# 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.application`package 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.

The directory you add to your PYTHONPATH needs to be the directory containing your package's directory! For example, if your TwistedQuotes directory is /my/stuff/TwistedQuotes, you can `export PYTHONPATH=/my/stuff:$PYTHONPATH` in UNIX, or edit the `PYTHONPATH` environment variable to add `/my/stuff;` at the beginning through the System Properties dialog on Windows.

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.)

Twisted Quotes Central Abstraction

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.)

Twisted Quotes Protocol Implementation

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.)

Twisted Quotes TAP construction module

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.

Twisted Quotes Plug-in registration

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!

The plug-in system is a relatively new part of Twisted, and not as many things use it as they should yet. Watch this space for new developments regarding plug-ins, other systems that you can plug your code into, and more documentation for people wanting to write systems that can be plugged in to!