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  <title>Multi-Processing Modules (MPMs)</title>

<summary>
<p>This document describes what a Multi-Processing Module is and
how they are using by the Apache HTTP Server.</p>
</summary>

<section id="introduction"><title>Introduction</title>

    <p>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 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.</p>

    <p>Apache 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.</p>

    <p>Extending the modular design to this level of the server
    allows two important benefits:</p>

    <ul>
      <li>Apache can more cleanly and efficiently support a wide
      variety of operating systems. In particular, the Windows
      version of Apache is now much more efficient, since
      <module>mpm_winnt</module> can use native
      networking features in place of the POSIX layer used in
      Apache 1.3. This benefit also extends to other operating
      systems that implement specialized MPMs.</li>

      <li>The server can be better customized for the needs of the
      particular site. For example, sites that need a great deal of
      scalability can choose to use a threaded MPM like 
      <module>worker</module>, while sites requiring
      stability or compatibility with older software can use a
      <module>prefork</module>. In addition,
      special features like serving different hosts under different
      userids (<module>perchild</module>) can be
      provided.</li>
    </ul>

    <p>At the user level, MPMs appear much like other Apache
    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 <a href="mod/">module index page</a>.</p>

</section>

<section id="choosing"><title>Choosing an MPM</title>

    <p>MPMs must be chosen during configuration, and compiled into
    the server. Compilers are capable of optimizing a lot of
    functions if threads are used, but only if they know that
    threads are being used. Because some MPMs use threads on Unix
    and others don't, Apache will always perform better if the MPM
    is chosen at configuration time and built into Apache.</p>

    <p>To actually choose the desired MPM, use the argument
    --with-mpm= <em>NAME</em> with the ./configure script.
    <em>NAME</em> is the name of the desired MPM.</p>

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

<section id="defaults"><title>MPM Defaults</title>

<p>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.</p>

<table>
<tr><td>BeOS</td><td><module>beos</module></td></tr>
<tr><td>OS/2</td><td><module>mpmt_os2</module></td></tr>
<tr><td>Unix</td><td><module>prefork</module></td></tr>
<tr><td>Windows</td><td><module>mpm_winnt</module></td></tr>
</table>
</section>

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