'Multi Processing Modules', generally called MPMs, are modules that get the primary attention in Apache. Apache has been known for its extensibility through modules and this is one of the main reason that it has been favoured worldwide, apart from its rock stabilty. Modules that can be added at a later stage or when in need makes Apache more stable due to the decreased load.
MPMs are supposed to do the rigorous work of binding to the port specified, accepts the connection requests, generate the child processes according to the load of the server and dispatch the children for the incoming connections. They are loaded along with 'httpd' at startup time. Many a MPM exist, but one and only one can exist in a running Apache installation. The default MPM for Unix is the 'Prefork' module. The default MPMs which exist for other platforms are :
BeOS : beos
Netware : mpm_netware
OS/2 : mpmt_os2
Windows : mpm_winnt
The main difference between MPMs and normal modules is that only one of the former can be used and multiple ones can be loaded in the latter. MPMs must be chosen during install and can be compiled into the binary using the '--with-mpm=NAME' option. If any of the MPMs are not specified, then the default MPM, 'prefork', will be compiled. Apache in Windows is now more efficient since it does not need to use the POSIX compliance and can use the native networking features of the OS. In the Windows environment,the MPM 'mpm_winnt' is used as the default.
Two of the MPMs specified in 'httpd.conf' are 'prefork' and 'worker'. These two MPMs exists for different specifications. The 'worker' MPM was introduced in Apache2. It uses a multiprocess-multithreaded structure. Multi-process means the number of child servers started and multi-threads, the number of connections made by each child-process. The child servers starts the threads according to the directives 'ThreadsPerChild', 'MinSpareThreads' and 'MaxSpareThreads'. By using a threaded structure, each child server can handle more than one connection, upto the limit specified by 'MaxSpareThreads'. The parent process is responsible for starting the child processes. The child instances inturn starts the number of threads specified by 'ThreadsPerChild ' and one additional thread for listening to incoming requests. The main drawback is that it makes more demand on your virtual memory and since one child server handles more than one thread (each thread equals one connection), anything that effects a particular child process has the same effect on the connections. In short, one crashed child process means more than one lost connection. But in the case of 'prefork' module, the concept of threads doesn't exist. A seperate child process get started for each incoming connection, provided within the limit specified. This concept is more geared towards stability since each child process has to handle only its own connection.
'Multi Processing Modules' are just a small part of the really big world of modules in Apache. More about module configuration in the next part. More to come.