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 oneLog::Log4perl::Appender::Synchronized
appender, it is important to specify different keys for them, as otherwise every newLog::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
andmylongkey2
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, Syncer
# 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.
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>