FastCGI "is a binary protocol for interfacing interactive programs with a web server" (from Wikipedia).

In the same vain as my nginx in 15 minutes post, I thought i'd outline FastCGI, and its implementation on nginx with PHPFPM.

A FastCGI server is independent of your web server. You delegate your request to it, it processes it, and returns a response.

FastCGI is a protocol. An implementation of said protocol can be written in any language. PHPFPM is a process manager that implements the FastCGI protocol with a number of optimizations. It is now part of the PHP core and is well used on the web.

Whereas previous incarnations of CGI created a new request per process the FastCGI protocol processes multiple requests within the same process (multiplexing). This allows for concurrency and handling of higher loads.

The FastCGI protocol seeks to resolves many similar issues in the CGI protocol that nginx seeks to resolve in earlier versions of Apache.

nginx

nginx integrates with the FastCGI protocol throught its fastcgi module. That is to say it knows how to interface with a FastCGI server that implements the FastCGI protocol. This makes connecting to a FastCGI server extremely simple.

Communication occurs via interprocess communication (IPC). For a simple setup you can connect to a unix socket. For a more complex setup you might want to communicate with multiple servers using TCP sockets.

The most important fastcgi_param is SCRIPT_FILENAME which indicates as to where on the filesystem a file should be loaded for a particular request.

I choose to define my server root as follows:

set $root_path '/path/to/files';
root $root_path;

I then utilize the $root variable within my location blocks for static image files as well as my location block for passing php files to my FastCGI server.

Within the latter block I use:

fastcgi_param SCRIPT_FILENAME $root_path$fastcgi_script_name;

This maps a request for myfile.php to /path/to/files/myfile.php

You can utilize the fastcgi_split_path_info directive to allow for customized URL structures.

As long as you specify a regular expression with two capturing blocks you could direct a request to http://domain.com/request/my_file/hash to /path/to/files/myfile.php with relative ease.

You are only limited by your knowledge of regular expressions :)

Another interesting tidbit regarding the nginx integration is fastcgi_intercept_errors on.

This directive allows for a response from the FastCGI server to be directed to the appropriate location block based on your defined error_page directive.

Through this directive it is easy to display a custom 404 page for example should FastCGI return an error response.

PHPFPM

PHPFPM is a "process manager to manage the FastCGI SAPI (Server API) in PHP" (source).

In essence, what that means is that PHPFPM manages the creation of PHP processes (as required) to process the requests sent to it by the webserver.

It implements the FastCGI protocol such that for example, data sent to it as a fastcgi_param (when using nginx) can be processed and utilized appropriately.

When running PHPFPM on a local machine (server) it 'runs' the server to which one directs their PHP requests.

For usage with nginx you would connect to the server through a unix socket using the fastcgi_pass directive (as outlined above).

fastcgi_pass unix:/var/run/php5-fpm.sock;

Summary

nginx, PHP, and FastCGI are a power combination in web application development because they integrate so seemlessly together with one another.

Configuration is simple, and they allow for deployment of dynamic websites that can scale with ease.