To address the scalability shortcomings of CGI, Open Market developed FastCGI and first introduced it in their webserver product in the mid-1990s. Open Market originally developed FastCGI in part as a competitive response to Netscape's proprietary, in-process API (NSAPI) for developing Web applications.
Although initially developed by Open Market, FastCGI was implemented by a number of other webserver makers. The FastCGI approach, however, competed against other techniques which also aimed to speed and simplify server-subprogram communications. Apache modules such as mod_perl and mod_php appeared around the same time, and they also quickly gained popularity. Today, all of these various approaches (including CGI) remain in common use.
To service an incoming request, the web server sends Environment information and the page request itself to a FastCGI process over a socket (in the case of local FastCGI processes on the web server) or TCP connection (for remote FastCGI processes in a server farm). Responses are returned from the process to the web server over the same connection, and the web server subsequently delivers that response to the end-user. The connection may be closed at the end of a response, but both the web server and the FastCGI service processes persist.
Each individual FastCGI process can handle many requests over its lifetime, thereby avoiding the overhead of per-request process creation and termination. Processing of multiple requests simultaneously can be achieved in several ways: by using a single connection with internal multiplexing (ie. multiple requests over a single connection); by using multiple connections; or by a combination of these techniques. Multiple FastCGI servers can be configured, increasing stability and scalability.
Web site administrators and programmers can find that the separation of web applications from the web server in FastCGI has many advantages over embedded interpreters (mod_perl, mod_php, etc.). This separation allows server and application processes to be restarted independently – an important consideration for busy web sites. It also enables the implementation of per-application / hosting service security policies, which is an important requirement for ISPs and web hosting companies. Different types of incoming requests can be distributed to specific FastCGI servers which have been equipped to handle those particular types of requests efficiently.
Recent frameworks such as Ruby on Rails, Catalyst, Django and Kepler (etc...) allow use with either the embedded interpreters (mod_ruby, mod_perl, mod_python or mod_lua, for example), or FastCGI.
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