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Multi-Processing Modules (MPMs) - Apache HTTP Server Version 2.4








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Apache > HTTP Server > Documentation > Version 2.4Multi-Processing Modules (MPMs)

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

 Introduction
 MPM Defaults
 Building an MPM as a static module
 Building an MPM as a DSO module
See alsoComments


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:

    
      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
      mpm_winnt 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.

      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
      worker or event, while sites requiring
      stability or compatibility with older software can use a
      prefork.
    

    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.



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


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

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

If the answer to both questions is 'yes', the default MPM is
event.

If The answer to #1 is 'yes', but the answer to #2 is 'no', the
default will be worker.

If the answer to both questions is 'no', then the default MPM will be
prefork.

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



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.



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.

    LoadModule mpm_prefork_module modules/mod_mpm_prefork.so


    Attempting to LoadModule
    more than one MPM will result in a startup failure with the
    following error.

    AH00534: httpd: Configuration error: More than one MPM
    loaded.

    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.



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CommentsNotice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.

Copyright 2016 The Apache Software Foundation.Licensed under the Apache License, Version 2.0.
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