Writing a New Plug-In for mktap

Getting Started

Twisted is a very general and powerful tool. It can power anything connected to a network, from your corporate message-broadcasting network to your desktop IRC client. This is great for integrating lots of different tools, but can make it very difficult to document and understand how the whole platform is supposed to work. A side effect of this is that it's hard to get started with a project using Twisted, because it's hard to find out where to start.

This guide is to help you understand one of the main ways to get started working on a Twisted server application. It probably won't answer your specific questions about how to do things like schedule functions to call in the future or listen on a socket; there are other documents that address these concerns and you can read them later. Client applications are not likely to use the infrastructure described in this document, and a simpler alternative exists for servers, covered in the Application howto.

Twisted and You: Where Does Your Code Fit In?

If you're like most people that have asked me questions about this, you've probably come to Twisted thinking of it as a library of code to help you write an application. It can be, but it is much more useful to think of your code as the library. Twisted is a framework.

The difference between a framework and a library is that a developer's code will run a library's functions; a framework runs the developer's functions, instead. The difference is subtle, but significant; there are a range of resources which have to be allocated and managed regarding start-up and shut-down of an process, such as spawning of threads and handling events. You don't have to use Twisted this way. It is quite possible to write applications that use Twisted almost exclusively as a library. If you use it as a framework, though, Twisted will help you by managing these resources itself.

The central framework classes that you will deal with, both as a Twisted developer and administrator, are services, which implement twisted.application.service.IService. twisted.application.service.Application creates the root of a tree of services that form a twisted application. There is one Application instance per Twisted process, and it is the top-level manager of resources and handler of events in the Twisted framework. (Unlike some other frameworks, developers do not subclass Application; rather by adding services it, you register event handlers to be called by it.) To store configuration data, as well as other information, Twisted serializes Application instances, storing all services that have been registered with them. Since the whole Application instance is serialized, Twisted configuration files are significantly more comprehensive than those for other systems. These files store everything related to a running Application instance; in essence the full state of a running process.

The central concept that a Twisted system administrator will work with are files that contain Application instances serialized in various formats optimized for different uses. .TAP files are optimized for speed of loading and saving, .TAX files are editable by administrators familiar with XML syntax, and .TAS files are generated Python source code, most useful for developers. The two command-line programs which work with these files are mktap and twistd. The mktap utility create .TA* files from simple command-line arguments, and the twistd daemon will load and run those files. Alternatively, a Python script can be used to create the Application instance and this script can be run directly using twistd -y script.py, as long as the file script.py has a Application object called application on the module level. Applications are covered in more depth in the Application howto.

There are many ways in which your code will be called by various parts of the Twisted framework by the time you're done. The initial one we're going to focus on here is a plug-in for the mktap utility. mktap produces complete, runnable Application instances, so no additional work is necessary to make your code work with twistd. First we will go through the process of creating a plug-in that Twisted can find, then we make it adhere to the mktap interface. Finally we will load that plug-in with a server.

What is a Plug-In?

Python makes it very easy to dynamically load and evaluate programs. The plug-in system for Twisted, twisted.python.plugin, is a way to find (without loading) and then load plug-ins for particular systems.

Unlike other plug-in systems, like the well known ones associated with The Gimp, Photoshop, and Apache twisted.python.plugin is generic. Any one of the Twisted dot-products can define mechanisms for extensibility using plug-ins. Two Twisted dot-products already load such plug-ins. The twisted.applicationpackage loads Twisted Application builder modules (TAP plug-ins) and the twisted.lore package loads document formatting modules.

Twisted finds its plug-ins by using pre-existing Python concepts; the load path, and packages. Every top-level Python package (that is, a directory whose parent is on sys.path and which contains an __init__.py) can potentially contain some number of plug-ins. Packages which contain plug-ins are called drop-ins, because you drop them into your sys.path. The only difference between a package and a drop-in is the existence of a file named plugins.tml (TML for Twisted Module List) that contains some special Python expressions to identify the location of sub-packages or modules which can be loaded.

If you look at twisted/plugins.tml, you will notice that Twisted is a drop-in for itself! You can browse through it for lots of examples of plug-ins being registered.

The most prevalent kind of plug-in is the TAP (Twisted Application builder) type. These are relatively simple to get started with. Let's look at an excerpt from Twisted's own plugins.tml for an example of registering one:

# ...

register("Twisted Web Automated TAP builder",
         "twisted.tap.web",
         description="""
         Builds a Twisted Application instance that contains a general-purpose
         web server, which can serve from a filesystem or application resource.
         """            ,
         type="tap",
         tapname="web")

# ...

plugins.tml will be a list of calls to one function:

register(name, module, type=plugin_type,
         description=user_description
         [, **plugin_specific_data])

• name is a free-form string, to be displayed to the user in presentation contexts (like a web page, or a list-box in a GUI).
• module is a string which must be the fully-qualified name of a Python module.
• type is the name of the system you are plugging in to. Be sure to spell this right, or Twisted won't find your plug-in at all!
• **plugin_specific_data is a dictionary of information associated with the plug-in, specific to the type of plug-in it is. Note that some plug-in types may require a specific bit of data in order to work.

Note the tapname parameter given in the example above. This parameter is an example of **plugin_specific_data. The parameter tapname is only used by "tap"-type modules. It indicates what name to use on the mktap command line. In English, this particular call to register means When the user types mktap web, it selects the module twisted.tap.web to handle the rest of the arguments.

Now that you understand how to register a plug-in, let's move along to writing your first one.

Twisted Quotes: A Case Study

As an example, we are going to work on a Quote of the Day application, TwistedQuotes. Aspects of this application will be explored in more depth throughout in the Twisted documentation.

TwistedQuotes is a very simple plugin which is a great demonstration of Twisted's power. It will export a small kernel of functionality -- Quote of the Day -- which can be accessed through every interface that Twisted supports: web pages, e-mail, instant messaging, a specific Quote of the Day protocol, and more.

Before you Begin

First, make a directory, TwistedQuotes, where you're going to keep your code. If you installed Twisted from source, the path of least resistance is probably just to make a directory inside your Twisted-X.X.X directory, which will already be in your sys.path. If you want to put it elsewhere, make sure that your TwistedQuotes directory is a package on your python path.

You will then need to add an __init__.py to this directory, to mark it as a package. (For more information on exactly how Python packages work, read this section of the Python tutorial.) In order to test that everything is working, start up the Python interactive interpreter, or your favorite IDE, and verify that the package imports properly.

Python 2.1.3 (#1, Apr 20 2002, 22:45:31) 
[GCC 2.95.4 20011002 (Debian prerelease)] on linux2
Type "copyright", "credits" or "license" for more information.
>>> import TwistedQuotes
>>> # No traceback means you're fine.

A Look at the Heart of the Application

(You'll need to put this code into a file called quoters.py in your TwistedQuotes directory.)

from twisted.python import components

from random import choice


class IQuoter(components.Interface):
    """An object that returns quotes."""

    def getQuote(self):
        """Return a quote."""


class StaticQuoter:
    """Return a static quote."""

    __implements__ = IQuoter

    def __init__(self, quote):
        self.quote = quote

    def getQuote(self):
        return self.quote


class FortuneQuoter:
    """Load quotes from a fortune-format file."""

    __implements__ = IQuoter

    def __init__(self, filenames):
        self.filenames = filenames

    def getQuote(self):
        return choice(open(choice(self.filenames)).read().split('\n%\n'))

This code listing shows us what the Twisted Quotes system is all about. The code doesn't have any way of talking to the outside world, but it provides a library which is a clear and uncluttered abstraction: give me the quote of the day.

Note that this module does not import any Twisted functionality at all! The reason for doing things this way is integration. If your business objects are not stuck to your user interface, you can make a module that can integrate those objects with different protocols, GUIs, and file formats. Having such classes provides a way to decouple your components from each other, by allowing each to be used independently.

In this manner, Twisted itself has minimal impact on the logic of your program. Although the Twisted dot products are highly interoperable, they also follow this approach. You can use them independently because they are not stuck to each other. They communicate in well-defined ways, and only when that communication provides some additional feature. Thus, you can use twisted.web with twisted.enterprise, but neither requires the other, because they are integrated around the concept of Deferreds. (Don't worry we'll get to each of those features in later documentation.)

Your Twisted applications should follow this style as much as possible. Have (at least) one module which implements your specific functionality, independant of any user-interface code.

Next, we're going to need to associate this abstract logic with some way of displaying it to the user. We'll do this by writing a Twisted server protocol, which will respond to the clients that connect to it by sending a quote to the client and then closing the connection. Note: don't get too focused on the details of this -- different ways to interface with the user are 90% of what Twisted does, and there are lots of documents describing the different ways to do it.

(You'll need to put this code into a file called quoteproto.py in your TwistedQuotes directory.)

from twisted.internet.protocol import Factory, Protocol

class QOTD(Protocol):

    def connectionMade(self):
        self.transport.write(self.factory.quoter.getQuote()+'\r\n')
        self.transport.loseConnection()

class QOTDFactory(Factory):

    protocol = QOTD

    def __init__(self, quoter):
        self.quoter = quoter

This is a very straightforward Protocol implementation, and the pattern described above is repeated here. The Protocol contains essentially no logic of its own, just enough to tie together an object which can generate quotes (a Quoter) and an object which can relay bytes to a TCP connection (a Transport). When a client connects to this server, a QOTD instance is created, and its connectionMade method is called.

The QOTDFactory's role is to specify to the Twisted framework how to create a Protocol instance that will handle the connection. Twisted will not instantiate a QOTDFactory; you will do that yourself later, in the mktap plug-in below.

Note: you can read more specifics of Protocol and Factory in the Writing Servers HOWTO.

Once we have an abstraction -- a Quoter -- and we have a mechanism to connect it to the network -- the QOTD protocol -- the next thing to do is to put the last link in the chain of functionality between abstraction and user. This last link will allow a user to choose a Quoter and configure the protocol.

Practically speaking, this link is an interface for a savvy user who will run the server. (In this case, you; when you have more users, a system administrator.) For the purposes of this example we will first implement a mktap interface. Like most system administrator tools, this is command-line oriented. (It is possible to implement a graphical front-end to mktap, using the same plug-in structure, but this has not been done yet.)

Creating the extension to mktap is done through implementing a module that follows the mktap plug-in interface, and then registering it to be found and loaded by twisted.python.plugin. As described above, registration is done by adding a call to register in the file TwistedQuotes/plugins.tml

(You'll need to put this code into a file called quotetap.py in your TwistedQuotes directory.)

from twisted.application import internet # services that run TCP/SSL/etc.
from TwistedQuotes import quoteproto    # Protocol and Factory
from TwistedQuotes import quoters       # "give me a quote" code

from twisted.python import usage        # twisted command-line processing


class Options(usage.Options):
    optParameters = [["port", "p", 8007,
                      "Port number to listen on for QOTD protocol."],
                     ["static", "s", "An apple a day keeps the doctor away.",
                      "A static quote to display."],
                     ["file", "f", None,
                      "A fortune-format text file to read quotes from."]]


def makeService(config):
    """Return a service that will be attached to the application."""
    if config["file"]:                  # If I was given a "file" option...
        # Read quotes from a file, selecting a random one each time,
        quoter = quoters.FortuneQuoter([config['file']])
    else:                               # otherwise,
        # read a single quote from the command line (or use the default).
        quoter = quoters.StaticQuoter(config['static'])
    port = int(config["port"])          # TCP port to listen on
    factory = quoteproto.QOTDFactory(quoter) # here we create a QOTDFactory
    # Finally, set up our factory, with its custom quoter, to create QOTD
    # protocol instances when events arrive on the specified port.
    return internet.TCPServer(port, factory)

This module has to conform to a fairly simple interface. It must have a class called Options which is a subclass of twisted.python.usage.Options. It must also have a function makeService(config), which will be passed an instance of the Options class defined in the module itself, TwistedQuotes.quotetap.Options. Command-line options given on the mktap command line fill in the values in Options and are used in makeService to make the actual connections between objects. makeService is expected to return an object implementing IService. This can be a Service subclass, a MultiService collection of sub-services, a TCPServer serving a protocol factory, and so on.

A more detailed discussion of twisted.python.usage.Options can be found in the document Using usage.Options.

Now that we've implemented all the necessary pieces, we can finish putting them together by writing a TML file which allows the mktap utility to find our protocol module.

register("Quote of the Day TAP Builder",
         "TwistedQuotes.quotetap",
         description="""
         Example of a TAP builder module.
         """            ,
         type="tap",
         tapname="qotd")

Now the QOTD server is ready to be instantiated! Let's start up a server and get a quote from it.

% mktap qotd       
Saving qotd application to qotd.tap...
Saved.
% twistd -f qotd.tap 
% nc localhost 8007
An apple a day keeps the doctor away.
% kill `cat twistd.pid`

Let's walk through the above example. First, we run mktap specifying the Application type (qotd) to create. mktap reads in our plugins.tml file, instantiates an Application object, fills in the appropriate data, and serializes it out to a qotd.tap file. Next, we launch the server using the twistd daemon, passing qotd.tap as a command line option. The server launches, listens on the default port from quotetap.py. Next, we run nc to connect to the running server. In this step, the QOTDFactory creates a Quoter instance, which responds to our network connection by sending a quote string (in this case, the default quote) over our connection, and then closes the connection. Finally, we shutdown the server by killing it via a saved out process id file.

(nc is the netcat utility, which no UNIX system should be without.)

So we just saw Twisted in action as a framework. With relatively little code, we've got a server that can respond to a request over a network, with two potential alternative back-ends (fortune files and static text).

After reading this (and following along with your own example, of course), you should be familiar with the process of getting your own Twisted code with unique functionality in it running inside of a server. You should be familiar with the concept of a drop-in and a plug-in, and understand both how to create them and how to install them from other people on your system.

By following the rules set out at the beginning of this HOWTO, we have accidentally implemented another piece of useful functionality.

% mktap
Usage:    mktap [options] <command> [command options]

Options:
  -x, --xml      DEPRECATED: same as --type=xml
  -s, --source   DEPRECATED: same as --type=source
  -e, --encrypted  Encrypt file before writing
  -p, --progress   Show progress of plugin loading
  -d, --debug      Show debug information for plugin loading
  -u, --uid=     [default: 1000]
  -g, --gid=     [default: 1000]
  -a, --append=  An existing .tap file to append the plugin to, rather than
                 creating a new one.
  -t, --type=    The output format to use; this can be 'pickle', 'xml', or
                 'source'. [default: pickle]
      --help     display this message
Commands:
    ftp              An FTP server.
    im               A multi-protocol chat client.
    inetd
    mail             An email service.
    manhole          An interactive remote debugger service.
    news             News Server
    portforward      A simple port-forwarder.
    qotd             Example of a TAP builder module.
    socks            A SOCKSv4 proxy service.
    ssh
    telnet           A simple, telnet-based remote debugging service.
    toc              An AIM TOC service.
    web              A general-purpose web server which can serve from a
                     filesystem or application resource.
    words            A chat service.

Not only does our Options class get instantiated by mktap directly, the user can query mktap for interactive help! This is just one small benefit to using Twisted as it was designed. As more tools that use the tap style of plug-in, more useful functionality will become available from Twisted Quotes. For example, a graphical tool could provide not just help messages at the command line, but a listing of all available TAP types and forms for each, for the user to enter information.

It is this kind of power that results from using a dynamic, powerful framework like Twisted. I hope that you take your newfound knowledge and discover all kinds of cool things like this that you get for free just by using it!