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# The Basics

## Application

Twisted programs usually work with twisted.application.service.Application. This class usually holds all persistent configuration of a running server, such as:

- ports to bind to,
- places where connections to must be kept or attempted,
- periodic actions to do,
- and almost everything else to do with your Application.

It is the root object in a tree of services implementing twisted.application.service.IService.

Other howtos describe how to write custom code for `Application`s, but this one describes how to use already written code (which can be part of Twisted or from a third-party Twisted plugin developer). The Twisted distribution comes with an important tool to deal with `Application`s: `twistd(1)`.

`Application`s are just Python objects, which can be created and manipulated in the same ways as any other object.

## twistd

The Twisted Daemon is a program that knows how to run Applications. Strictly speaking, `twistd` is not necessary. Fetching the application, getting the `IService` component, calling `startService()`, scheduling `stopService()` when the reactor shuts down, and then calling `reactor.run()` could be done manually.

However, `twistd` supplies many options which are highly useful for program set up:

- choosing a reactor (for more on reactors, see Choosing a Reactor),
- logging configuration (see the logging documentation for more),
- daemonizing (forking to the background),
- and more.

`twistd` supports all Applications mentioned above – and an additional one. Sometimes it is convenient to write the code for building a class in straight Python. One big source of such Python files is the examples directory. When a straight Python file which defines an `Application` object called `application` is used, use the `-y` option.

When `twistd` runs, it records its process id in a `twistd.pid` file (this can be configured via a command line switch). In order to shutdown the `twistd` process, kill that pid. The usual way to do this would be:

    kill `cat twistd.pid`

To prevent `twistd` from daemonizing, you can pass it the `--no-daemon` option (or `-n`, in conjunction with other short options).

As always, the gory details are in the manual page.

## OS Integration

If you have an Application that runs with `twistd`, you can deploy it on RedHat Linux or Debian GNU/Linux based systems using the `tap2deb` or `tap2rpm` tools. These take a Twisted `Application` file (of any of the supported formats — Python source, XML or pickle), and build a Debian or RPM package (respectively) that installs the `Application` as a system service. The package includes the `Application` file, a default `/etc/init.d/` script that starts and stops the process with twistd, and post-installation scripts that configure the `Application` to be run in the appropriate init levels.

Note

`tap2rpm` and `tap2deb` do not package your entire application and dependent code, just the Twisted Application file. You will need to find some other way to package your Python code, such as distutils‘ `bdist_rpm` command.

For more savvy users, these tools also generate the source package, allowing you to modify and polish things which automated software cannot detect (such as dependencies or relationships to virtual packages).