phpman > perldoc > Log::Log4perl::Appender(3pm)

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"
          filename => '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
    "filename" 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 behavior 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::Appender::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 behavior. 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 behavior 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.

  Composite Appenders
    Composite appenders relay their messages to sub-appenders after providing some filtering or
    synchronizing functionality on incoming messages. Examples are
    Log::Log4perl::Appender::Synchronized, Log::Log4perl::Appender::Limit, and
    Log::Log4perl::Appender::Buffer. Check their manual pages for details.

    Composite appender objects are regular Log::Log4perl::Appender objects, but they have the
    composite flag set:

        $app->composite(1);

    and they define a post_init() method, which sets the appender it relays its messages to:

        ###########################################
        sub post_init {
        ############################################
            my($self) = @_;

            if(! exists $self->{appender}) {
                die "No appender defined for " . __PACKAGE__;
            }

            my $appenders = Log::Log4perl->appenders();
            my $appender = Log::Log4perl->appenders()->{$self->{appender}};

            if(! defined $appender) {
                die "Appender $self->{appender} not defined (yet) when " .
                    __PACKAGE__ . " needed it";
            }

            $self->{app} = $appender;
        }

    The reason for this post-processing step is that the relay appender might not be defined yet
    when the composite appender gets defined. This can happen if Log4perl is initialized with a
    configuration file (which is the most common way to initialize Log4perl), because appenders
    spring into existence in unpredictable order.

    For example, if you define a Synchronized appender like

        log4perl.appender.Syncer            = Log::Log4perl::Appender::Synchronized
        log4perl.appender.Syncer.appender   = Logfile

    then Log4perl will set the appender's "appender" attribute to the *name* of the appender to
    finally relay messages to. After the Log4perl configuration file has been processed, Log4perl
    will remember to call the composite appender's post_init() method, which will grab the relay
    appender instance referred to by the name (Logfile) and set it in its "app" attribute. This is
    exactly what the code snippet above does.

    But if you initialize Log4perl by its API, you need to remember to perform these steps. Here's
    the lineup:

        use Log::Log4perl qw(get_logger :levels);

        my $fileApp = Log::Log4perl::Appender->new(
                    'Log::Log4perl::Appender::File',
                    name     => 'MyFileApp',
                    filename => 'mylog',
                    mode     => 'append',
                    );
        $fileApp->layout(
                    Log::Log4perl::Layout::PatternLayout::Multiline->new(
                            '%d{yyyy-MM-dd HH:mm:ss} %p [%c] #%P> %m%n')
                    );
          # Make the appender known to the system (without assigning it to
          # any logger
        Log::Log4perl->add_appender( $fileApp );

        my $syncApp = Log::Log4perl::Appender->new(
                    'Log::Log4perl::Appender::Synchronized',
                    name       => 'MySyncApp',
                    appender   => 'MyFileApp',
                    key        => 'nem',
                    );
        $syncApp->post_init();
        $syncApp->composite(1);

          # The Synchronized appender is now ready, assign it to a logger
          # and start logging.
        get_logger("")->add_appender($syncApp);

        get_logger("")->level($DEBUG);
        get_logger("wonk")->debug("waah!");

    The composite appender's log() function will typically cache incoming messages until a certain
    trigger condition is met and then forward a bulk of messages to the relay appender.

    Caching messages is surprisingly tricky, because you want them to look like they came from the
    code location they were originally issued from and not from the location that triggers the
    flush. Luckily, Log4perl offers a cache mechanism for messages, all you need to do is call the
    base class' log() function with an additional reference to a scalar, and then save its content
    to your composite appender's message buffer afterwards:

        ###########################################
        sub log {
        ###########################################
            my($self, %params) = @_;

            # ... some logic to decide whether to cache or flush

                # Adjust the caller stack
            local $Log::Log4perl::caller_depth =
                  $Log::Log4perl::caller_depth + 2;

                # We need to cache.
                # Ask the appender to save a cached message in $cache
            $self->{relay_app}->SUPER::log(\%params,
                                 $params{log4p_category},
                                 $params{log4p_level}, \my $cache);

                # Save it in the appender's message buffer
            push @{ $self->{buffer} }, $cache;
        }

    Note that before calling the log() method of the relay appender's base class (and thus
    introducing two additional levels on the call stack), we need to adjust the call stack to allow
    Log4perl to render cspecs like the %M or %L correctly. The cache will then contain a correctly
    rendered message, according to the layout of the target appender.

    Later, when the time comes to flush the cached messages, a call to the relay appender's base
    class' log_cached() method with the cached message as an argument will forward the correctly
    rendered message:

        ###########################################
        sub log {
        ###########################################
            my($self, %params) = @_;

            # ... some logic to decide whether to cache or flush

                # Flush pending messages if we have any
            for my $cache (@{$self->{buffer}}) {
                $self->{relay_app}->SUPER::log_cached($cache);
            }
        }

SEE ALSO
    Log::Dispatch

LICENSE
    Copyright 2002-2013 by Mike Schilli <m AT perlmeister.com> and Kevin Goess <cpan AT goess.org>.

    This library is free software; you can redistribute it and/or modify it under the same terms as
    Perl itself.

AUTHOR
    Please contribute patches to the project on Github:

        http://github.com/mschilli/log4perl

    Send bug reports or requests for enhancements to the authors via our

    MAILING LIST (questions, bug reports, suggestions/patches): log4perl-devel AT lists.net

    Authors (please contact them via the list above, not directly): Mike Schilli
    <m AT perlmeister.com>, Kevin Goess <cpan AT goess.org>

    Contributors (in alphabetical order): Ateeq Altaf, Cory Bennett, Jens Berthold, Jeremy Bopp,
    Hutton Davidson, Chris R. Donnelly, Matisse Enzer, Hugh Esco, Anthony Foiani, James FitzGibbon,
    Carl Franks, Dennis Gregorovic, Andy Grundman, Paul Harrington, Alexander Hartmaier David Hull,
    Robert Jacobson, Jason Kohles, Jeff Macdonald, Markus Peter, Brett Rann, Peter Rabbitson, Erik
    Selberg, Aaron Straup Cope, Lars Thegler, David Viner, Mac Yang.