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MAN page from RedHat EL 8 perl-Log-Log4perl-1.50-1.el8.noarch.rpm

Appender::Synchronized

Section: User Contributed Perl Documentation (3)
Updated: 2020-07-22
Index 

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 scenariofor this would be a process spawning children, each of which inheritsthe parent's Log::Log4perl configuration.

In most cases, you won't need an external synchronisation tool likeLog::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 systemmakes 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 yourloggers and your appenders. An appender itself, "Log::Log4perl::Appender::Synchronized" just takes two additionalarguments:

appender
Specifies the name of the appender it synchronizes access to. Theappender 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 ensureatomic operations. It defaults to "_l4p". If you define more thanone "Log::Log4perl::Appender::Synchronized" appender, it is important to specify different keys for them, as otherwise everynew "Log::Log4perl::Appender::Synchronized" appender will nukepreviously defined semaphores. The maximum key length is fourcharacters, longer keys will be truncated to 4 characters --- "mylongkey1" and "mylongkey2" are interpreted to be the same:"mylo" (thanks to David Viner <dvinerAATTyahoo-inc.com> forpointing this out).

"Log::Log4perl::Appender::Synchronized" uses Log::Log4perl::Util::Semaphoreinternally to perform locking with semaphores provided by theoperating system used. 

Performance tips

The "Log::Log4perl::Appender::Synchronized" serializes access to aprotected resource globally, slowing down actions otherwise performed inparallel.

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

To control access to different appender instances, it often makes senseto define different keys for different synchronizing appenders. In thisway, 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 unnecessaryserialization 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 bespecified as a perl sub {}, unless you want to calculate what 0755 meansin decimal.

Changing ownership or group settings for a semaphore will obviously onlywork if the current user ID owns the semaphore already or if the currentuser is "root". The "destroy" option causes the current process to destroy the semaphore on exit. Spawned children of the process won'tinherit this behavior. 

Semaphore user and group IDs with mod_perl

Setting user and group IDs is especially important when the Synchronizedappender is used with mod_perl. If Log4perl gets initialized by a startuphandler, which runs as root, and not as the user who will later usethe 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 justpasses 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 appenderthresholds. This behaviour might change in the future. 

LICENSE

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

This library is free software; you can redistribute it and/or modifyit 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-develAATTlists.sourceforge.net

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

Contributors (in alphabetical order):Ateeq Altaf, Cory Bennett, Jens Berthold, Jeremy Bopp, HuttonDavidson, Chris R. Donnelly, Matisse Enzer, Hugh Esco, AnthonyFoiani, James FitzGibbon, Carl Franks, Dennis Gregorovic, AndyGrundman, 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.


 

Index

NAME
SYNOPSIS
DESCRIPTION
Performance tips
Advanced configuration
Semaphore user and group IDs with mod_perl
DEVELOPMENT NOTES
LICENSE
AUTHOR

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