apache/docs/manual/mpm.xml

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<manualpage metafile="mpm.xml.meta">

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

<summary>
<p>This document describes what a Multi-Processing Module is and
how they are used 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 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.</p>

    <p>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.</p>

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

    <ul>
      <li>Apache httpd can more cleanly and efficiently support a wide
      variety of operating systems. In particular, the Windows
      version of the server is now much more efficient, since
      <module>mpm_winnt</module> can use native
      networking features in place of the POSIX layer used in
      Apache httpd 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> or <module>event</module>, while sites requiring
      stability or compatibility with older software can use a
      <module>prefork</module>.</li>
    </ul>

    <p>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 <a href="mod/">module index page</a>.</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 border="1" style="zebra">
<columnspec><column width=".2"/><column width=".2"/></columnspec>
<tr><td>Netware</td><td><module>mpm_netware</module></td></tr>
<tr><td>OS/2</td><td><module>mpmt_os2</module></td></tr>
<tr><td>Unix</td><td><module>prefork</module>, <module>worker</module>, or
    <module>event</module>, depending on platform capabilities</td></tr>
<tr><td>Windows</td><td><module>mpm_winnt</module></td></tr>
</table>

<note><p>Here, 'Unix' is used to mean Unix-like operating systems, such as
Linux, BSD, Solaris, Mac OS X, etc.</p></note>

<p>In the case of Unix, the decision as to which MPM is installed is
based on two questions:</p>
<p>1. Does the system support threads?</p>
<p>2. Does the system support thread-safe polling (Specifically, the
kqueue and epoll functions)?</p>

<p>If the answer to both questions is 'yes', the default MPM is
<module>event</module>.</p>

<p>If The answer to #1 is 'yes', but the answer to #2 is 'no', the
default will be <module>worker</module>.</p>

<p>If the answer to both questions is 'no', then the default MPM will be
<module>prefork</module>.</p>

<p>In practical terms, this means that the default will almost always be
<module>event</module>, as all modern operating systems support these
two features.</p>

</section>

<section id="static"><title>Building an MPM as a static module</title>

    <p>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.</p>

    <p>To override the default MPM choice, use the
    <code>--with-mpm=<em>NAME</em></code> option of the
    <program>configure</program> 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="dynamic"><title>Building an MPM as a DSO module</title>

    <p>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 <directive module="mod_so">LoadModule</directive> directive
    for the MPM instead of by rebuilding the server.</p>

    <highlight language="config">
    LoadModule mpm_prefork_module modules/mod_mpm_prefork.so
    </highlight>

    <p>Attempting to <directive module="mod_so">LoadModule</directive>
    more than one MPM will result in a startup failure with the
    following error.</p>

    <example>AH00534: httpd: Configuration error: More than one MPM
    loaded.</example>

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

    <p>The default MPM, either selected automatically or specified with the
    <code>--with-mpm</code> option of the <program>configure</program>
    script, will be loaded in the generated server configuration file.  Edit the
    <directive module="mod_so">LoadModule</directive> directive to select a
    different MPM.</p>

</section>

</manualpage>