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    prefork - Optimized module loading for forking or non-forking processes

    In a module that normally delays module loading with require

      # Module Foo::Bar only uses This::That 25% of the time.
      # We want to preload in in forking scenarios (like mod_perl), but
      # we want to delay loading in non-forking scenarios (like CGI)
      use prefork 'This::That';
  sub do_something {
          my $arg = shift;
      # Load the module at run-time as normal
          if ( $special_case ) {
              require This::That;
  # Register a module to be loaded before forking directly

    In a script or module that is going to be forking.

      package Module::Forker;
  # Enable forking mode
      use prefork ':enable';
  # Or call it directly

    In a third-party run-time loader

      package Runtime::Loader;
  use prefork ();
      prefork::notify( \&load_everything );
  sub load_everything { ... }

    The task of optimizing module loading in Perl tends to move in two
    different directions, depending on the context.

    In a procedural context, such as scripts and CGI-type situations, you
    can improve the load times and memory usage by loading a module at
    run-time, only once you are sure you will need it.

    In the other common load profile for perl applications, the application
    will start up and then fork off various worker processes. To take full
    advantage of memory copy-on-write features, the application should load
    as many modules as possible before forking to prevent them consuming
    memory in multiple worker processes.

    Unfortunately, the strategies used to optimise for these two load
    profiles are diametrically opposed. What improves a situation for one
    tends to make life worse for the other.

    The "prefork" pragma is intended to allow module writers to optimise
    module loading for both scenarios with as little additional code as
    possible. is intended to serve as a central and optional marshalling
    point for state detection (are we running in compile-time or run-time
    mode) and to act as a relatively light-weight module loader.

  Loaders and Forkers
    "prefork" is intended to be used in two different ways.

    The first is by a module that wants to indicate that another module
    should be loaded before forking. This is known as a "Loader".

    The other is a script or module that will be initiating the forking. It
    will tell that it is either going to fork, or is about to
    fork, or for some other reason all modules previously mentioned by the
    Loaders should be loaded immediately.

  Usage as a Pragma
    A Loader can register a module to be loaded using the following

      use prefork 'My::Module';

    The same thing can be done in such a way as to not require prefork being
    installed, but taking advantage of it if it is.

      eval "use prefork 'My::Module';";

    A Forker can indicate that it will be forking with the following

      use prefork ':enable';

    In any use of "prefork" as a pragma, you can only pass a single value as
    argument. Any additional arguments will be ignored. (This may throw an
    error in future versions).

  Compatbility with mod_perl and others
    Part of the design of "prefork", and its minimalistic nature, is that it
    is intended to work easily with existing modules, needing only small

    For example, "prefork" itself will detect the $ENV{MOD_PERL} environment
    variable and automatically start in forking mode.

    prefork has support for integrating with third-party modules, such as
    Class::Autouse. The "notify" function allows these run-time loaders to
    register callbacks, to be called once prefork enters forking mode.

    The synopsis entry above describes adding support for as a
    dependency. To allow your third-party module loader without a dependency
    and only if it is installed use the following:

      eval { require prefork; }
      prefork::notify( \&function ) unless $@;

    From the Loader side, it is fairly simple. prefork becomes a dependency
    for your module, and you use it as a pragma as documented above.

    For the Forker, you have two options. Use as a dependency or optional

    In the dependency case, you add prefork as a dependency and use it as a
    pragma with the ':enable' option.

    To add only optional support for prefork, without requiring it to be
    installed, you should wait until the moment just before you fork and
    then call "prefork::enable" directly ONLY if it is loaded.

      # Load modules if any use the prefork pragma.
      prefork::enable() if $INC{};

    This will cause the modules to be loaded ONLY if there are any modules
    that need to be loaded. The main advantage of the dependency version is
    that you only need to enable the module once, and not before each fork.

    If you wish to have your own module leverage off the forking-detection
    that prefork provides, you can also do the following.

      use prefork;
      if ( $prefork::FORKING ) {
          # Complete some preparation task

  Modules that are prefork-aware

  prefork $module
    The 'prefork' function indicates that a module should be loaded before
    the process will fork. If already in forking mode the module will be
    loaded immediately.

    Otherwise it will be added to a queue to be loaded later if it recieves
    instructions that it is going to be forking.

    Returns true on success, or dies on error.

    The "enable" function indicates to the prefork module that the process
    is going to fork, possibly immediately.

    When called, will immediately load all outstanding modules,
    and will set a flag so that any further 'prefork' calls will load the
    module at that time.

    Returns true, dieing as normal is there is a problem loading a module.

  notify &function
    The "notify" function is used to integrate support for modules other
    than itself.

    A module loader calls the notify function, passing it a reference to a
    "CODE" reference (either anon or a function reference). "prefork" will
    store this CODE reference, and execute it immediately as soon as it
    knows it is in forking-mode, but after it loads its own modules.

    Callbacks are called in the order they are registered.

    Normally, this will happen as soon as the "enable" function is called.

    However, you should be aware that if prefork is already in preforking
    mode at the time that the notify function is called, will
    execute the function immediately.

    This means that any third party module loader should be fully loaded and
    initialised before the callback is provided to "notify".

    Returns true if the function is stored, or dies if not passed a "CODE"
    reference, or the callback is already set in the notify queue.

    - Add checks for more pre-forking situations

    Bugs should be always submitted via the CPAN bug tracker, located at


    For other issues, or commercial enhancement or support, contact the

    Adam Kennedy <>

    Thank you to Phase N Australia (<>) for permitting
    the open sourcing and release of this distribution.

    Copyright 2004 - 2009 Adam Kennedy.

    This program is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.

    The full text of the license can be found in the LICENSE file included
    with this module.