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@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.
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.