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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> |