===========================================
How to use Django with FastCGI, SCGI or AJP
===========================================

Although the `current preferred setup`_ for running Django is Apache_ with
`mod_python`_, many people use shared hosting, on which protocols such as
FastCGI, SCGI or AJP are the only viable options. In some setups, these protocols
also allow better security -- and, possibly, better performance -- than mod_python.

.. admonition:: Note

    This document primarily focuses on FastCGI. Other protocols, such as SCGI
    and AJP, are also supported, through the ``flup`` Python package. See the
    "Protocols" section below for specifics about SCGI and AJP.

Essentially, FastCGI is an efficient way of letting an external application
serve pages to a Web server. The Web server delegates the incoming Web requests
(via a socket) to FastCGI, which executes the code and passes the response back
to the Web server, which, in turn, passes it back to the client's Web browser.

Like mod_python, FastCGI allows code to stay in memory, allowing requests to be
served with no startup time. Unlike mod_python (or `mod_perl`_), a FastCGI
process doesn't run inside the Web server process, but in a separate,
persistent process.

.. _current preferred setup: ../modpython/
.. _Apache: http://httpd.apache.org/
.. _mod_python: http://www.modpython.org/
.. _mod_perl: http://perl.apache.org/

.. admonition:: Why run code in a separate process?

    The traditional ``mod_*`` arrangements in Apache embed various scripting
    languages (most notably PHP, Python and Perl) inside the process space of
    your Web server. Although this lowers startup time -- because code doesn't
    have to be read off disk for every request -- it comes at the cost of
    memory use. For mod_python, for example, every Apache process gets its own
    Python interpreter, which uses up a considerable amount of RAM.

    Due to the nature of FastCGI, it's even possible to have processes that run
    under a different user account than the Web server process. That's a nice
    security benefit on shared systems, because it means you can secure your
    code from other users.

Prerequisite: flup
==================

Before you can start using FastCGI with Django, you'll need to install flup_,
which is a Python library for dealing with FastCGI. Version 0.5 or newer should
work fine.

.. _flup: http://www.saddi.com/software/flup/

Starting your FastCGI server
============================

FastCGI operates on a client-server model, and in most cases you'll be starting
the FastCGI process on your own. Your Web server (be it Apache, lighttpd, or
otherwise) only contacts your Django-FastCGI process when the server needs a
dynamic page to be loaded. Because the daemon is already running with the code
in memory, it's able to serve the response very quickly.

.. admonition:: Note

    If you're on a shared hosting system, you'll probably be forced to use
    Web server-managed FastCGI processes. See the section below on running
    Django with Web server-managed processes for more information.

A Web server can connect to a FastCGI server in one of two ways: It can use
either a Unix domain socket (a "named pipe" on Win32 systems), or it can use a
TCP socket. What you choose is a manner of preference; a TCP socket is usually
easier due to permissions issues.

To start your server, first change into the directory of your project (wherever
your ``manage.py`` is), and then run ``manage.py`` with the ``runfcgi`` option::

    ./manage.py runfcgi [options]

If you specify ``help`` as the only option after ``runfcgi``, it'll display a
list of all the available options.

You'll need to specify either a ``socket``, ``protocol`` or both ``host`` and
``port``. Then, when you set up your Web server, you'll just need to point it
at the host/port or socket you specified when starting the FastCGI server.

Protocols
---------

Django supports all the protocols that flup_ does, namely fastcgi_, `SCGI`_ and
`AJP1.3`_ (the Apache JServ Protocol, version 1.3). Select your preferred
protocol by using the ``protocol=<protocol_name>`` option with
``./manage.py runfcgi`` -- where ``<protocol_name>`` may be one of: ``fcgi``
(the default), ``scgi`` or ``ajp``. For example::

    ./manage.py runfcgi protocol=scgi

.. _flup: http://www.saddi.com/software/flup/
.. _fastcgi: http://www.fastcgi.com/
.. _SCGI: http://python.ca/scgi/protocol.txt
.. _AJP1.3: http://tomcat.apache.org/connectors-doc/ajp/ajpv13a.html

Examples
--------

Running a threaded server on a TCP port::

    ./manage.py runfcgi method=threaded host=127.0.0.1 port=3033

Running a preforked server on a Unix domain socket::

    ./manage.py runfcgi method=prefork socket=/home/user/mysite.sock pidfile=django.pid

Run without daemonizing (backgrounding) the process (good for debugging)::

    ./manage.py runfcgi daemonize=false socket=/tmp/mysite.sock maxrequests=1

Stopping the FastCGI daemon
---------------------------

If you have the process running in the foreground, it's easy enough to stop it:
Simply hitting ``Ctrl-C`` will stop and quit the FastCGI server. However, when
you're dealing with background processes, you'll need to resort to the Unix
``kill`` command.

If you specify the ``pidfile`` option to your ``manage.py runfcgi``, you can
kill the running FastCGI daemon like this::

    kill `cat $PIDFILE`

...where ``$PIDFILE`` is the ``pidfile`` you specified.

To easily restart your FastCGI daemon on Unix, try this small shell script::

    #!/bin/bash

    # Replace these three settings.
    PROJDIR="/home/user/myproject"
    PIDFILE="$PROJDIR/mysite.pid"
    SOCKET="$PROJDIR/mysite.sock"

    cd $PROJDIR
    if [ -f $PIDFILE ]; then
        kill `cat -- $PIDFILE`
        rm -f -- $PIDFILE
    fi

    exec /usr/bin/env - \
      PYTHONPATH="../python:.." \
      ./manage.py runfcgi socket=$SOCKET pidfile=$PIDFILE

Apache setup
============

To use Django with Apache and FastCGI, you'll need Apache installed and
configured, with `mod_fastcgi`_ installed and enabled. Consult the Apache
documentation for instructions.

Once you've got that set up, point Apache at your Django FastCGI instance by
editing the ``httpd.conf`` (Apache configuration) file. You'll need to do two
things:

    * Use the ``FastCGIExternalServer`` directive to specify the location of
      your FastCGI server.
    * Use ``mod_rewrite`` to point URLs at FastCGI as appropriate.

.. _mod_fastcgi: http://www.fastcgi.com/mod_fastcgi/docs/mod_fastcgi.html

Specifying the location of the FastCGI server
---------------------------------------------

The ``FastCGIExternalServer`` directive tells Apache how to find your FastCGI
server. As the `FastCGIExternalServer docs`_ explain, you can specify either a
``socket`` or a ``host``. Here are examples of both::

    # Connect to FastCGI via a socket / named pipe.
    FastCGIExternalServer /home/user/public_html/mysite.fcgi -socket /home/user/mysite.sock

    # Connect to FastCGI via a TCP host/port.
    FastCGIExternalServer /home/user/public_html/mysite.fcgi -host 127.0.0.1:3033

In either case, the file ``/home/user/public_html/mysite.fcgi`` doesn't
actually have to exist. It's just a URL used by the Web server internally -- a
hook for signifying which requests at a URL should be handled by FastCGI. (More
on this in the next section.)

.. _FastCGIExternalServer docs: http://www.fastcgi.com/mod_fastcgi/docs/mod_fastcgi.html#FastCgiExternalServer

Using mod_rewrite to point URLs at FastCGI
------------------------------------------

The second step is telling Apache to use FastCGI for URLs that match a certain
pattern. To do this, use the `mod_rewrite`_ module and rewrite URLs to
``mysite.fcgi`` (or whatever you specified in the ``FastCGIExternalServer``
directive, as explained in the previous section).

In this example, we tell Apache to use FastCGI to handle any request that
doesn't represent a file on the filesystem and doesn't start with ``/media/``.
This is probably the most common case, if you're using Django's admin site::

    <VirtualHost 12.34.56.78>
      ServerName example.com
      DocumentRoot /home/user/public_html
      Alias /media /home/user/python/django/contrib/admin/media
      RewriteEngine On
      RewriteRule ^/(media.*)$ /$1 [QSA,L,PT]
      RewriteCond %{REQUEST_FILENAME} !-f
      RewriteRule ^/(.*)$ /mysite.fcgi/$1 [QSA,L]
    </VirtualHost>

.. _mod_rewrite: http://httpd.apache.org/docs/2.0/mod/mod_rewrite.html

Django will automatically use the pre-rewrite version of the URL when
constructing URLs with the ``{% url %}`` template tag (and similar methods).

lighttpd setup
==============

lighttpd is a lightweight Web server commonly used for serving static files. It
supports FastCGI natively and, thus, is a good choice for serving both static
and dynamic pages, if your site doesn't have any Apache-specific needs.

Make sure ``mod_fastcgi`` is in your modules list, somewhere after
``mod_rewrite`` and ``mod_access``, but not after ``mod_accesslog``. You'll
probably want ``mod_alias`` as well, for serving admin media.

Add the following to your lighttpd config file::

    server.document-root = "/home/user/public_html"
    fastcgi.server = (
        "/mysite.fcgi" => (
            "main" => (
                # Use host / port instead of socket for TCP fastcgi
                # "host" => "127.0.0.1",
                # "port" => 3033,
                "socket" => "/home/user/mysite.sock",
                "check-local" => "disable",
            )
        ),
    )
    alias.url = (
        "/media/" => "/home/user/django/contrib/admin/media/",
    )

    url.rewrite-once = (
        "^(/media.*)$" => "$1",
        "^/favicon\.ico$" => "/media/favicon.ico",
        "^(/.*)$" => "/mysite.fcgi$1",
    )

Running multiple Django sites on one lighttpd
---------------------------------------------

lighttpd lets you use "conditional configuration" to allow configuration to be
customized per host. To specify multiple FastCGI sites, just add a conditional
block around your FastCGI config for each site::

    # If the hostname is 'www.example1.com'...
    $HTTP["host"] == "www.example1.com" {
        server.document-root = "/foo/site1"
        fastcgi.server = (
           ...
        )
        ...
    }

    # If the hostname is 'www.example2.com'...
    $HTTP["host"] == "www.example2.com" {
        server.document-root = "/foo/site2"
        fastcgi.server = (
           ...
        )
        ...
    }

You can also run multiple Django installations on the same site simply by
specifying multiple entries in the ``fastcgi.server`` directive. Add one
FastCGI host for each.

Running Django on a shared-hosting provider with Apache
=======================================================

Many shared-hosting providers don't allow you to run your own server daemons or
edit the ``httpd.conf`` file. In these cases, it's still possible to run Django
using Web server-spawned processes.

.. admonition:: Note

    If you're using Web server-spawned processes, as explained in this section,
    there's no need for you to start the FastCGI server on your own. Apache
    will spawn a number of processes, scaling as it needs to.

In your Web root directory, add this to a file named ``.htaccess`` ::

    AddHandler fastcgi-script .fcgi
    RewriteEngine On
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteRule ^(.*)$ mysite.fcgi/$1 [QSA,L]

Then, create a small script that tells Apache how to spawn your FastCGI
program. Create a file ``mysite.fcgi`` and place it in your Web directory, and
be sure to make it executable::

    #!/usr/bin/python
    import sys, os

    # Add a custom Python path.
    sys.path.insert(0, "/home/user/python")

    # Switch to the directory of your project. (Optional.)
    # os.chdir("/home/user/myproject")

    # Set the DJANGO_SETTINGS_MODULE environment variable.
    os.environ['DJANGO_SETTINGS_MODULE'] = "myproject.settings"

    from django.core.servers.fastcgi import runfastcgi
    runfastcgi(method="threaded", daemonize="false")

Restarting the spawned server
-----------------------------

If you change any Python code on your site, you'll need to tell FastCGI the
code has changed. But there's no need to restart Apache in this case. Rather,
just reupload ``mysite.fcgi``, or edit the file, so that the timestamp on the
file will change. When Apache sees the file has been updated, it will restart
your Django application for you.

If you have access to a command shell on a Unix system, you can accomplish this
easily by using the ``touch`` command::

    touch mysite.fcgi

Serving admin media files
=========================

Regardless of the server and configuration you eventually decide to use, you will
also need to give some thought to how to serve the admin media files. The
advice given in the modpython_ documentation is also applicable in the setups
detailed above.

.. _modpython: ../modpython/#serving-the-admin-files

Forcing the URL prefix to a particular value
============================================

Because many of these fastcgi-based solutions require rewriting the URL at
some point inside the webserver, the path information that Django sees may not
resemble the original URL that was passed in. This is a problem if the Django
application is being served from under a particular prefix and you want your
URLs from the ``{% url %}`` tag to look like the prefix, rather than the
rewritten version, which might contain, for example, ``mysite.fcgi``.

Django makes a good attempt to work out what the real script name prefix
should be. In particular, if the webserver sets the ``SCRIPT_URL`` (specific
to Apache's mod_rewrite), or ``REDIRECT_URL`` (set by a few servers, including
Apache + mod_rewrite in some situations), Django will work out the original
prefix automatically.

In the cases where Django cannot work out the prefix correctly and where you
want the original value to be used in URLs, you can set the
``FORCE_SCRIPT_NAME`` setting in your main ``settings`` file. This sets the
script name uniformly for every URL served via that settings file. Thus you'll
need to use different settings files if you want different sets of URLs to
have different script names in this case, but that is a rare situation.

As an example of how to use it, if your Django configuration is serving all of
the URLs under ``'/'`` and you wanted to use this setting, you would set
``FORCE_SCRIPT_NAME = ''`` in your settings file.

