Appender(3) User Contributed Perl Documentation Appender(3)NAMELog::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 existance 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
COPYRIGHT AND LICENSE
Copyright 2002-2009 by Mike Schilli <m@perlmeister.com> and Kevin Goess
<cpan@goess.org>.
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.14.1 2011-05-02 Appender(3)
