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Appender::Synchronized(3pm) User Contributed Perl Documentation 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.
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 AT 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 AT perlmeister.com> and Kevin Goess <cpan AT 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 AT lists.net
Authors (please contact them via the list above, not directly): Mike Schilli
<m AT perlmeister.com>, Kevin Goess <cpan AT 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.
perl v5.32.1 2021-02-14 Appender::Synchronized(3pm)
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On Apache
Under GNU General Public License
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