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 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 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 (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.