Writing a twistd Plugin¶
This document describes adding subcommands to
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
twistdplugins; in particular, you should know how to expose your program’s functionality as a Service.
- In order to parse command line arguments, the
twistdplugin mechanism relies on
twisted.python.usage, which is documented in Using usage.Options .
After reading this document, the reader should be able to expose
their Service-using application as a subcommand
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
file, which is a simple script to be used with the
-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
- myproject - Python package
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
twisted containing a directory
plugins , and add a file
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
myproject.py (because that would conflict with
your project’s module name).
tapname attribute of your IServiceMaker provider
will be used as the subcommand name in a command
twistd [subcommand] [args...] , and
options attribute (which should be
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()
twistd --help should
myproject in the list of available subcommands,
followed by the description that we specified in the
twistd -n myproject would,
assuming we defined a
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
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