Re: [log4perl-devel] Log::Log4perl MDC at object level

[Mike S. <m...@pe...>]

On Wed, 12 Nov 2008, Anatoliy wrote:

> I'm going to use POE library for performing multitasks and I'd like to
> use POEs session specific values in the MDC.
>
> POE doesn't use threads/processes, so I can't make it with global variables.
> Is there any thing equal to MDC but for using with $logger object? not
> Log::Log4perl class. Thank you for Log::Log4perl anyway :)

You're welcome, and that's a very good question, I've added an anwer to
the FAQ and made some changes to Log4perl:

=head2 How can I access POE heap values from Log4perl's layout?

POE is a framework for creating multitasked applications running in a
single process and a single thread. POE's threads equivalents are
'sessions' and since they run quasi-simultaneously, you can't use
Log4perl's global NDC/MDC to hold session-specific data.

However, POE already maintains a data store for every session. It is called
'heap' and is just a hash storing session-specific data in key-value pairs.
To access this per-session heap data from a Log4perl layout, define a
custom cspec and reference it with the newly defined pattern in the layout:

     use strict;
     use POE;
     use Log::Log4perl qw(:easy);

     Log::Log4perl->init( \ q{
         log4perl.logger = DEBUG, Screen
         log4perl.appender.Screen = Log::Log4perl::Appender::Screen
         log4perl.appender.Screen.layout = PatternLayout
         log4perl.appender.Screen.layout.ConversionPattern = %U %m%n
         log4perl.PatternLayout.cspec.U = \
             sub { POE::Kernel->get_active_session->get_heap()->{ user } }
     } );

     for (qw( Huey Lewey Dewey )) {
         POE::Session->create(
             inline_states => {
                 _start    => sub {
                     $_[HEAP]->{user} = $_;
                     POE::Kernel->yield('hello');
                 },
                 hello     => sub {
                     DEBUG "I'm here now";
                 }
             }
         );
     }

     POE::Kernel->run();
     exit;

The code snippet above defines a new layout placeholder (called
'cspec' in Log4perl) %U which calls a subroutine, retrieves the active
session, gets its heap and looks up the entry specified ('user').

Starting with Log::Log4perl 1.20, cspecs also support parameters in 
curly braces, so you can say

     log4perl.appender.Screen.layout.ConversionPattern = %U{user} %U{id} %m%n
     log4perl.PatternLayout.cspec.U = \
             sub { POE::Kernel->get_active_session-> \
                   get_heap()->{ $_[0]->{curlies} } }

and print the POE session heap entries 'user' and 'id' with every logged
message. For more details on cpecs, read the PatternLayout manual.

Hope that helps!

-- Mike

Mike Schilli
m...@pe...