FastCGI is a protocol for interfacing interactive programs with a web server. FastCGI’s main aim is to reduce the overhead associated with interfacing the web server and CGI programs, allowing a server to handle more web page requests at once.
Also, PHP is not recommended with multithreaded Apache2 (worker MPM) because of performance and some 3rd party PHP extensions are not not guaranteed thread-safe.
Why use mod_fastcgi instead of mod_perl / mod_php?
From the wikipedia article:
Instead of creating a new process for every request, FastCGI can use a single persistent process which handles many requests over its lifetime. Processing of multiple requests simultaneously is achieved either by using a single connection with internal multiplexing (ie. multiple requests over a single connection) and/or by using multiple connections. Many such processes can exist, something that can increase stability and scalability. FastCGI also allows programs to get the web server to do certain simple operations, like reading in a file, before the request is handed over. Environment information and page requests are sent from the web server to the process over a TCP connection (for remote processes) or Unix domain sockets (for local processes). Responses are returned from the process to the web server over the same connection. The connection may be closed at the end of a response, but the web server and the process are left standing.
Many web site administrators and programmers are finding that the separation of web applications from the web server in FastCGI (and the simpler SCGI) has many desirable 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 facilitates per-application security policies — important for ISPs and web hosting companies.
In this quick tutorial, you will learn about Apache 2 + mod_fastcgi + PHP installation and configuration under Red Hat Enterprise Linux / CentOS Linux version 5.x+.
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