Log::ger::Manual::Tutorial::790_WritingALayoutPlugin - Writing a layout plugin


version 0.033.000


The function of a layout plugin is to take the log message and format it, usually into a string. The most often used layout is Log::ger::Layout::Pattern which allows you to add things like timestamp, newline, level, location (source filename and line number) along with the log message itself.

This is not to be confused with a format plugin. A format plugin takes arguments from a log statement and produce a log message from it. You can say that format plugin is a form of formatting that is applied first, before the layout plugin.

Let's create a silly example layout plugin that can convert your log message to uppercase or lowercase. We'll call it ConvertCase. Create Log::ger::Layout::ConvertCase as follows:

 # in lib/Log/ger/Layout/
 package Log::ger::Layout::ConvertCase;

 use strict;
 use warnings;

 sub get_hooks {
     my %plugin_conf = @_;

     $plugin_conf{case} or die "Please specify case";
     $plugin_conf{case} =~ /\A(upper|lower)\z/
         or die "Invalid value for 'case', please use 'upper' or 'lower'";

     return {
         create_layouter => [
             __PACKAGE__, # key
             50,          # priority
             sub {        # hook
                 my %hook_args = @_;
                 my $layouter = sub {
                     $plugin_conf{case} eq 'upper' ? uc($_[0]) : lc($_[0]);


The plugin module needs to define get_hooks which returns a hashref of hook names and hook records. For the list of available hooks (as well as basically the same information presented here), see Log::ger::Manual::Internals. For a layout plugin, the relevant hook is create_layouter. This hook will be called when Log::ger wants to construct a layouter.

The hook record is an arrayref of 3 elements:

 [$key, $prio, $coderef]

$key is usually the name of the module (__PACKAGE__). $prio is priority for ordering when there are multiple plugins for the same hook, a number between 0-100 (the lower the number, the higher the priority), normally 50. $coderef is the actual hook. Our hook will receive a hash arguments (%hook_args) and is expected to return the result:

 [$layouter, ...]

We are only concerned with the first element, hence will not discuss the rest. The layouter will be passed:

 ($fmsg, \%per_target_conf, $lnum, $lname, \%per_msg_conf)

where $fmsg is formatted message from the formatter, %per_target_conf are arguments given to Log::ger->get_logger or to Log::ger's import(), $lnum is numeric level, $lname is string level, and %per_msg_conf is optional. In the example above, we are only concerned with $fmsg ($_[0]).

To see our plugin in action, try this simple program:

 use Log::ger;
 use Log::ger::Layout ConvertCase => (case => 'upper');
 log_warn "Hello, World!";

When run, it will print:


For examples of more involved layout plugins, see: Log::ger::Layout::Pattern, Log::ger::Layout::JSON.






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