Section: User Contributed Perl Documentation (3)
Updated: 2021-02-08
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Log::Log4perl::Appender::Buffer - Buffering Appender  


    use Log::Log4perl qw(:easy);

    my $conf = qq(
    log4perl.category                  = DEBUG, Buffer

        # Regular Screen Appender
    log4perl.appender.Screen           = Log::Log4perl::Appender::Screen
    log4perl.appender.Screen.stdout    = 1
    log4perl.appender.Screen.layout    = PatternLayout
    log4perl.appender.Screen.layout.ConversionPattern = %d %p %c %m %n

        # Buffering appender, using the appender above as outlet
    log4perl.appender.Buffer               = Log::Log4perl::Appender::Buffer
    log4perl.appender.Buffer.appender      = Screen
    log4perl.appender.Buffer.trigger_level = ERROR


    DEBUG("This message gets buffered.");
    INFO("This message gets buffered also.");

    # Time passes. Nothing happens. But then ...

    print "It's GO time!!!\n";

    ERROR("This message triggers a buffer flush.");



"Log::Log4perl::Appender::Buffer" takes these arguments:
Specifies the name of the appender it buffers messages for. The appender specified must be defined somewhere in the configuration file, not necessarily before the definition of "Log::Log4perl::Appender::Buffer".
Specifies the maximum number of messages the appender will hold in its ring buffer. "max_messages" is optional. By default, "Log::Log4perl::Appender::Buffer" will not limit the number of messages buffered. This might be undesirable in long-running processes accumulating lots of messages before a flush happens. If "max_messages" is set to a numeric value, "Log::Log4perl::Appender::Buffer" will displace old messages in its buffer to make room if the buffer is full.
If trigger_level is set to one of Log4perl's levels (see Log::Log4perl::Level), a "trigger" function will be defined internally to flush the buffer if a message with a priority of $level or higher comes along. This is just a convenience function. Defining

    log4perl.appender.Buffer.trigger_level = ERROR

is equivalent to creating a trigger function like

    log4perl.appender.Buffer.trigger = sub {   \
        my($self, $params) = @_;               \
        return $params->{log4p_level} >=       \
               $Log::Log4perl::Level::ERROR; }

See the next section for defining generic trigger functions.

"trigger" holds a reference to a subroutine, which "Log::Log4perl::Appender::Buffer" will call on every incoming message with the same parameters as the appender's "log()" method:

        my($self, $params) = @_;

$params references a hash containing the message priority (key "l4p_level"), the message category (key "l4p_category") and the content of the message (key "message").

If the subroutine returns 1, it will trigger a flush of buffered messages.




"Log::Log4perl::Appender::Buffer" is a composite appender. Unlike other appenders, it doesn't log any messages, it just passes them on to its attached sub-appender. For this reason, it doesn't need a layout (contrary to regular appenders). If it defines none, messages are passed on unaltered.

Custom filters are also applied to the composite appender only. They are not applied to the sub-appender. Same applies to appender thresholds. This behaviour might change in the future.  


Copyright 2002-2013 by Mike Schilli <> and Kevin Goess <>.

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


Please contribute patches to the project on Github:

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

MAILING LIST (questions, bug reports, suggestions/patches):

Authors (please contact them via the list above, not directly): Mike Schilli <>, Kevin Goess <>

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.