fastCGI

FastCGI is a binary communication protocol designed to interface web servers with applications, created as a faster alternative to the traditional Common Gateway Interface (CGI). It enables long‑lived application processes and multiplexed request handling, reducing the overhead of starting a new process for every request and allowing multiple requests to be processed concurrently.

In operation, a web server communicates with a FastCGI application over a socket, typically a TCP or Unix domain socket. The server sends request data in a sequence of records, including environment variables (FCGI_PARAMS) and the request body (FCGI_STDIN). The application processes the request and returns output through FCGI_STDOUT, followed by FCGI_END_REQUEST to signal completion. The web server then translates the returned data into the HTTP response sent to the client. The protocol is language‑agnostic, enabling software written in many languages to work with any compatible server.

Deployment commonly involves a web server such as Nginx or Apache connected to a FastCGI application or process manager. Popular configurations use PHP‑FPM (FastCGI Process Manager) to handle PHP requests, while other languages provide their own FastCGI runtimes or libraries (for example, Perl, Python, and Ruby implementations). A FastCGI process pool runs as persistent workers, which improves throughput and latency compared with spawning new processes per request.

Key features and considerations include the ability to multiplex multiple requests on a single connection (optional and not universally supported), and the protocol’s emphasis on performance and language neutrality. Security considerations include proper isolation and permissions for sockets and processes. FastCGI remains a widely used approach in scalable web deployments, especially in setups where persistent application processes are desirable.