man Log::Log4perl::Appender::Synchronized () - Synchronizing other appenders


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


    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

    WARN("This message is guaranteed to be complete.");


If multiple processes are using the same CWLog::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 CWLog::Log4perl::Appender::Synchronized as a gateway between your loggers and your appenders. An appender itself, CWLog::Log4perl::Appender::Synchronized just takes two additional arguments: 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 CWLog::Log4perl::Appender::Synchronized. This optional argument specifies the key for the semaphore that CWLog::Log4perl::Appender::Synchronized uses internally to ensure atomic operations. It defaults to CW_l4p. If you define more than one CWLog::Log4perl::Appender::Synchronized appender, it is important to specify different keys for them, as otherwise every new CWLog::Log4perl::Appender::Synchronized appender will nuke previously defined semaphores. The maximum key length is four characters, longer keys will be truncated to 4 characters CWmylongkey1 and CWmylongkey2 are interpreted to be the same: CWmylo (thanks to David Viner <> for pointing this out).

CWLog::Log4perl::Appender::Synchronized uses CWIPC::Shareable internally to perform locking with semaphores provided by the operating system used.

Performance tips

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

Unless specified otherwise, all instances of CWLog::Log4perl::Appender::Synchronized objects in the system will use the same global IPC key CW_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 CW.key = l4p1 and CW.key = l4p2 lines, both Synchronized appenders would be using the default CW_l4p key, causing unnecessary serialization of output written to different files.


CWLog::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.


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.


2003, Mike Schilli <>