Application
Twisted programs usually work
with twisted.application.service.Application.
This class usually holds all persistent configuration of a running
server -- ports to bind to, places where connections to must be kept
or attempted, periodic actions to do and almost everything else. It is
the root object in a tree of services implementing IService.
Other HOWTOs describe how to write custom code for Applications,
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
Applications, twistd.
Applications 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.
This program
is twistd(1). 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. twistd(1), however, supplies
many options which are highly useful for program set up.
twistd supports choosing a reactor (for more on
reactors, see Choosing a Reactor), logging
to a logfile, daemonizing 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 doc/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 (usually
you would do kill `cat twistd.pid`).
As always, the gory details are in the manual page.
OS Integration
If you have an Application that runs
with twistd, you can easily 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.
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).