[log4perl-devel] Log::Log4perl Recipe of the Week (4)

[<msc...@ao...>]

Welcome to the Log::Log4perl recipe of the week. Today, we'll talk about customizing your loggers to include global data in all of your log messages:

Say, you're writing a web application and want all
your log messages to include the current client's
IP address. Most certainly, you don't want to
include it in each and every log message like in

    $logger->debug( $r->connection->remote_ip,
                    " Retrieving user data from DB" );

do you? Instead, you want to set it in a global 
data structure and have Log::Log4perl include it
automatically via a PatternLayout setting in the
configuration file:

    log4perl.appender.FileApp.layout.ConversionPattern = \   
      %X{ip} %m%n

The conversion specifier %X{ip} references an 
entry under the key "ip" in the global "MDC"
(mapped diagnostic context) table, which you've
set once via

    Log::Log4perl::MDC->put("ip", $r->connection->remote_ip);

at the start of the request handler. Note that 
this is a *static* (class) method, there's no
logger object involved. You can use this method 
with as many key/value pairs as you like as long
as you reference them under different names.

The mappings are stored in a global hash table 
within Log::Log4perl. Luckily, because the thread
model in 5.8.0 doesn't share global variables
between threads unless they're explicitly marked
as such, there's no problem with multi-threaded
environments like mod_perl.

For more details on the MDC, please refer to
the "Mapped Diagnostic Context (MDC)" section in the 
Log::Log4perl manpage and Log::Log4perl::MDC.

As always, this recipe is going to be archived in the Log::Log4perl FAQ.

Have fun!

-- 
-- Mike

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# Mike Schilli             #
# log...@pe... #
# http://perlmeister.com   #
# log4perl.sourceforge.net #
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