=======
FastCGI
=======

A very popular deployment setup on servers like `lighttpd`_ and `nginx`_
is FastCGI.  To use your WSGI application with any of them you will need
a FastCGI server first.

The most popular one is `flup`_ which we will use for this guide.  Make
sure to have it installed.

Creating a `.fcgi` file
=======================

First you need to create the FastCGI server file.  Let's call it
`yourapplication.fcgi`::

    #!/usr/bin/python
    from flup.server.fcgi import WSGIServer
    from yourapplication import make_app
    
    if __name__ == '__main__':
        application = make_app()
        WSGIServer(application).run()

This is enough for Apache to work, however ngingx and older versions of
lighttpd need a socket to be explicitly passed to communicate with the FastCGI
server.  For that to work you need to pass the path to the socket to the
:class:`~flup.server.fcgi.WSGIServer`::

    WSGIServer(application, bindAddress='/path/to/fcgi.sock').run()

The path has to be the exact same path you define in the server
config.

Save the `yourapplication.fcgi` file somewhere you will find it again.
It makes sense to have that in `/var/www/yourapplication` or something
similar.

Make sure to set the executable bit on that file so that the servers
can execute it::

    # chmod +x /var/www/yourapplication/yourapplication.fcgi

Configuring lighttpd
====================

A basic FastCGI configuration for lighttpd looks like this::

    fastcgi.server = ("/yourapplication.fcgi" =>
        ((
            "socket" => "/tmp/yourapplication-fcgi.sock",
            "bin-path" => "/var/www/yourapplication/yourapplication.fcgi",
            "check-local" => "disable",
            "max-procs" -> 1
        ))
    )

    alias.url = (
        "/static/" => "/path/to/your/static"
    )

    url.rewrite-once = (
        "^(/static.*)$" => "$1",
        "^(/.*)$" => "/yourapplication.fcgi$1"

Remember to enable the FastCGI, alias and rewrite modules. This configuration
binds the application to `/yourapplication`.  If you want the application to
work in the URL root you have to work around a lighttpd bug with the
:class:`~werkzeug.contrib.fixers.LighttpdCGIRootFix` middleware.

Make sure to apply it only if you are mounting the application the URL
root. Also, see the Lighty docs for more information on `FastCGI and Python
<http://redmine.lighttpd.net/wiki/lighttpd/Docs:ModFastCGI>`_ (note that
explicitly passing a socket to run() is no longer necessary).

Configuring nginx
=================

Installing FastCGI applications on nginx is a bit tricky because by default
some FastCGI parameters are not properly forwarded.

A basic FastCGI configuration for nginx looks like this::

    location /yourapplication/ {
        include fastcgi_params;
        if ($uri ~ ^/yourapplication/(.*)?) {
            set $path_url $1;
        }
        fastcgi_param PATH_INFO $path_url;
        fastcgi_param SCRIPT_NAME /yourapplication;
        fastcgi_pass unix:/tmp/yourapplication-fcgi.sock;
    }

This configuration binds the application to `/yourapplication`.  If you want
to have it in the URL root it's a bit easier because you don't have to figure
out how to calculate `PATH_INFO` and `SCRIPT_NAME`::

    location /yourapplication/ {
        include fastcgi_params;
        fastcgi_param PATH_INFO $fastcgi_script_name;
        fastcgi_param SCRIPT_NAME "";
        fastcgi_pass unix:/tmp/yourapplication-fcgi.sock;
    }

Since Nginx doesn't load FastCGI apps, you have to do it by yourself.  You
can either write an `init.d` script for that or execute it inside a screen
session::

    $ screen
    $ /var/www/yourapplication/yourapplication.fcgi

Debugging
=========

FastCGI deployments tend to be hard to debug on most webservers.  Very often the
only thing the server log tells you is something along the lines of "premature
end of headers".  In order to debug the application the only thing that can
really give you ideas why it breaks is switching to the correct user and
executing the application by hand.

This example assumes your application is called `application.fcgi` and that your
webserver user is `www-data`::

    $ su www-data
    $ cd /var/www/yourapplication
    $ python application.fcgi
    Traceback (most recent call last):
      File "yourapplication.fcg", line 4, in <module>
    ImportError: No module named yourapplication

In this case the error seems to be "yourapplication" not being on the python
path.  Common problems are:

-   relative paths being used.  Don't rely on the current working directory
-   the code depending on environment variables that are not set by the
    web server.
-   different python interpreters being used.

.. _lighttpd: http://www.lighttpd.net/
.. _nginx: http://nginx.net/
.. _flup: http://trac.saddi.com/flup