Writing a New Plug-In for mktap

  1. Goals
  2. What is a Plug-In?
  3. Applications and plugins in Twisted
  4. Writing a mktap plugin: the Twisted Quotes example

This document does not describe current Twisted best practice. The progress of this document is currently being tracked in issue 1139.

Goals

This document describes serialized Python configuration objects which can be used for running Twisted applications.

Users of this document should be have a Twisted server application that they are going to deploy. In general, if you are going to write a mktap plugin, it is the last stage of developing and deploying a Twisted application. The majority of users will use the .tac Python configuration files without needing to use the mktap serialization. Therefore, people writing Twisted applications should review the Application howto before this document, and consider using the .tac mechanism described there.

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.

See The Twisted Plugin system for more information on using twisted.python.plugin.

Applications and plugins in Twisted

Application objects

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.

Application objects and related infrastructure are covered in more depth in the Application howto.

TAP plugins

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])

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.

Writing a mktap plugin: the Twisted Quotes example

As an example, we are going to work on a Quote of the Day application, TwistedQuotes described in the Designing Twisted Applications document.

Set up the project directory

See the description of setting up the TwistedQuotes example.

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

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)
Twisted Quotes TAP construction module - listings/TwistedQuotes/quotetap.py

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.

Once the TwistedQuotes application design is complete, we pull it 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")
Twisted Quotes Plug-in registration - listings/TwistedQuotes/plugins.tml

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.)

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.

Index

Version: 2.2.0