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<?xml version='1.0' encoding='UTF-8' ?>
<!DOCTYPE manualpage SYSTEM "./style/manualpage.dtd">
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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>
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