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Appender(3pm) User Contributed Perl Documentation 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.
perl v5.32.1 2021-02-14 Appender(3pm)
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On Apache
Under GNU General Public License
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