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Apache > HTTP Server > Documentation > Version 2.4

Multi-Processing Modules (MPMs)

This document describes what a Multi-Processing Module is and how they are used by the Apache HTTP Server.

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Introduction

The Apache HTTP Server is designed to be a powerful and flexible web server that can work on a very wide variety of platforms in a range of different environments. Different platforms and different environments often require different features, or may have different ways of implementing the same feature most efficiently. Apache httpd has always accommodated a wide variety of environments through its modular design. This design allows the webmaster to choose which features will be included in the server by selecting which modules to load either at compile-time or at run-time.

Apache HTTP Server 2.0 extends this modular design to the most basic functions of a web server. The server ships with a selection of Multi-Processing Modules (MPMs) which are responsible for binding to network ports on the machine, accepting requests, and dispatching children to handle the requests.

Extending the modular design to this level of the server allows two important benefits:

At the user level, MPMs appear much like other Apache httpd modules. The main difference is that one and only one MPM must be loaded into the server at any time. The list of available MPMs appears on the module index page.

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MPM Defaults

The following table lists the default MPMs for various operating systems. This will be the MPM selected if you do not make another choice at compile-time.

Netwarempm_netware
OS/2mpmt_os2
Unixprefork, worker, or event, depending on platform capabilities
Windowsmpm_winnt
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Building an MPM as a static module

MPMs can be built as static modules on all platforms. A single MPM is chosen at build time and linked into the server. The server must be rebuilt in order to change the MPM.

To override the default MPM choice, use the --with-mpm=NAME option of the configure script. NAME is the name of the desired MPM.

Once the server has been compiled, it is possible to determine which MPM was chosen by using ./httpd -l. This command will list every module that is compiled into the server, including the MPM.

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Building an MPM as a DSO module

On Unix and similar platforms, MPMs can be built as DSO modules and dynamically loaded into the server in the same manner as other DSO modules. Building MPMs as DSO modules allows the MPM to be changed by updating the LoadModule directive for the MPM instead of by rebuilding the server.

This feature is enabled using the --enable-mpms-shared option of the configure script. With argument all, all possible MPMs for the platform will be installed. Alternately, a list of MPMs can be specified as the argument.

The default MPM, either selected automatically or specified with the --with-mpm option of the configure script, will be loaded in the generated server configuration file. Edit the LoadModule directive to select a different MPM.