Apache HTTP Server - traditional non-threaded model

 Each Apache Multi-Processing Module provides a different "flavor" of
 web server binary, compiled with a different processing model.
 .
 The prefork MPM provides a non-threaded implementation using a variable
 number of processes where each process handles only one connection
 at the same time. It is not as fast as threaded models, but is considered to
 be more stable. It is appropriate for sites that need to maintain
 compatibility with non-thread-safe libraries, and is the best MPM for
 isolating each request, so that a problem with a single request will not
 affect any other.