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Log::Log4perl::Appender::Synchronized.3pm

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.

Usually, you should avoid this scenario and have each child have its own Log::Log4perl configuration, ensuring that each e.g. writes to a different logfile.

In cases where you need additional synchronization, however, 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 "IPC::Shareable" 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 IPC::Shareable module, use the options property and hand it a reference to a hash of options.

By default, the setting is equal to

       { create=>1, destroy=>1 }
 
 

which has the appender create the semaphore at startup and remove it at shutdown.

In order to add another option, e.g. a setting of 0775 for IPC::Shareable's "mode" parameter, add

     log4perl.appender.Syncer1.options  = \
       sub { { create=>1, destroy=>1, mode=>0775 } }
 
 

to the Log4perl configuration file.

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.

LEGALESE

Copyright 2003 by Mike Schilli, all rights reserved. This program is free software, you can redistribute it and/or modify it under the same terms as Perl itself.

AUTHOR

2003, Mike Schilli <m@perlmeister.com>