# Writing a twistd Plugin

This document describes adding subcommands to the `twistd` command, as a way to facilitate the deployment of your applications. (This feature was added in Twisted 2.5)

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.

## A note on .tap files

Readers may be confused about a historical file type associated with Twisted, the `.tap` file. This was a kind of file that was generated by a program named `mktap` and which `twistd` can read. `.tap` files are deprecated; this document has nothing to do with them, although the technology described herein is very closely related to the old system. Existing plugins that were written for the mktap system are compatible with this `twistd` plugin system; the following commands,

    $ mktap [foo] [options...]
    $ twistd -n -f [foo].tap

are equivalent to the command:

    $ twistd -n [foo] [options...]

## 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 should 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 implements

    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."]]

    class MyServiceMaker(object):
        implements(IServiceMaker, IPlugin)
        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"]]

    class MyServerServiceMaker(object):
        implements(IServiceMaker, IPlugin)
        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

## 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

Index

Version: 8.2.0