and you want to support a wide variety of authentication patterns, Twisted provides an easy-to-use mixin for your Options subclass: strcred.AuthOptionMixin . The following code is an example of using this mixin:

from twisted.cred import credentials, portal, strcred
from twisted.python import usage
from twisted.plugin import IPlugin
from twisted.application.service import IServiceMaker
from myserver import myservice


class ServerOptions(usage.Options, strcred.AuthOptionMixin):
    # This part is optional; it tells AuthOptionMixin what
    # kinds of credential interfaces the user can give us.
    supportedInterfaces = (credentials.IUsernamePassword,)

    optParameters = [
        ["port", "p", 1234, "Server port number"],
        ["host", "h", "localhost", "Server hostname"]]


@implementer(IServiceMaker, IPlugin)
class MyServerServiceMaker(object):
    tapname = "myserver"
    description = "This server does nothing productive."
    options = ServerOptions

    def makeService(self, options):
        """Construct a service object."""
        # The realm is a custom object that your server defines.
        realm = myservice.MyServerRealm(options["host"])

        # The portal is something Cred can provide, as long as
        # you have a list of checkers that you'll support. This
        # list is provided my AuthOptionMixin.
        portal = portal.Portal(realm, options["credCheckers"])

        # OR, if you know you might get multiple interfaces, and
        # only want to give your application one of them, you
        # also have that option with AuthOptionMixin:
        interface = credentials.IUsernamePassword
        portal = portal.Portal(realm, options["credInterfaces"][interface])

        # The protocol factory is, like the realm, something you implement.
        factory = myservice.ServerFactory(realm, portal)

        # Finally, return a service that will listen for connections.
        return internet.TCPServer(int(options["port"]), factory)


# As in our example above, we have to construct an object that
# provides the IPlugin and IServiceMaker interfaces.
serviceMaker = MyServerServiceMaker()

Now that you have your TAP configured to support any authentication we can throw at it, you’re ready to use it. Here is an example of starting your server using the /etc/passwd file for authentication. (Clearly, this won’t work on servers with shadow passwords.)

$ twistd myserver --auth passwd:/etc/passwd

For a full list of cred plugins supported, see twisted.plugins , or use the command-line help:

$ twistd myserver --help-auth
$ twistd myserver --help-auth-type passwd

Deploy your Application Using Python Packages

To deploy your application one possibility is to wrap it up in a Python package. For this you need to write a special file setup.py, which contains metadata of the package. You would have to extend the layout of your files like this:

• MyProject - Top level directory

  • setup.py - Description file for the package

    • myproject - Python package

      • __init__.py

    • twisted

      • plugins

        • myproject_plugins.py - Dropin file containing the actual plugin

from setuptools import setup, find_packages

setup(
    name='MyApplication',
    version='0.1dev',
    # it is necesary to extend the found package list with the twisted.plugin
    # directory. It cannot be automatically detected, because it should not
    # contain a __init__.py file.
    packages=find_packages() + ['twisted.plugins'],
    install_requires=[
        'twisted',
    ],
)

To create the Python package from the directory the standard setup tools can be used:

$ python3 setup.py sdist

This command creates a dist directory in your project folder with the compressed archive file MyApplication-0.1dev.tar.gz. This archive contains all the code and additional files if specified. This file can be copied and used for deployment.

To install the application just use pip. It will also install all requirements specified in setup.py.

$ pip install MyApplication-0.1dev.tar.gz

For more information about packaging in Python have a look at the official Python packaging user guide or hitchhiker’s guide to packaging.

Conclusion

You should now be able to

• Create a twistd plugin

• Incorporate authentication into your plugin

• Use it from your development environment

• Install it correctly and use it in deployment