Appender(3)

NAME
Log::Log4perl::Appender - Log appender class

SYNOPSIS
  use Log::Log4perl;

      # Define a logger
  my $logger = Log::Log4perl->get_logger("abc.def.ghi");

      # Define a layout
  my $layout = Log::Log4perl::Layout::PatternLayout->new(
                   "%d (%F:%L)> %m");

      # Define an appender
  my $appender = Log::Log4perl::Appender->new(
                   "Log::Log4perl::Appender::Screen",
                   name => 'dumpy');

      # Set the appender's layout
  $appender->layout($layout);
  $logger->add_appender($appender);

DESCRIPTION
This class is a wrapper around the "Log::Log4perl::Appender" appender set.

It also supports the <Log::Dispatch::*> collections of appenders. The module hides the idiosyncrasies of "Log::Dispatch" (e.g. every dispatcher gotta have a name, but there's no accessor to retrieve it) from "Log::Log4perl" and yet re-uses the extremely useful variety of dispatchers already created and tested in "Log::Dispatch".

FUNCTIONS

  Log::Log4perl::Appender->new($dispatcher_class_name, ...);
The constructor "new()" takes the name of the appender class to be created as a string (!) argument, optionally followed by a number of appender-specific parameters, for example:

      # Define an appender
  my $appender = Log::Log4perl::Appender->new(
      "Log::Log4perl::Appender::File"
      file => 'out.log');

In case of "Log::Dispatch" appenders, if no "name" parameter is specified, the appender object will create a unique one (format "appNNN"), which can be retrieved later via the "name()" method:

  print "The appender's name is ", $appender->name(), "\n";

Other parameters are specific to the appender class being used. In the case above, the "file" parameter specifies the name of the "Log::Log4perl::Appender::File" dispatcher used.

However, if, for instance, you're using a "Log::Dispatch::Email" dispatcher to send you email, you'll have to specify "from" and "to" email addresses. Every dispatcher is different. Please check the "Log::Dispatch::*" documentation for the appender used for details on specific requirements.

The "new()" method will just pass these parameters on to a newly created "Log::Dispatch::*" object of the specified type.

When it comes to logging, the "Log::Log4perl::Appender" will transparently relay all messages to the "Log::Dispatch::*" object it carries in its womb.

  $appender->layout($layout);
The "layout()" method sets the log layout used by the appender to the format specified by the "Log::Log4perl::Layout::*" object which is passed to it as a reference. Currently there's two layouts available:

    Log::Log4perl::Layout::SimpleLayout
    Log::Log4perl::Layout::PatternLayout

Please check the Log::Log4perl::Layout::SimpleLayout and Log::Log4perl::Layout::PatternLayout manual pages for details.

Supported Appenders
Here's the list of appender modules currently available via "Log::Dispatch", if not noted otherwise, written by Dave Rolsky:

       Log::Dispatch::ApacheLog
       Log::Dispatch::DBI (by Tatsuhiko Miyagawa)
       Log::Dispatch::Email,
       Log::Dispatch::Email::MailSend,
       Log::Dispatch::Email::MailSendmail,
       Log::Dispatch::Email::MIMELite
       Log::Dispatch::File
       Log::Dispatch::FileRotate (by Mark Pfeiffer)
       Log::Dispatch::Handle
       Log::Dispatch::Screen
       Log::Dispatch::Syslog
       Log::Dispatch::Tk (by Dominique Dumont)

"Log4perl" doesn't care which ones you use, they're all handled in the same way via the "Log::Log4perl::Appender" interface. Please check the well-written manual pages of the "Log::Dispatch" hierarchy on how to use each one of them.

Parameters passed on to the appender's log() method
When calling the appender's log()-Funktion, Log::Log4perl will submit a list of key/value pairs. Entries to the following keys are guaranteed to be present:

message
    Text of the rendered message

log4p_category
    Name of the category of the logger that triggered the event.

log4p_level
    Log::Log4perl level of the event

Pitfalls
Since the "Log::Dispatch::File" appender truncates log files by default, and most of the time this is not what you want, we've instructed "Log::Log4perl" to change this behaviour by slipping it the "mode => append" parameter behind the scenes. So, effectively with "Log::Log4perl" 0.23, a configuration like

    log4perl.category = INFO, FileAppndr
    log4perl.appender.FileAppndr          = Log::Dispatch::File
    log4perl.appender.FileAppndr.filename = test.log
    log4perl.appender.FileAppndr.layout   = Log::Log4perl::Layout::SimpleLayout

will always append to an existing logfile "test.log" while if you specifically request clobbering like in

    log4perl.category = INFO, FileAppndr
    log4perl.appender.FileAppndr          = Log::Dispatch::File
    log4perl.appender.FileAppndr.filename = test.log
    log4perl.appender.FileAppndr.mode     = write
    log4perl.appender.FileAppndr.layout   = Log::Log4perl::Layout::SimpleLayout

it will overwrite an existing log file "test.log" and start from scratch.

Appenders Expecting Message Chunks
Instead of simple strings, certain appenders are expecting multiple fields as log messages. If a statement like

    $logger->debug($ip, $user, "signed in");

causes an off-the-shelf "Log::Log4perl::Screen" appender to fire, the appender will just concatenate the three message chunks passed to it in order to form a single string. The chunks will be separated by a string defined in $Log::Log4perl::JOIN_MSG_ARRAY_CHAR (defaults to the empty string "").

However, different appenders might choose to interpret the message above differently: An appender like "Log::Log4perl::Appender::DBI" might take the three arguments passed to the logger and put them in three separate rows into the DB.

The "warp_message" appender option is used to specify the desired behaviour. If no setting for the appender property

    # *** Not defined ***
    # log4perl.appender.SomeApp.warp_message

is defined in the Log4perl configuration file, the appender referenced by "SomeApp" will fall back to the standard behaviour and join all message chunks together, separating them by $Log::Log4perl::JOIN_MSG_ARRAY_CHAR.

If, on the other hand, it is set to a false value, like in

    log4perl.appender.SomeApp.layout=NoopLayout
    log4perl.appender.SomeApp.warp_message = 0

then the message chunks are passed unmodified to the appender as an array reference. Please note that you need to set the appender's layout to "Log::Log4perl::Layout::NoopLayout" which just leaves the messages chunks alone instead of formatting them or replacing conversion specifiers.

Please note that the standard appenders in the Log::Dispatch hierarchy will choke on a bunch of messages passed to them as an array reference. You can't use "warp_message = 0" (or the function name syntax defined below) on them. Only special appenders like Log::Log4perl::Appender::DBI can deal with this.

If (and now we're getting fancy) an appender expects message chunks, but we would like to pre-inspect and probably modify them before they're actually passed to the appender's "log" method, an inspection subroutine can be defined with the appender's "warp_message" property:

    log4perl.appender.SomeApp.layout=NoopLayout
    log4perl.appender.SomeApp.warp_message = sub { \
                                           $#_ = 2 if @_ > 3; \
                                           return @_; }

The inspection subroutine defined by the "warp_message" property will receive the list of message chunks, like they were passed to the logger and is expected to return a corrected list. The example above simply limits the argument list to a maximum of three by cutting off excess elements and returning the shortened list.

Also, the warp function can be specified by name like in

    log4perl.appender.SomeApp.layout=NoopLayout
    log4perl.appender.SomeApp.warp_message = main::filter_my_message

In this example, "filter_my_message" is a function in the "main" package, defined like this:

    my $COUNTER = 0;

    sub filter_my_message {
        my @chunks = @_;
        unshift @chunks, ++$COUNTER;
        return @chunks;
    }

The subroutine above will add an ever increasing counter as an additional first field to every message passed to the "SomeApp" appender -- but not to any other appender in the system.

SEE ALSO
Log::Dispatch

AUTHOR
Mike Schilli, <log4perl@perlmeister.com>