FastCGI is a hybrid solution to serving web applications written in a wide variety of programming languages. It sits somewhere between CGI, which spawns a new instance of the web application for each request, and the various web server modules (such as mod_php, mod_python and mod_wsgi) which take care of pre-spawning a pool of interpreters and web applications from within the web server, which will then handle requests.
FastCGI, too, can take care of spawning a pool of web application instances to handle requests. It can also facilitate communications between a web server and an external, already running web application. Unlike CGI, FastCGI does not spawn a new process for each request, and unlike the various web server modules it is not completely embedded within the web server. Instead it uses TCP/IP or Unix sockets to communicate to the web application. This makes it possible to create advanced setups such as spreading out requests over multiple servers, limiting web applications' rights and system resources using the standard Unix tool-set, etc.
FastCGI is available for a large range of web servers, is fast and is powerful and versatile in its capabilities. It is however not well documented, nor easy to set up. This document covers the basic idea behind FastCGI, setting up FastCGI for Apache (v2) and hooking it up to a simple Python web application. Such a light-weight setup, which requires nothing more than a FastCGI Python library, can provide an alternative Python web development environment for people who feel that the Python web development frameworks (Django, Pylons, TurboGears and even Web.py) are too bloated.