Multiple Process Modules(MPM)

The original reason for creating Apache 2.0 was scalability, and the first solution was a hybrid web server; one that has both processes and threads. This solution provides the reliability that comes with not having everything in one process, combined with the scalability that threads provide. The problem with this is that there is no perfect way to map requests to either a thread or a process.

On platforms such as like Linux, it is best to have multiple processes each with multiple threads serving the requests so that if a single thread dies, the rest of the server will continue to serve more requests. Other platforms such as Windows don't handle multiple processes well, so one process with multiple threads is required. Older platforms which do not have threads also had to be taken into account. For these platforms, it is necessary to continue with the 1.3 method of pre-forking processes to handle requests.

There are multiple ways to deal with the mapping issue, but the cleanest is to enhance the module features of Apache. Apache 2.0 sees the introduction of 'Multiple-Processing Modules' (MPMs) - modules which determine how requests are mapped to threads or processes. The majority of users will never write an MPM or even know they exist. Each server uses a single MPM, and the correct one for a given platform is determined at compile time.

References:
1.Multi-Processing Modules
http://httpd.apache.org/docs/2.2/mpm.html