NAME

           Log::Log4perl::Appender::Synchronized - Synchronizing other appenders

SYNOPSIS

           use Log::Log4perl qw(:easy);

           my $conf = qq(
           log4perl.category                   = WARN, Syncer

               # File appender (unsynchronized)
           log4perl.appender.Logfile           = Log::Log4perl::Appender::File
           log4perl.appender.Logfile.autoflush = 1
           log4perl.appender.Logfile.filename  = test.log
           log4perl.appender.Logfile.mode      = truncate
           log4perl.appender.Logfile.layout    = SimpleLayout

               # Synchronizing appender, using the file appender above
           log4perl.appender.Syncer            = Log::Log4perl::Appender::Synchronized
           log4perl.appender.Syncer.appender   = Logfile
       );

           Log::Log4perl->init(\$conf);
           WARN("This message is guaranteed to be complete.");

DESCRIPTION

       If multiple processes are using the same "Log::Log4perl" appender without synchronization,
       overwrites might happen. A typical scenario for this would be a process spawning children,
       each of which inherits the parent's Log::Log4perl configuration.

       In most cases, you won't need an external synchronisation tool like
       Log::Log4perl::Appender::Synchronized at all. Log4perl's file appender,
       Log::Log4perl::Appender::File, for example, provides the "syswrite" mechanism for making
       sure that even long log lines won't interleave.  Short log lines won't interleave anyway,
       because the operating system makes sure the line gets written before a task switch occurs.

       In cases where you need additional synchronization, however, you can use
       "Log::Log4perl::Appender::Synchronized" as a gateway between your loggers and your
       appenders. An appender itself, "Log::Log4perl::Appender::Synchronized" just takes two
       additional arguments:

       "appender"
           Specifies the name of the appender it synchronizes access to. The appender specified
           must be defined somewhere in the configuration file, not necessarily before the
           definition of "Log::Log4perl::Appender::Synchronized".

       "key"
           This optional argument specifies the key for the semaphore that
           "Log::Log4perl::Appender::Synchronized" uses internally to ensure atomic operations.
           It defaults to "_l4p". If you define more than one
           "Log::Log4perl::Appender::Synchronized" appender, it is important to specify different
           keys for them, as otherwise every new "Log::Log4perl::Appender::Synchronized" appender
           will nuke previously defined semaphores. The maximum key length is four characters,
           longer keys will be truncated to 4 characters -- "mylongkey1" and "mylongkey2" are
           interpreted to be the same: "mylo" (thanks to David Viner <dviner@yahoo-inc.com> for
           pointing this out).

       "Log::Log4perl::Appender::Synchronized" uses Log::Log4perl::Util::Semaphore internally to
       perform locking with semaphores provided by the operating system used.

   Performance tips
       The "Log::Log4perl::Appender::Synchronized" serializes access to a protected resource
       globally, slowing down actions otherwise performed in parallel.

       Unless specified otherwise, all instances of "Log::Log4perl::Appender::Synchronized"
       objects in the system will use the same global IPC key "_l4p".

       To control access to different appender instances, it often makes sense to define
       different keys for different synchronizing appenders. In this way, Log::Log4perl
       serializes access to each appender instance separately:

           log4perl.category                   = WARN, Syncer1, Syncer2

               # File appender 1 (unsynchronized)
           log4perl.appender.Logfile1           = Log::Log4perl::Appender::File
           log4perl.appender.Logfile1.filename  = test1.log
           log4perl.appender.Logfile1.layout    = SimpleLayout

               # File appender 2 (unsynchronized)
           log4perl.appender.Logfile2           = Log::Log4perl::Appender::File
           log4perl.appender.Logfile2.filename  = test2.log
           log4perl.appender.Logfile2.layout    = SimpleLayout

               # Synchronizing appender, using the file appender above
           log4perl.appender.Syncer1            = Log::Log4perl::Appender::Synchronized
           log4perl.appender.Syncer1.appender   = Logfile1
           log4perl.appender.Syncer1.key        = l4p1

               # Synchronizing appender, using the file appender above
           log4perl.appender.Syncer2            = Log::Log4perl::Appender::Synchronized
           log4perl.appender.Syncer2.appender   = Logfile2
           log4perl.appender.Syncer2.key        = l4p2

       Without the ".key = l4p1" and ".key = l4p2" lines, both Synchronized appenders would be
       using the default "_l4p" key, causing unnecessary serialization of output written to
       different files.

   Advanced configuration
       To configure the underlying Log::Log4perl::Util::Semaphore module in a different way than
       with the default settings provided by Log::Log4perl::Appender::Synchronized, use the
       following parameters:

           log4perl.appender.Syncer1.destroy  = 1
           log4perl.appender.Syncer1.mode     = sub { 0775 }
           log4perl.appender.Syncer1.uid      = hugo
           log4perl.appender.Syncer1.gid      = 100

       Valid options are "destroy" (Remove the semaphore on exit), "mode" (permissions on the
       semaphore), "uid" (uid or user name the semaphore is owned by), and "gid" (group id the
       semaphore is owned by),

       Note that "mode" is usually given in octal and therefore needs to be specified as a perl
       sub {}, unless you want to calculate what 0755 means in decimal.

       Changing ownership or group settings for a semaphore will obviously only work if the
       current user ID owns the semaphore already or if the current user is "root". The "destroy"
       option causes the current process to destroy the semaphore on exit. Spawned children of
       the process won't inherit this behavior.

   Semaphore user and group IDs with mod_perl
       Setting user and group IDs is especially important when the Synchronized appender is used
       with mod_perl. If Log4perl gets initialized by a startup handler, which runs as root, and
       not as the user who will later use the semaphore, the settings for uid, gid, and mode can
       help establish matching semaphore ownership and access rights.

DEVELOPMENT NOTES

       "Log::Log4perl::Appender::Synchronized" is a composite appender.  Unlike other appenders,
       it doesn't log any messages, it just passes them on to its attached sub-appender.  For
       this reason, it doesn't need a layout (contrary to regular appenders).  If it defines
       none, messages are passed on unaltered.

       Custom filters are also applied to the composite appender only.  They are not applied to
       the sub-appender. Same applies to appender thresholds. This behaviour might change in the
       future.

LICENSE

       Copyright 2002-2013 by Mike Schilli <m@perlmeister.com> and Kevin Goess <cpan@goess.org>.

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

AUTHOR

       Please contribute patches to the project on Github:

           http://github.com/mschilli/log4perl

       Send bug reports or requests for enhancements to the authors via our

       MAILING LIST (questions, bug reports, suggestions/patches):
       log4perl-devel@lists.sourceforge.net

       Authors (please contact them via the list above, not directly): Mike Schilli
       <m@perlmeister.com>, Kevin Goess <cpan@goess.org>

       Contributors (in alphabetical order): Ateeq Altaf, Cory Bennett, Jens Berthold, Jeremy
       Bopp, Hutton Davidson, Chris R. Donnelly, Matisse Enzer, Hugh Esco, Anthony Foiani, James
       FitzGibbon, Carl Franks, Dennis Gregorovic, Andy Grundman, Paul Harrington, Alexander
       Hartmaier  David Hull, Robert Jacobson, Jason Kohles, Jeff Macdonald, Markus Peter, Brett
       Rann, Peter Rabbitson, Erik Selberg, Aaron Straup Cope, Lars Thegler, David Viner, Mac
       Yang.