Appender::SynchronizedUser Contributed Perl DocumentaAppender::Synchronized(3)NAMELog::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@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.
COPYRIGHT AND LICENSE
Copyright 2002-2009 by Mike Schilli <m@perlmeister.com> and Kevin Goess
<cpan@goess.org>.
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.14.1 2011-05-02 Appender::Synchronized(3)
