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man : Log::Log4perl::FAQ(3p)

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FAQ(3p)        User Contributed Perl Documentation        FAQ(3p)

       Log::Log4perl::FAQ - Frequently Asked Questions on Log::Log4perl

       This FAQ shows a wide variety of commonly encountered logging tasks and
       how to solve them in the most elegant way with Log::Log4perl. Most of
       the time, this will be just a matter of smartly configuring your
       Log::Log4perl configuration files.

   Why use Log::Log4perl instead of any other logging module on CPAN?
       That's a good question. There's dozens of logging modules on CPAN.
       When it comes to logging, people typically think: "Aha. Writing out
       debug and error messages. Debug is lower than error. Easy. I'm gonna
       write my own." Writing a logging module is like a rite of passage for
       every Perl programmer, just like writing your own templating system.

       Of course, after getting the basics right, features need to be added.
       You'd like to write a timestamp with every message. Then timestamps
       with microseconds. Then messages need to be written to both the screen
       and a log file.

       And, as your application grows in size you might wonder: Why doesn't my
       logging system scale along with it? You would like to switch on logging
       in selected parts of the application, and not all across the board,
       because this kills performance. This is when people turn to
       Log::Log4perl, because it handles all of that.

       Avoid this costly switch.

       Use "Log::Log4perl" right from the start. "Log::Log4perl"'s ":easy"
       mode supports easy logging in simple scripts:

           use Log::Log4perl qw(:easy);

           DEBUG "A low-level message";
           ERROR "Won't make it until level gets increased to ERROR";

       And when your application inevitably grows, your logging system grows
       with it without you having to change any code.

       Please, don't re-invent logging. "Log::Log4perl" is here, it's easy to
       use, it scales, and covers many areas you haven't thought of yet, but
       will enter soon.

   What's the easiest way to use Log4perl?
       If you just want to get all the comfort of logging, without much
       overhead, use Stealth Loggers. If you use Log::Log4perl in ":easy" mode

           use Log::Log4perl qw(:easy);

       you'll have the following functions available in the current package:

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       Just make sure that every package of your code where you're using them
       in pulls in "use Log::Log4perl qw(:easy)" first, then you're set.
       Every stealth logger's category will be equivalent to the name of the
       package it's located in.

       These stealth loggers will be absolutely silent until you initialize
       Log::Log4perl in your main program with either

               # Define any Log4perl behaviour

       (using a full-blown Log4perl config file) or the super-easy method

               # Just log to STDERR

       or the parameter-style method with a complexity somewhat in between:

               # Append to a log file
           Log::Log4perl->easy_init( { level   => $DEBUG,
                                       file    => ">>test.log" } );

       For more info, please check out "Stealth Loggers" in Log::Log4perl.

   How can I simply log all my ERROR messages to a file?
       After pulling in the "Log::Log4perl" module, just initialize its
       behaviour by passing in a configuration to its "init" method as a
       string reference. Then, obtain a logger instance and write out a
       message with its "error()" method:

           use Log::Log4perl qw(get_logger);

               # Define configuration
           my $conf = q(
               log4perl.logger                    = ERROR, FileApp
               log4perl.appender.FileApp          = Log::Log4perl::Appender::File
               log4perl.appender.FileApp.filename = test.log
               log4perl.appender.FileApp.layout   = PatternLayout
               log4perl.appender.FileApp.layout.ConversionPattern = %d> %m%n

               # Initialize logging behaviour
           Log::Log4perl->init( \$conf );

               # Obtain a logger instance
           my $logger = get_logger("Bar::Twix");
           $logger->error("Oh my, a dreadful error!");
           $logger->warn("Oh my, a dreadful warning!");

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       This will append something like

           2002/10/29 20:11:55> Oh my, a dreadful error!

       to the log file "test.log". How does this all work?

       While the Log::Log4perl "init()" method typically takes the name of a
       configuration file as its input parameter like in

           Log::Log4perl->init( "/path/mylog.conf" );

       the example above shows how to pass in a configuration as text in a
       scalar reference.

       The configuration as shown defines a logger of the root category, which
       has an appender of type "Log::Log4perl::Appender::File" attached. The

           log4perl.logger = ERROR, FileApp

       doesn't list a category, defining a root logger. Compare that with

           log4perl.logger.Bar.Twix = ERROR, FileApp

       which would define a logger for the category "Bar::Twix", showing
       probably different behaviour. "FileApp" on the right side of the
       assignment is an arbitrarily defined variable name, which is only used
       to somehow reference an appender defined later on.

       Appender settings in the configuration are defined as follows:

           log4perl.appender.FileApp          = Log::Log4perl::Appender::File
           log4perl.appender.FileApp.filename = test.log

       It selects the file appender of the "Log::Log4perl::Appender"
       hierarchy, which will append to the file "test.log" if it already
       exists. If we wanted to overwrite a potentially existing file, we would
       have to explicitly set the appropriate "Log::Log4perl::Appender::File"
       parameter "mode":

           log4perl.appender.FileApp          = Log::Log4perl::Appender::File
           log4perl.appender.FileApp.filename = test.log
           log4perl.appender.FileApp.mode     = write

       Also, the configuration defines a PatternLayout format, adding the
       nicely formatted current date and time, an arrow (>) and a space before
       the messages, which is then followed by a newline:

           log4perl.appender.FileApp.layout   = PatternLayout
           log4perl.appender.FileApp.layout.ConversionPattern = %d> %m%n

       Obtaining a logger instance and actually logging something is typically
       done in a different system part as the Log::Log4perl initialisation
       section, but in this example, it's just done right after init for the

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       sake of compactness:

               # Obtain a logger instance
           my $logger = get_logger("Bar::Twix");
           $logger->error("Oh my, a dreadful error!");

       This retrieves an instance of the logger of the category "Bar::Twix",
       which, as all other categories, inherits behaviour from the root logger
       if no other loggers are defined in the initialization section.

       The "error()" method fires up a message, which the root logger catches.
       Its priority is equal to or higher than the root logger's priority
       (ERROR), which causes the root logger to forward it to its attached
       appender. By contrast, the following

           $logger->warn("Oh my, a dreadful warning!");

       doesn't make it through, because the root logger sports a higher
       setting (ERROR and up) than the WARN priority of the message.

   How can I install Log::Log4perl on Microsoft Windows?
       Log::Log4perl is fully supported on the Win32 platform. It has been
       tested with Activestate perl 5.6.1 under Windows 98 and rumor has it
       that it also runs smoothly on all other major flavors (Windows NT,
       2000, XP, etc.).

       It also runs nicely with ActiveState 5.8.0, and, believe me, we had to
       jump through some major hoops for that.

       Typically, Win32 systems don't have the "make" utility installed, so
       the standard "perl Makefile.PL; make install" on the downloadable
       distribution won't work. But don't despair, there's a very easy

       The "Log::Log4perl" homepage provides a so-called PPD file for
       ActiveState's "ppm" installer, which comes with ActiveState perl by

       Install on ActiveState 5.6.*
           The DOS command line

               ppm install ""

           will contact the Log4perl homepage, download the latest
           "Log::Log4perl" distribution and install it. If your ActiveState
           installation lacks any of the modules "Log::Log4perl" depends upon,
           "ppm" will automatically contact ActivateState and download them
           from their CPAN-like repository.

       Install on ActiveState 5.8.*
           ActiveState's "Programmer's Package Manager" can be called from
           Window's Start Menu: Start->Programs->>ActiveState ActivePerl
           5.8>Perl Package Manager will invoke ppm. Since Log::Log4perl
           hasn't made it yet into the standard ActiveState repository (and

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           you probably don't want their outdated packages anyway), just tell
           ppm the first time you call it to add the Log4perl repository

               ppm> repository add

           Then, just tell it to install Log::Log4perl and it will resolve all
           dependencies automatically and fetch them from
  if it can't find them in the main

               ppm> install Log-Log4perl

       That's it! Afterwards, just create a Perl script like

           use Log::Log4perl qw(:easy);

           my $logger = get_logger("Twix::Bar");
           $logger->debug("Watch me!");

       and run it. It should print something like

           2002/11/06 01:22:05 Watch me!

       If you find that something doesn't work, please let us know at -- we'll apprechiate it. Have fun!

   How can I include global (thread-specific) data in my log messages?
       Say, you're writing a web application and want all your log messages to
       include the current client's IP address. Most certainly, you don't want
       to include it in each and every log message like in

           $logger->debug( $r->connection->remote_ip,
                           " Retrieving user data from DB" );

       do you? Instead, you want to set it in a global data structure and have
       Log::Log4perl include it automatically via a PatternLayout setting in
       the configuration file:

           log4perl.appender.FileApp.layout.ConversionPattern = %X{ip} %m%n

       The conversion specifier %X{ip} references an entry under the key "ip"
       in the global "MDC" (mapped diagnostic context) table, which you've set
       once via

           Log::Log4perl::MDC->put("ip", $r->connection->remote_ip);

       at the start of the request handler. Note that this is a static (class)
       method, there's no logger object involved.  You can use this method
       with as many key/value pairs as you like as long as you reference them
       under different names.

       The mappings are stored in a global hash table within Log::Log4perl.
       Luckily, because the thread model in 5.8.0 doesn't share global

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       variables between threads unless they're explicitly marked as such,
       there's no problem with multi-threaded environments.

       For more details on the MDC, please refer to "Mapped Diagnostic Context
       (MDC)" in Log::Log4perl and Log::Log4perl::MDC.

   My application is already logging to a file. How can I duplicate all
       messages to also go to the screen?
       Assuming that you already have a Log4perl configuration file like

           log4perl.logger                    = DEBUG, FileApp

           log4perl.appender.FileApp          = Log::Log4perl::Appender::File
           log4perl.appender.FileApp.filename = test.log
           log4perl.appender.FileApp.layout   = PatternLayout
           log4perl.appender.FileApp.layout.ConversionPattern = %d> %m%n

       and log statements all over your code, it's very easy with Log4perl to
       have the same messages both printed to the logfile and the screen. No
       reason to change your code, of course, just add another appender to the
       configuration file and you're done:

           log4perl.logger                    = DEBUG, FileApp, ScreenApp

           log4perl.appender.FileApp          = Log::Log4perl::Appender::File
           log4perl.appender.FileApp.filename = test.log
           log4perl.appender.FileApp.layout   = PatternLayout
           log4perl.appender.FileApp.layout.ConversionPattern = %d> %m%n

           log4perl.appender.ScreenApp          = Log::Log4perl::Appender::Screen
           log4perl.appender.ScreenApp.stderr   = 0
           log4perl.appender.ScreenApp.layout   = PatternLayout
           log4perl.appender.ScreenApp.layout.ConversionPattern = %d> %m%n

       The configuration file above is assuming that both appenders are active
       in the same logger hierarchy, in this case the "root" category.  But
       even if you've got file loggers defined in several parts of your
       system, belonging to different logger categories, each logging to
       different files, you can gobble up all logged messages by defining a
       root logger with a screen appender, which would duplicate messages from
       all your file loggers to the screen due to Log4perl's appender
       inheritance. Check


       for details. Have fun!

   How can I make sure my application logs a message when it dies
       Whenever you encounter a fatal error in your application, instead of
       saying something like

           open FILE, "<blah" or die "Can't open blah -- bailing out!";

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       just use Log::Log4perl's fatal functions instead:

           my $log = get_logger("Some::Package");
           open FILE, "<blah" or $log->logdie("Can't open blah -- bailing out!");

       This will both log the message with priority FATAL according to your
       current Log::Log4perl configuration and then call Perl's "die()"
       afterwards to terminate the program. It works the same with stealth
       loggers (see "Stealth Loggers" in Log::Log4perl), all you need to do is

           use Log::Log4perl qw(:easy);
           open FILE, "<blah" or LOGDIE "Can't open blah -- bailing out!";

       What can you do if you're using some library which doesn't use
       Log::Log4perl and calls "die()" internally if something goes wrong? Use
       a $SIG{__DIE__} pseudo signal handler

           use Log::Log4perl qw(get_logger);

           $SIG{__DIE__} = sub {
               if($^S) {
                   # We're in an eval {} and don't want log
                   # this message but catch it later
               my $logger = get_logger("");
               die @_; # Now terminate really

       This will catch every "die()"-Exception of your application or the
       modules it uses. In case you want to It will fetch a root logger and
       pass on the "die()"-Message to it.  If you make sure you've configured
       with a root logger like this:

               log4perl.category         = FATAL, Logfile
               log4perl.appender.Logfile = Log::Log4perl::Appender::File
               log4perl.appender.Logfile.filename = fatal_errors.log
               log4perl.appender.Logfile.layout = \
               log4perl.appender.Logfile.layout.ConversionPattern = %F{1}-%L (%M)> %m%n

       then all "die()" messages will be routed to a file properly. The line


       in the pseudo signal handler above merits a more detailed explanation.
       With the setup above, if a module calls "die()" in one of its
       functions, the fatal message will be logged in the signal handler and
       not in the original function -- which will cause the %F, %L and %M

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       placeholders in the pattern layout to be replaced by the filename, the
       line number and the function/method name of the signal handler, not the
       error-throwing module. To adjust this, Log::Log4perl has the
       $caller_depth variable, which defaults to 0, but can be set to positive
       integer values to offset the caller level. Increasing it by one will
       cause it to log the calling function's parameters, not the ones of the
       signal handler.  See "Using Log::Log4perl from wrapper classes" in
       Log::Log4perl for more details.

   How can I hook up the LWP library with Log::Log4perl?
       Or, to put it more generally: How can you utilize a third-party
       library's embedded logging and debug statements in Log::Log4perl?  How
       can you make them print to configurable appenders, turn them on and
       off, just as if they were regular Log::Log4perl logging statements?

       The easiest solution is to map the third-party library logging
       statements to Log::Log4perl's stealth loggers via a typeglob

       As an example, let's take LWP, one of the most popular Perl modules,
       which makes handling WWW requests and responses a breeze.  Internally,
       LWP uses its own logging and debugging system, utilizing the following
       calls inside the LWP code (from the LWP::Debug man page):

               # Function tracing

               # High-granular state in functions
           LWP::Debug::debug('url ok');

               # Data going over the wire
           LWP::Debug::conns("read $n bytes: $data");

       First, let's assign Log::Log4perl priorities to these functions: I'd
       suggest that "debug()" messages have priority "INFO", "trace()" uses
       "DEBUG" and "conns()" also logs with "DEBUG" -- although your mileage
       may certainly vary.

       Now, in order to transpartently hook up LWP::Debug with Log::Log4perl,
       all we have to do is say

           package LWP::Debug;
           use Log::Log4perl qw(:easy);

           *trace = *INFO;
           *conns = *DEBUG;
           *debug = *DEBUG;

           package main;
           # ... go on with your regular program ...

       at the beginning of our program. In this way, every time the, say,
       "LWP::UserAgent" module calls "LWP::Debug::trace()", it will
       implicitely call INFO(), which is the "info()" method of a stealth

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       logger defined for the Log::Log4perl category "LWP::Debug". Is this
       cool or what?

       Here's a complete program:

           use LWP::UserAgent;
           use HTTP::Request::Common;
           use Log::Log4perl qw(:easy);

               { category => "LWP::Debug",
                 level    => $DEBUG,
                 layout   => "%r %p %M-%L %m%n",

           package LWP::Debug;
           use Log::Log4perl qw(:easy);
           *trace = *INFO;
           *conns = *DEBUG;
           *debug = *DEBUG;

           package main;
           my $ua = LWP::UserAgent->new();
           my $resp = $ua->request(GET "");

           if($resp->is_success()) {
               print "Success: Received ",
                     length($resp->content()), "\n";
           } else {
               print "Error: ", $resp->code(), "\n";

       This will generate the following output on STDERR:

           174 INFO LWP::UserAgent::new-164 ()
           208 INFO LWP::UserAgent::request-436 ()
           211 INFO LWP::UserAgent::send_request-294 GET
           212 DEBUG LWP::UserAgent::_need_proxy-1123 Not proxied
           405 INFO LWP::Protocol::http::request-122 ()
           859 DEBUG LWP::Protocol::collect-206 read 233 bytes
           863 DEBUG LWP::UserAgent::request-443 Simple response: Found
           869 INFO LWP::UserAgent::request-436 ()
           871 INFO LWP::UserAgent::send_request-294
           872 DEBUG LWP::UserAgent::_need_proxy-1123 Not proxied
           873 INFO LWP::Protocol::http::request-122 ()
           1016 DEBUG LWP::UserAgent::request-443 Simple response: Found
           1020 INFO LWP::UserAgent::request-436 ()
           1022 INFO LWP::UserAgent::send_request-294
           1023 DEBUG LWP::UserAgent::_need_proxy-1123 Not proxied
           1024 INFO LWP::Protocol::http::request-122 ()
           1382 DEBUG LWP::Protocol::collect-206 read 632 bytes

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           2605 DEBUG LWP::Protocol::collect-206 read 77 bytes
           2607 DEBUG LWP::UserAgent::request-443 Simple response: OK
           Success: Received 42584

       Of course, in this way, the embedded logging and debug statements
       within LWP can be utilized in any Log::Log4perl way you can think of.
       You can have them sent to different appenders, block them based on the
       category and everything else Log::Log4perl has to offer.

       Only drawback of this method: Steering logging behaviour via category
       is always based on the "LWP::Debug" package. Although the logging
       statements reflect the package name of the issuing module properly, the
       stealth loggers in "LWP::Debug" are all of the category "LWP::Debug".
       This implies that you can't control the logging behaviour based on the
       package that's initiating a log request (e.g. LWP::UserAgent) but only
       based on the package that's actually executing the logging statement,
       "LWP::Debug" in this case.

       To work around this conundrum, we need to write a wrapper function and
       plant it into the "LWP::Debug" package. It will determine the caller
       and create a logger bound to a category with the same name as the
       caller's package:

           package LWP::Debug;

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

           sub l4p_wrapper {
               my($prio, @message) = @_;
               $Log::Log4perl::caller_depth += 2;
               get_logger(scalar caller(1))->log($prio, @message);
               $Log::Log4perl::caller_depth -= 2;

           no warnings 'redefine';
           *trace = sub { l4p_wrapper($INFO, @_); };
           *debug = *conns = sub { l4p_wrapper($DEBUG, @_); };

           package main;
           # ... go on with your main program ...

       This is less performant than the previous approach, because every log
       request will request a reference to a logger first, then call the
       wrapper, which will in turn call the appropriate log function.

       This hierarchy shift has to be compensated for by increasing
       $Log::Log4perl::caller_depth by 2 before calling the log function and
       decreasing it by 2 right afterwards. Also, the "l4p_wrapper" function
       shown above calls caller(1) which determines the name of the package
       two levels down the calling hierarchy (and therefore compensates for
       both the wrapper function and the anonymous subroutine calling it).

       "no warnings 'redefine'" suppresses a warning Perl would generate
       otherwise upon redefining "LWP::Debug"'s "trace()", "debug()" and

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       "conns()" functions. In case you use a perl prior to 5.6.x, you need to
       manipulate $^W instead.

       To make things easy for you when dealing with LWP, Log::Log4perl 0.47
       introduces "Log::Log4perl->infiltrate_lwp()" which does exactly the

   What if I need dynamic values in a static Log4perl configuration file?
       Say, your application uses Log::Log4perl for logging and therefore
       comes with a Log4perl configuration file, specifying the logging
       behaviour.  But, you also want it to take command line parameters to
       set values like the name of the log file.  How can you have both a
       static Log4perl configuration file and a dynamic command line

       As of Log::Log4perl 0.28, every value in the configuration file can be
       specified as a Perl hook. So, instead of saying

           log4perl.appender.Logfile.filename = test.log

       you could just as well have a Perl subroutine deliver the value

           log4perl.appender.Logfile.filename = sub { logfile(); };

       given that "logfile()" is a valid function in your "main" package
       returning a string containing the path to the log file.

       Or, think about using the value of an environment variable:

           log4perl.appender.DBI.user = sub { $ENV{USERNAME} };

       When "Log::Log4perl->init()" parses the configuration file, it will
       notice the assignment above because of its "sub {...}" pattern and
       treat it in a special way: It will evaluate the subroutine (which can
       contain arbitrary Perl code) and take its return value as the right
       side of the assignment.

       A typical application would be called like this on the command line:

           app                # log file is "test.log"
           app -l mylog.txt   # log file is "mylog.txt"

       Here's some sample code implementing the command line interface above:

           use Log::Log4perl qw(get_logger);
           use Getopt::Std;

           getopt('l:', \our %OPTS);

           my $conf = q(
           log4perl.category.Bar.Twix         = WARN, Logfile
           log4perl.appender.Logfile          = Log::Log4perl::Appender::File
           log4perl.appender.Logfile.filename = sub { logfile(); };

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           log4perl.appender.Logfile.layout   = SimpleLayout


           my $logger = get_logger("Bar::Twix");

           sub logfile {
               if(exists $OPTS{l}) {
                   return $OPTS{l};
               } else {
                   return "test.log";

       Every Perl hook may contain arbitrary perl code, just make sure to
       fully qualify eventual variable names (e.g. %main::OPTS instead of

       SECURITY NOTE: this feature means arbitrary perl code can be embedded
       in the config file.  In the rare case where the people who have access
       to your config file are different from the people who write your code
       and shouldn't have execute rights, you might want to call


       before you call init(). This will prevent Log::Log4perl from executing
       any Perl code in the config file (including code for custom conversion
       specifiers (see "Custom cspecs" in

   How can I roll over my logfiles automatically at midnight?
       Long-running applications tend to produce ever-increasing logfiles.
       For backup and cleanup purposes, however, it is often desirable to move
       the current logfile to a different location from time to time and start
       writing a new one.

       This is a non-trivial task, because it has to happen in sync with the
       logging system in order not to lose any messages in the process.

       Luckily, Mark Pfeiffer's "Log::Dispatch::FileRotate" appender works
       well with Log::Log4perl to rotate your logfiles in a variety of ways.

       Note, however, that having the application deal with rotating a log
       file is not cheap. Among other things, it requires locking the log file
       with every write to avoid race conditions.  There are good reasons to
       use external rotators like "newsyslog" instead.  See the entry "How can
       I rotate a logfile with newsyslog?" in the FAQ for more information on
       how to configure it.

       When using "Log::Dispatch::FileRotate", all you have to do is specify

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       it in your Log::Log4perl configuration file and your logfiles will be
       rotated automatically.

       You can choose between rolling based on a maximum size ("roll if
       greater than 10 MB") or based on a date pattern ("roll everyday at
       midnight").  In both cases, "Log::Dispatch::FileRotate" allows you to
       define a number "max" of saved files to keep around until it starts
       overwriting the oldest ones. If you set the "max" parameter to 2 and
       the name of your logfile is "test.log", "Log::Dispatch::FileRotate"
       will move "test.log" to "test.log.1" on the first rollover. On the
       second rollover, it will move "test.log.1" to "test.log.2" and then
       "test.log" to "test.log.1". On the third rollover, it will move
       "test.log.1" to "test.log.2" (therefore discarding the old
       "test.log.2") and "test.log" to "test.log.1". And so forth. This way,
       there's always going to be a maximum of 2 saved log files around.

       Here's an example of a Log::Log4perl configuration file, defining a
       daily rollover at midnight (date pattern "yyyy-MM-dd"), keeping a
       maximum of 5 saved logfiles around:

           log4perl.category         = WARN, Logfile
           log4perl.appender.Logfile = Log::Dispatch::FileRotate
           log4perl.appender.Logfile.filename    = test.log
           log4perl.appender.Logfile.max         = 5
           log4perl.appender.Logfile.DatePattern = yyyy-MM-dd
           log4perl.appender.Logfile.TZ          = PST
           log4perl.appender.Logfile.layout = \
           log4perl.appender.Logfile.layout.ConversionPattern = %d %m %n

       Please see the "Log::Dispatch::FileRotate" documentation for details.
       "Log::Dispatch::FileRotate" is available on CPAN.

   What's the easiest way to turn off all logging, even with a lengthy
       Log4perl configuration file?
       In addition to category-based levels and appender thresholds,
       Log::Log4perl supports system-wide logging thresholds. This is the
       minimum level the system will require of any logging events in order
       for them to make it through to any configured appenders.

       For example, putting the line

           log4perl.threshold = ERROR

       anywhere in your configuration file will limit any output to any
       appender to events with priority of ERROR or higher (ERROR or FATAL
       that is).

       However, in order to suppress all logging entirely, you need to use a
       priority that's higher than FATAL: It is simply called "OFF", and it is
       never used by any logger. By definition, it is higher than the highest
       defined logger level.

       Therefore, if you keep the line

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           log4perl.threshold = OFF

       somewhere in your Log::Log4perl configuration, the system will be quiet
       as a graveyard. If you deactivate the line (e.g. by commenting it out),
       the system will, upon config reload, snap back to normal operation,
       providing logging messages according to the rest of the configuration
       file again.

   I keep getting duplicate log messages! What's wrong?
       Having several settings for related categories in the Log4perl
       configuration file sometimes leads to a phenomenon called "message
       duplication". It can be very confusing at first, but if thought through
       properly, it turns out that Log4perl behaves as advertised. But, don't
       despair, of course there's a number of ways to avoid message
       duplication in your logs.

       Here's a sample Log4perl configuration file that produces the

           log4perl.logger.Cat        = ERROR, Screen
           log4perl.logger.Cat.Subcat = WARN, Screen

           log4perl.appender.Screen   = Log::Log4perl::Appender::Screen
           log4perl.appender.Screen.layout = SimpleLayout

       It defines two loggers, one for category "Cat" and one for
       "Cat::Subcat", which is obviously a subcategory of "Cat".  The parent
       logger has a priority setting of ERROR, the child is set to the lower
       "WARN" level.

       Now imagine the following code in your program:

           my $logger = get_logger("Cat.Subcat");

       What do you think will happen? An unexperienced Log4perl user might
       think: "Well, the message is being sent with level WARN, so the
       "Cat::Subcat" logger will accept it and forward it to the attached
       "Screen" appender. Then, the message will percolate up the logger
       hierarchy, find the "Cat" logger, which will suppress the message
       because of its ERROR setting."  But, perhaps surprisingly, what you'll
       get with the code snippet above is not one but two log messages written
       to the screen:

           WARN - Warning!
           WARN - Warning!

       What happened? The culprit is that once the logger "Cat::Subcat"
       decides to fire, it will forward the message unconditionally to all
       directly or indirectly attached appenders. The "Cat" logger will never
       be asked if it wants the message or not -- the message will just be
       pushed through to the appender attached to "Cat".

       One way to prevent the message from bubbling up the logger hierarchy is

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       to set the "additivity" flag of the subordinate logger to 0:

           log4perl.logger.Cat            = ERROR, Screen
           log4perl.logger.Cat.Subcat     = WARN, Screen
           log4perl.additivity.Cat.Subcat = 0

           log4perl.appender.Screen   = Log::Log4perl::Appender::Screen
           log4perl.appender.Screen.layout = SimpleLayout

       The message will now be accepted by the "Cat::Subcat" logger, forwarded
       to its appender, but then "Cat::Subcat" will suppress any further
       action. While this setting avoids duplicate messages as seen before, it
       is often not the desired behaviour. Messages percolating up the
       hierarchy are a useful Log4perl feature.

       If you're defining different appenders for the two loggers, one other
       option is to define an appender threshold for the higher-level
       appender. Typically it is set to be equal to the logger's level

           log4perl.logger.Cat           = ERROR, Screen1
           log4perl.logger.Cat.Subcat    = WARN, Screen2

           log4perl.appender.Screen1   = Log::Log4perl::Appender::Screen
           log4perl.appender.Screen1.layout = SimpleLayout
           log4perl.appender.Screen1.Threshold = ERROR

           log4perl.appender.Screen2   = Log::Log4perl::Appender::Screen
           log4perl.appender.Screen2.layout = SimpleLayout

       Since the "Screen1" appender now blocks every message with a priority
       less than ERROR, even if the logger in charge lets it through, the
       message percolating up the hierarchy is being blocked at the last
       minute and not appended to "Screen1".

       So far, we've been operating well within the boundaries of the Log4j
       standard, which Log4perl adheres to. However, if you would really,
       really like to use a single appender and keep the message percolation
       intact without having to deal with message duplication, there's a non-
       standard solution for you:

           log4perl.logger.Cat        = ERROR, Screen
           log4perl.logger.Cat.Subcat = WARN, Screen

           log4perl.appender.Screen   = Log::Log4perl::Appender::Screen
           log4perl.appender.Screen.layout = SimpleLayout

           log4perl.oneMessagePerAppender = 1

       The "oneMessagePerAppender" flag will suppress duplicate messages to
       the same appender. Again, that's non-standard. But way cool :).

   How can I configure Log::Log4perl to send me email if something happens?
       Some incidents require immediate action. You can't wait until someone

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       checks the log files, you need to get notified on your pager right

       The easiest way to do that is by using the
       "Log::Dispatch::Email::MailSend" module as an appender. It comes with
       the "Log::Dispatch" bundle and allows you to specify recipient and
       subject of outgoing emails in the Log4perl configuration file:

           log4perl.category = FATAL, Mailer
           log4perl.appender.Mailer         = Log::Dispatch::Email::MailSend
           log4perl.appender.Mailer.subject = Something's broken!
           log4perl.appender.Mailer.layout  = SimpleLayout

       The message of every log incident this appender gets will then be
       forwarded to the given email address. Check the
       "Log::Dispatch::Email::MailSend" documentation for details. And please
       make sure there's not a flood of email messages sent out by your
       application, filling up the receipient's inbox.

       There's one caveat you need to know about: The "Log::Dispatch::Email"
       hierarchy of appenders turns on buffering by default. This means that
       the appender will not send out messages right away but wait until a
       certain threshold has been reached. If you'd rather have your alerts
       sent out immeditately, use

           log4perl.appender.Mailer.buffered = 0

       to turn buffering off.

   How can I write my own appender?
       First off, Log::Log4perl comes with a set of standard appenders. Then,
       there's a lot of Log4perl-compatible appenders already available on
       CPAN: Just run a search for "Log::Dispatch" on
       and chances are that what you're looking for has already been
       developed, debugged and been used successfully in production -- no need
       for you to reinvent the wheel.

       Also, Log::Log4perl ships with a nifty database appender named
       Log::Log4perl::Appender::DBI -- check it out if talking to databases is
       your desire.

       But if you're up for a truly exotic task, you might have to write an
       appender yourself. That's very easy -- it takes no longer than a couple
       of minutes.

       Say, we wanted to create an appender of the class
       "ColorScreenAppender", which logs messages to the screen in a
       configurable color. Just create a new class in

           package ColorScreenAppender;

       Now let's assume that your Log::Log4perl configuration file "test.conf"

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       looks like this:

           log4perl.logger = INFO, ColorApp


           log4perl.appender.ColorApp.layout = PatternLayout
           log4perl.appender.ColorApp.layout.ConversionPattern=%d %m %n

       This will cause Log::Log4perl on "init()" to look for a class
       ColorScreenAppender and call its constructor new(). Let's add new() to

           sub new {
               my($class, %options) = @_;

               my $self = { %options };
               bless $self, $class;

               return $self;

       To initialize this appender, Log::Log4perl will call and pass all
       attributes of the appender as defined in the configuration file to the
       constructor as name/value pairs (in this case just one):

           ColorScreenAppender->new(color => "blue");

       The new() method listed above stores the contents of the %options hash
       in the object's instance data hash (referred to by $self).  That's all
       for initializing a new appender with Log::Log4perl.

       Second, ColorScreenAppender needs to expose a "log()" method, which
       will be called by Log::Log4perl every time it thinks the appender
       should fire. Along with the object reference (as usual in Perl's object
       world), log() will receive a list of name/value pairs, of which only
       the one under the key "message" shall be of interest for now since it
       is the message string to be logged. At this point, Log::Log4perl has
       already taken care of joining the message to be a single string.

       For our special appender ColorScreenAppender, we're using the
       Term::ANSIColor module to colorize the output:

           use Term::ANSIColor;

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

               print colored($params{message},

       The color (as configured in the Log::Log4perl configuration file) is

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       available as $self->{color} in the appender object. Don't forget to


       at the end of and you're done. Install the new
       appender somewhere where perl can find it and try it with a test script

           use Log::Log4perl qw(:easy);

       to see the new colored output. Is this cool or what?

       And it gets even better: You can write dynamically generated appender
       classes using the "Class::Prototyped" module. Here's an example of an
       appender prepending every outgoing message with a configurable number
       of bullets:

           use Class::Prototyped;

           my $class = Class::Prototyped->newPackage(
             bullets => 1,
             log     => sub {
               my($self, %params) = @_;
               print "*" x $self->bullets(),

           use Log::Log4perl qw(:easy);

           Log::Log4perl->init(\ q{
             log4perl.logger = INFO, Bully


             log4perl.appender.Bully.layout = PatternLayout
             log4perl.appender.Bully.layout.ConversionPattern=%m %n

               # ... prints: "***Boo!\n";
           INFO "Boo!";

   How can I drill down on references before logging them?
       If you've got a reference to a nested structure or object, then you
       probably don't want to log it as "HASH(0x81141d4)" but rather dump it
       as something like

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           $VAR1 = {
                     'a' => 'b',
                     'd' => 'e'

       via a module like Data::Dumper. While it's syntactically correct to say


       this call imposes a huge performance penalty on your application if the
       message is suppressed by Log::Log4perl, because Data::Dumper will
       perform its expensive operations in any case, because it doesn't know
       that its output will be thrown away immediately.

       As of Log::Log4perl 0.28, there's a better way: Use the message output
       filter format as in

           $logger->debug( {filter => \&Data::Dumper::Dumper,
                            value  => $ref} );

       and Log::Log4perl won't call the filter function unless the message
       really gets written out to an appender. Just make sure to pass the
       whole slew as a reference to a hash specifying a filter function (as a
       sub reference) under the key "filter" and the value to be passed to the
       filter function in "value").  When it comes to logging, Log::Log4perl
       will call the filter function, pass the "value" as an argument and log
       the return value.  Saves you serious cycles.

   How can I collect all FATAL messages in an extra log file?
       Suppose you have employed Log4perl all over your system and you've
       already activated logging in various subsystems. On top of that,
       without disrupting any other settings, how can you collect all FATAL
       messages all over the system and send them to a separate log file?

       If you define a root logger like this:

           log4perl.logger                  = FATAL, File
           log4perl.appender.File           = Log::Log4perl::Appender::File
           log4perl.appender.File.filename  = /tmp/fatal.txt
           log4perl.appender.File.layout    = PatternLayout
           log4perl.appender.File.layout.ConversionPattern= %d %m %n
               # !!! Something's missing ...

       you'll be surprised to not only receive all FATAL messages issued
       anywhere in the system, but also everything else -- gazillions of
       ERROR, WARN, INFO and even DEBUG messages will end up in your fatal.txt
       logfile!  Reason for this is Log4perl's (or better: Log4j's) appender
       additivity.  Once a lower-level logger decides to fire, the message is
       going to be forwarded to all appenders upstream -- without further
       priority checks with their attached loggers.

       There's a way to prevent this, however: If your appender defines a
       minimum threshold, only messages of this priority or higher are going
       to be logged. So, just add

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           log4perl.appender.File.Threshold = FATAL

       to the configuration above, and you'll get what you wanted in the first
       place: An overall system FATAL message collector.

   How can I bundle several log messages into one?
       Would you like to tally the messages arriving at your appender and dump
       out a summary once they're exceeding a certain threshold?  So that
       something like


       won't be logged as


       but as

           [3] Blah

       instead? If you'd like to hold off on logging a message until it has
       been sent a couple of times, you can roll that out by creating a
       buffered appender.

       Let's define a new appender like

           package TallyAppender;

           sub new {
               my($class, %options) = @_;

               my $self = { maxcount => 5,

               bless $self, $class;

               $self->{last_message}        = "";
               $self->{last_message_count}  = 0;

               return $self;

       with two additional instance variables "last_message" and
       "last_message_count", storing the content of the last message sent and
       a counter of how many times this has happened. Also, it features a
       configuration parameter "maxcount" which defaults to 5 in the snippet
       above but can be set in the Log4perl configuration file like this:

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           log4perl.logger = INFO, A
           log4perl.appender.A.maxcount = 3

       The main tallying logic lies in the appender's "log" method, which is
       called every time Log4perl thinks a message needs to get logged by our

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

                   # Message changed? Print buffer.
               if($self->{last_message} and
                  $params{message} ne $self->{last_message}) {
                   print "[$self->{last_message_count}]: " .
                   $self->{last_message_count} = 1;
                   $self->{last_message} = $params{message};

               $self->{last_message} = $params{message};

                   # Threshold exceeded? Print, reset counter
               if($self->{last_message_count} >=
                  $self->{maxcount}) {
                   print "[$self->{last_message_count}]: " .
                   $self->{last_message_count} = 0;
                   $self->{last_message}       = "";

       We basically just check if the oncoming message in $param{message} is
       equal to what we've saved before in the "last_message" instance
       variable. If so, we're increasing "last_message_count".  We print the
       message in two cases: If the new message is different than the buffered
       one, because then we need to dump the old stuff and store the new. Or,
       if the counter exceeds the threshold, as defined by the "maxcount"
       configuration parameter.

       Please note that the appender always gets the fully rendered message
       and just compares it as a whole -- so if there's a date/timestamp in
       there, that might confuse your logic. You can work around this by
       specifying %m %n as a layout and add the date later on in the appender.
       Or, make the comparison smart enough to omit the date.

       At last, don't forget what happens if the program is being shut down.
       If there's still messages in the buffer, they should be printed out at
       that point. That's easy to do in the appender's DESTROY method, which
       gets called at object destruction time:

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           sub DESTROY {
               my($self) = @_;

               if($self->{last_message_count}) {
                   print "[$self->{last_message_count}]: " .

       This will ensure that none of the buffered messages are lost.  Happy

   I want to log ERROR and WARN messages to different files! How can I do
       Let's assume you wanted to have each logging statement written to a
       different file, based on the statement's priority. Messages with
       priority "WARN" are supposed to go to "/tmp/app.warn", events
       prioritized as "ERROR" should end up in "/tmp/app.error".

       Now, if you define two appenders "AppWarn" and "AppError" and assign
       them both to the root logger, messages bubbling up from any loggers
       below will be logged by both appenders because of Log4perl's message
       propagation feature. If you limit their exposure via the appender
       threshold mechanism and set "AppWarn"'s threshold to "WARN" and
       "AppError"'s to "ERROR", you'll still get "ERROR" messages in
       "AppWarn", because "AppWarn"'s "WARN" setting will just filter out
       messages with a lower priority than "WARN" -- "ERROR" is higher and
       will be allowed to pass through.

       What we need for this is a Log4perl Custom Filter, available with
       Log::Log4perl 0.30.

       Both appenders need to verify that the priority of the oncoming
       messages exactly matches the priority the appender is supposed to log
       messages of. To accomplish this task, let's define two custom filters,
       "MatchError" and "MatchWarn", which, when attached to their appenders,
       will limit messages passed on to them to those matching a given

           log4perl.logger = WARN, AppWarn, AppError

               # Filter to match level ERROR
           log4perl.filter.MatchError = Log::Log4perl::Filter::LevelMatch
           log4perl.filter.MatchError.LevelToMatch  = ERROR
           log4perl.filter.MatchError.AcceptOnMatch = true

               # Filter to match level WARN
           log4perl.filter.MatchWarn  = Log::Log4perl::Filter::LevelMatch
           log4perl.filter.MatchWarn.LevelToMatch  = WARN
           log4perl.filter.MatchWarn.AcceptOnMatch = true

               # Error appender
           log4perl.appender.AppError = Log::Log4perl::Appender::File

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           log4perl.appender.AppError.filename = /tmp/app.err
           log4perl.appender.AppError.layout   = SimpleLayout
           log4perl.appender.AppError.Filter   = MatchError

               # Warning appender
           log4perl.appender.AppWarn = Log::Log4perl::Appender::File
           log4perl.appender.AppWarn.filename = /tmp/app.warn
           log4perl.appender.AppWarn.layout   = SimpleLayout
           log4perl.appender.AppWarn.Filter   = MatchWarn

       The appenders "AppWarn" and "AppError" defined above are logging to
       "/tmp/app.warn" and "/tmp/app.err" respectively and have the custom
       filters "MatchWarn" and "MatchError" attached.  This setup will direct
       all WARN messages, issued anywhere in the system, to /tmp/app.warn (and
       ERROR messages to /tmp/app.error) -- without any overlaps.

   On our server farm, Log::Log4perl configuration files differ slightly from
       host to host. Can I roll them all into one?
       You sure can, because Log::Log4perl allows you to specify attribute
       values dynamically. Let's say that one of your appenders expects the
       host's IP address as one of its attributes. Now, you could certainly
       roll out different configuration files for every host and specify the
       value like

           log4perl.appender.MyAppender    = Log::Log4perl::Appender::SomeAppender
           log4perl.appender.MyAppender.ip =

       but that's a maintenance nightmare. Instead, you can have Log::Log4perl
       figure out the IP address at configuration time and set the appender's
       value correctly:

               # Set the IP address dynamically
           log4perl.appender.MyAppender    = Log::Log4perl::Appender::SomeAppender
           log4perl.appender.MyAppender.ip = sub { \
              use Sys::Hostname; \
              use Socket; \
              return inet_ntoa(scalar gethostbyname hostname); \

       If Log::Log4perl detects that an attribute value starts with something
       like "sub {...", it will interpret it as a perl subroutine which is to
       be executed once at configuration time (not runtime!) and its return
       value is to be used as the attribute value. This comes in handy for
       rolling out applications whichs Log::Log4perl configuration files show
       small host-specific differences, because you can deploy the unmodified
       application distribution on all instances of the server farm.

   Log4perl doesn't interpret my backslashes correctly!
       If you're using Log4perl's feature to specify the configuration as a
       string in your program (as opposed to a separate configuration file),
       chances are that you've written it like this:

           # *** WRONG! ***

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           Log::Log4perl->init( \ <<END_HERE);
               log4perl.logger = WARN, A1
               log4perl.appender.A1 = Log::Log4perl::Appender::Screen
               log4perl.appender.A1.layout = \
               log4perl.appender.A1.layout.ConversionPattern = %m%n

           # *** WRONG! ***

       and you're getting the following error message:

           Layout not specified for appender A1 at .../ line 342.

       What's wrong? The problem is that you're using a here-document with
       substitution enabled ("<<END_HERE") and that Perl won't interpret
       backslashes at line-ends as continuation characters but will
       essentially throw them out. So, in the code above, the layout line will
       look like

           log4perl.appender.A1.layout =

       to Log::Log4perl which causes it to report an error. To interpret the
       backslash at the end of the line correctly as a line-continuation
       character, use the non-interpreting mode of the here-document like in

           # *** RIGHT! ***

           Log::Log4perl->init( \ <<'END_HERE');
               log4perl.logger = WARN, A1
               log4perl.appender.A1 = Log::Log4perl::Appender::Screen
               log4perl.appender.A1.layout = \
               log4perl.appender.A1.layout.ConversionPattern = %m%n

           # *** RIGHT! ***

       (note the single quotes around 'END_HERE') or use "q{...}" instead of a
       here-document and Perl will treat the backslashes at line-end as

   I want to suppress certain messages based on their content!
       Let's assume you've plastered all your functions with Log4perl
       statements like

           sub some_func {

               INFO("Begin of function");

               # ... Stuff happens here ...

               INFO("End of function");

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       to issue two log messages, one at the beginning and one at the end of
       each function. Now you want to suppress the message at the beginning
       and only keep the one at the end, what can you do? You can't use the
       category mechanism, because both messages are issued from the same

       Log::Log4perl's custom filters (0.30 or better) provide an interface
       for the Log4perl user to step in right before a message gets logged and
       decide if it should be written out or suppressed, based on the message
       content or other parameters:

           use Log::Log4perl qw(:easy);

           Log::Log4perl::init( \ <<'EOT' );
               log4perl.logger             = INFO, A1
               log4perl.appender.A1        = Log::Log4perl::Appender::Screen
               log4perl.appender.A1.layout = \
               log4perl.appender.A1.layout.ConversionPattern = %m%n

               log4perl.filter.M1 = Log::Log4perl::Filter::StringMatch
               log4perl.filter.M1.StringToMatch = Begin
               log4perl.filter.M1.AcceptOnMatch = false

               log4perl.appender.A1.Filter = M1

       The last four statements in the configuration above are defining a
       custom filter "M1" of type "Log::Log4perl::Filter::StringMatch", which
       comes with Log4perl right out of the box and allows you to define a
       text pattern to match (as a perl regular expression) and a flag
       "AcceptOnMatch" indicating if a match is supposed to suppress the
       message or let it pass through.

       The last line then assigns this filter to the "A1" appender, which will
       call it every time it receives a message to be logged and throw all
       messages out not matching the regular expression "Begin".

       Instead of using the standard "Log::Log4perl::Filter::StringMatch"
       filter, you can define your own, simply using a perl subroutine:

           log4perl.filter.ExcludeBegin  = sub { !/Begin/ }
           log4perl.appender.A1.Filter   = ExcludeBegin

       For details on custom filters, check Log::Log4perl::Filter.

   My new module uses Log4perl -- but what happens if the calling program
       didn't configure it?
       If a Perl module uses Log::Log4perl, it will typically rely on the
       calling program to initialize it. If it is using Log::Log4perl in
       ":easy" mode, like in

           package MyMod;
           use Log::Log4perl qw(:easy);

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           sub foo {
               DEBUG("In foo");


       and the calling program doesn't initialize Log::Log4perl at all (e.g.
       because it has no clue that it's available), Log::Log4perl will
       silently ignore all logging messages. However, if the module is using
       Log::Log4perl in regular mode like in

           package MyMod;
           use Log::Log4perl qw(get_logger);

           sub foo {
               my $logger = get_logger("");


       and the main program is just using the module like in

           use MyMode;

       then Log::Log4perl will also ignore all logging messages but issue a
       warning like

           Log4perl: Seems like no initialization happened.
           Forgot to call init()?

       (only once!) to remind novice users to not forget to initialize the
       logging system before using it.  However, if you want to suppress this
       message, just add the ":nowarn" target to the module's "use
       Log::Log4perl" call:

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

       This will have Log::Log4perl silently ignore all logging statements if
       no initialization has taken place.

       If the module wants to figure out if some other program part has
       already initialized Log::Log4perl, it can do so by calling


       which will return a true value in case Log::Log4perl has been
       initialized and a false value if not.

   How can I synchronize access to an appender?
       If you're using the same instance of an appender in multiple processes,
       and each process is passing on messages to the appender in parallel,

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       you might end up with overlapping log entries.

       Typical scenarios include a file appender that you create in the main
       program, and which will then be shared between the parent and a forked
       child process. Or two separate processes, each initializing a Log4perl
       file appender on the same logfile.

       Log::Log4perl won't synchronize access to the shared logfile by
       default. Depending on your operating system's flush mechanism, buffer
       size and the size of your messages, there's a small chance of an

       The easiest way to prevent overlapping messages in logfiles written to
       by multiple processes is setting the file appender's "syswrite" flag
       along with a file write mode of "append".  This makes sure that
       "Log::Log4perl::Appender::File" uses "syswrite()" (which is guaranteed
       to run uninterrupted) instead of "print()" which might buffer the
       message or get interrupted by the OS while it is writing. And in
       "append" mode, the OS kernel ensures that multiple processes share one
       end-of-file marker, ensuring that each process writes to the real end
       of the file. (The value of "append" for the "mode" parameter is the
       default setting in Log4perl's file appender so you don't have to set it

             # Guarantees atomic writes

           log4perl.category.Bar.Twix          = WARN, Logfile

           log4perl.appender.Logfile           = Log::Log4perl::Appender::File
           log4perl.appender.Logfile.mode      = append
           log4perl.appender.Logfile.syswrite  = 1
           log4perl.appender.Logfile.filename  = test.log
           log4perl.appender.Logfile.layout    = SimpleLayout

       Another guaranteed way of having messages separated with any kind of
       appender is putting a Log::Log4perl::Appender::Synchronized composite
       appender in between Log::Log4perl and the real appender. It will make
       sure to let messages pass through this virtual gate one by one only.

       Here's a sample configuration to synchronize access to a file appender:

           log4perl.category.Bar.Twix          = WARN, Syncer

           log4perl.appender.Logfile           = Log::Log4perl::Appender::File
           log4perl.appender.Logfile.autoflush = 1
           log4perl.appender.Logfile.filename  = test.log
           log4perl.appender.Logfile.layout    = SimpleLayout

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

       "Log::Log4perl::Appender::Synchronized" uses the "IPC::Shareable"
       module and its semaphores, which will slow down writing the log
       messages, but ensures sequential access featuring atomic checks.  Check

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       Log::Log4perl::Appender::Synchronized for details.

   Can I use Log::Log4perl with log4j's Chainsaw?
       Yes, Log::Log4perl can be configured to send its events to log4j's
       graphical log UI Chainsaw.

       Here's how it works:

       o   Get Guido Carls' <> Log::Log4perl extension
           "Log::Log4perl::Layout::XMLLayout" from CPAN and install it:

               perl -MCPAN -eshell
               cpan> install Log::Log4perl::Layout::XMLLayout

       o   Install and start Chainsaw, which is part of the "log4j"
           distribution now (see ). Create a
           configuration file like

             <log4j:configuration debug="true">
               <plugin name="XMLSocketReceiver"
                 <param name="decoder" value="org.apache.log4j.xml.XMLDecoder"/>
                 <param name="Port" value="4445"/>
               <root> <level value="debug"/> </root>

           and name it e.g. "config.xml". Then start Chainsaw like

             java -Dlog4j.debug=true -Dlog4j.configuration=config.xml \
               -classpath ".:log4j-1.3alpha.jar:log4j-chainsaw-1.3alpha.jar" \

           and watch the GUI coming up.

       o   Configure Log::Log4perl to use a socket appender with an XMLLayout,
           pointing to the host/port where Chainsaw (as configured above) is
           waiting with its XMLSocketReceiver:

             use Log::Log4perl qw(get_logger);
             use Log::Log4perl::Layout::XMLLayout;

             my $conf = q(
               log4perl.category.Bar.Twix          = WARN, Appender
               log4perl.appender.Appender          = Log::Log4perl::Appender::Socket
               log4perl.appender.Appender.PeerAddr = localhost
               log4perl.appender.Appender.PeerPort = 4445
               log4perl.appender.Appender.layout   = Log::Log4perl::Layout::XMLLayout


               # Nasty hack to suppress encoding header
             my $app = Log::Log4perl::appenders->{"Appender"};

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             $app->layout()->{enc_set} = 1;

             my $logger = get_logger("Bar.Twix");

           The nasty hack shown in the code snippet above is currently
           (October 2003) necessary, because Chainsaw expects XML messages to
           arrive in a format like

             <log4j:event logger="Bar.Twix"
               <log4j:locationInfo class="main"

           without a preceding

             <?xml version = "1.0" encoding = "iso8859-1"?>

           which Log::Log4perl::Layout::XMLLayout applies to the first event
           sent over the socket.

       See figure 1 for a screenshot of Chainsaw in action, receiving events
       from the Perl script shown above.

       Many thanks to Chainsaw's Scott Deboy <> for
       his support!

   How can I run Log::Log4perl under mod_perl?
       In persistent environments it's important to play by the rules outlined
       in section "Initialize once and only once" in Log::Log4perl.  If you
       haven't read this yet, please go ahead and read it right now. It's very

       And no matter if you use a startup handler to init() Log::Log4perl or
       use the init_once() strategy (added in 0.42), either way you're very
       likely to have unsynchronized writes to logfiles.

       If Log::Log4perl is configured with a log file appender, and it is
       initialized via the Apache startup handler, the file handle created
       initially will be shared among all Apache processes. Similarly, with
       the init_once() approach: although every process has a separate L4p
       configuration, processes are gonna share the appender file names
       instead, effectively opening several different file handles on the same

       Now, having several appenders using the same file handle or having

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       several appenders logging to the same file unsynchronized, this might
       result in overlapping messages. Sometimes, this is acceptable. If it's
       not, here's two strategies:

       o   Use the Log::Log4perl::Appender::Synchronized appender to connect
           to your file appenders. Here's the writeup:

       o   Use a different logfile for every process like in

                log4perl.appender.A1.filename = sub { "mylog.$$.log" }

   My program already uses warn() and die(). How can I switch to Log4perl?
       If your program already uses Perl's "warn()" function to spew out error
       messages and you'd like to channel those into the Log4perl world, just
       define a "__WARN__" handler where your program or module resides:

           use Log::Log4perl qw(:easy);

           $SIG{__WARN__} = sub {
               local $Log::Log4perl::caller_depth =
                   $Log::Log4perl::caller_depth + 1;
               WARN @_;

       Why the "local" setting of $Log::Log4perl::caller_depth?  If you leave
       that out, "PatternLayout" conversion specifiers like %M or %F (printing
       the current function/method and source filename) will refer to where
       the __WARN__ handler resides, not the environment Perl's "warn()"
       function was issued from. Increasing "caller_depth" adjusts for this
       offset. Having it "local", makes sure the level gets set back after the
       handler exits.

       Once done, if your program does something like

           sub some_func {
               warn "Here's a warning";

       you'll get (depending on your Log::Log4perl configuration) something

           2004/02/19 20:41:02-main::some_func: Here's a warning at ./t line 25.

       in the appropriate appender instead of having a screen full of STDERR
       messages. It also works with the "Carp" module and its "carp()" and
       "cluck()" functions.

       If, on the other hand, catching "die()" and friends is required, a
       "__DIE__" handler is appropriate:

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           $SIG{__DIE__} = sub {
               if($^S) {
                   # We're in an eval {} and don't want log
                   # this message but catch it later
               LOGDIE @_;

       This will call Log4perl's "LOGDIE()" function, which will log a fatal
       error and then call die() internally, causing the program to exit.
       Works equally well with "Carp"'s "croak()" and "confess()" functions.

   Some module prints messages to STDERR. How can I funnel them to
       If a module you're using doesn't use Log::Log4perl but prints logging
       messages to STDERR instead, like

           package IgnorantModule;

           sub some_method {
               print STDERR "Parbleu! An error!\n";


       there's still a way to capture these messages and funnel them into
       Log::Log4perl, even without touching the module. What you need is a
       trapper module like

           package Trapper;

           use Log::Log4perl qw(:easy);

           sub TIEHANDLE {
               my $class = shift;
               bless [], $class;

           sub PRINT {
               my $self = shift;
               DEBUG @_;


       and a "tie" command in the main program to tie STDERR to the trapper

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       module along with regular Log::Log4perl initialization:

           package main;

           use Log::Log4perl qw(:easy);

               {level  => $DEBUG,
                file   => 'stdout',   # make sure not to use stderr here!
                layout => "%d %M: %m%n",

           tie *STDERR, "Trapper";

       Make sure not to use STDERR as Log::Log4perl's file appender here
       (which would be the default in ":easy" mode), because it would end up
       in an endless recursion.

       Now, calling


       will result in the desired output

           2004/05/06 11:13:04 IgnorantModule::some_method: Parbleu! An error!

   How come PAR (Perl Archive Toolkit) creates executables which then can't
       find their Log::Log4perl appenders?
       If not instructed otherwise, "Log::Log4perl" dynamically pulls in
       appender classes found in its configuration. If you specify


           use Log::Log4perl qw(get_logger);

           my $conf = q(
             log4perl.category.Bar.Twix = WARN, Logfile
             log4perl.appender.Logfile  = Log::Log4perl::Appender::Screen
             log4perl.appender.Logfile.layout = SimpleLayout

           my $logger = get_logger("Bar::Twix");

       then "Log::Log4perl::Appender::Screen" will be pulled in while the
       program runs, not at compile time. If you have PAR compile the script
       above to an executable binary via

           pp -o mytest

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       and then run "mytest" on a machine without having Log::Log4perl
       installed, you'll get an error message like

           ERROR: can't load appenderclass 'Log::Log4perl::Appender::Screen'
           Can't locate Log/Log4perl/Appender/ in @INC ...

       Why? At compile time, "pp" didn't realize that
       "Log::Log4perl::Appender::Screen" would be needed later on and didn't
       wrap it into the executable created. To avoid this, either say "use
       Log::Log4perl::Appender::Screen" in the script explicitely or compile
       it with

           pp -o mytest -M Log::Log4perl::Appender::Screen

       to make sure the appender class gets included.

   How can I access a custom appender defined in the configuration?
       Any appender defined in the configuration file or somewhere in the code
       can be accessed later via
       "Log::Log4perl->appender_by_name("appender_name")", which returns a
       reference the the appender object.

       Once you've got a hold of the object, it can be queried or modified to
       your liking. For example, see the custom "IndentAppender" defined
       below: After calling "init()" to define the Log4perl settings, the
       appender object is retrieved to call its "indent_more()" and
       "indent_less()" methods to control indentation of messages:

           package IndentAppender;

           sub new {
               bless { indent => 0 }, $_[0];

           sub indent_more  { $_[0]->{indent}++ }
           sub indent_less  { $_[0]->{indent}-- }

           sub log {
               my($self, %params) = @_;
               print " " x $self->{indent}, $params{message};

           package main;

           use Log::Log4perl qw(:easy);

           my $conf = q(
           log4perl.category          = DEBUG, Indented
           log4perl.appender.Indented = IndentAppender
           log4perl.appender.Indented.layout = Log::Log4perl::Layout::SimpleLayout


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           my $appender = Log::Log4perl->appender_by_name("Indented");

           DEBUG "No identation";
           DEBUG "One more";
           DEBUG "Two more";
           DEBUG "One less";

       As you would expect, this will print

           DEBUG - No identation
            DEBUG - One more
             DEBUG - Two more
            DEBUG - One less

       because the very appender used by Log4perl is modified dynamically at

   I don't know if Log::Log4perl is installed. How can I prepare my script?
       In case your script needs to be prepared for environments that may or
       may not have Log::Log4perl installed, there's a trick.

       If you put the following BEGIN blocks at the top of the program, you'll
       be able to use the DEBUG(), INFO(), etc. macros in Log::Log4perl's
       ":easy" mode.  If Log::Log4perl is installed in the target environment,
       the regular Log::Log4perl rules apply. If not, all of DEBUG(), INFO(),
       etc. are "stubbed" out, i.e. they turn into no-ops:

           use warnings;
           use strict;

           BEGIN {
               eval { require Log::Log4perl; };

               if($@) {
                   print "Log::Log4perl not installed - stubbing.\n";
                   no strict qw(refs);
                   *{"main::$_"} = sub { } for qw(DEBUG INFO WARN ERROR FATAL);
               } else {
                   no warnings;
                   print "Log::Log4perl installed - life is good.\n";
                   require Log::Log4perl::Level;

               # The regular script begins ...
           DEBUG "Hey now!";

       This snippet will first probe for Log::Log4perl, and if it can't be

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       found, it will alias DEBUG(), INFO(), with empty subroutines via
       typeglobs.  If Log::Log4perl is available, its level constants are
       first imported ($DEBUG, $INFO, etc.) and then "easy_init()" gets called
       to initialize the logging system.

   Can file appenders create files with different permissions?
       Typically, when "Log::Log4perl::Appender::File" creates a new file, its
       permissions are set to "rw-r--r--". Why? Because your environment's
       umask most likely defaults to 0022, that's the standard setting.

       What's a umask, you're asking? It's a template that's applied to the
       permissions of all newly created files. While calls like "open(FILE,
       ">foo")" will always try to create files in "rw-rw-rw- " mode, the
       system will apply the current umask template to determine the final
       permission setting. umask is a bit mask that's inverted and then
       applied to the requested permission setting, using a bitwise AND:

           $request_permission &~ $umask

       So, a umask setting of 0000 (the leading 0 simply indicates an octal
       value) will create files in "rw-rw-rw-" mode, a setting of 0277 will
       use "r--------", and the standard 0022 will use "rw-r--r--".

       As an example, if you want your log files to be created with
       "rw-r--rw-" permissions, use a umask of 0020 before calling

           use Log::Log4perl;

           umask 0020;
               # Creates log.out in rw-r--rw mode
           Log::Log4perl->init(\ q{
               log4perl.logger = WARN, File
               log4perl.appender.File = Log::Log4perl::Appender::File
               log4perl.appender.File.filename = log.out
               log4perl.appender.File.layout = SimpleLayout

   Using Log4perl in an END block causes a problem!
       It's not easy to get to this error, but if you write something like

           END { Log::Log4perl::get_logger()->debug("Hey there."); }

           use Log::Log4perl qw(:easy);

       it won't work. The reason is that "Log::Log4perl" defines an END block
       that cleans up all loggers. And perl will run END blocks in the reverse
       order as they're encountered in the compile phase, so in the scenario
       above, the END block will run after Log4perl has cleaned up its

       Placing END blocks using Log4perl after a "use Log::Log4perl" statement
       fixes the problem:

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           use Log::Log4perl qw(:easy);

           END { Log::Log4perl::get_logger()->debug("Hey there."); }

       In this scenario, the shown END block is executed before Log4perl
       cleans up and the debug message will be processed properly.

   Help! My appender is throwing a "Wide character in print" warning!
       This warning shows up when Unicode strings are printed without
       precautions. The warning goes away if the complaining appender is set
       to utf-8 mode:

             # Either in the log4perl configuration file:
         log4perl.appender.Logfile.filename = test.log
         log4perl.appender.Logfile.utf8     = 1

             # Or, in easy mode:
         Log::Log4perl->easy_init( {
           level => $DEBUG,
           file  => ":utf8> test.log"
         } );

       If the complaining appender is a screen appender, set its "utf8"

             log4perl.appender.Screen.stderr = 1
             log4perl.appender.Screen.utf8   = 1

       Alternatively, "binmode" does the trick:

             # Either STDOUT ...
           binmode(STDOUT, ":utf8);

             # ... or STDERR.
           binmode(STDERR, ":utf8);

       Some background on this: Perl's strings are either byte strings or
       Unicode strings. "Mike" is a byte string.  "\x{30DE}\x{30A4}\x{30AF}"
       is a Unicode string. Unicode strings are marked specially and are UTF-8
       encoded internally.

       If you print a byte string to STDOUT, all is well, because STDOUT is by
       default set to byte mode. However, if you print a Unicode string to
       STDOUT without precautions, "perl" will try to transform the Unicode
       string back to a byte string before printing it out. This is
       troublesome if the Unicode string contains 'wide' characters which
       can't be represented in Latin-1.

       For example, if you create a Unicode string with three japanese
       Katakana characters as in

           perl -le 'print "\x{30DE}\x{30A4}\x{30AF}"'

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       (coincidentally pronounced Ma-i-ku, the japanese pronounciation of
       "Mike"), STDOUT is in byte mode and the warning

           Wide character in print at ./ line 14.

       appears. Setting STDOUT to UTF-8 mode as in

           perl -le 'binmode(STDOUT, ":utf8"); print "\x{30DE}\x{30A4}\x{30AF}"'

       will silently print the Unicode string to STDOUT in UTF-8. To see the
       characters printed, you'll need a UTF-8 terminal with a font including
       japanese Katakana characters.

   How can I send errors to the screen, and debug messages to a file?
       Let's assume you want to maintain a detailed DEBUG output in a file and
       only messages of level ERROR and higher should be printed on the
       screen. Often times, developers come up with something like this:

            # Wrong!!!
           log4perl.logger = DEBUG, FileApp
           log4perl.logger = ERROR, ScreenApp
            # Wrong!!!

       This won't work, however. Logger definitions aren't additive, and the
       second statement will overwrite the first one. Log4perl versions below
       1.04 were silently accepting this, leaving people confused why it
       wouldn't work as expected.  As of 1.04, this will throw a fatal error
       to notify the user of the problem.

       What you want to do instead, is this:

           log4perl.logger                    = DEBUG, FileApp, ScreenApp

           log4perl.appender.FileApp          = Log::Log4perl::Appender::File
           log4perl.appender.FileApp.filename = test.log
           log4perl.appender.FileApp.layout   = SimpleLayout

           log4perl.appender.ScreenApp          = Log::Log4perl::Appender::Screen
           log4perl.appender.ScreenApp.stderr   = 0
           log4perl.appender.ScreenApp.layout   = SimpleLayout
              ### limiting output to ERROR messages
           log4perl.appender.ScreenApp.Threshold = ERROR

       Note that without the second appender's "Threshold" setting, both
       appenders would receive all messages prioritized DEBUG and higher. With
       the threshold set to ERROR, the second appender will filter the
       messages as required.

   Where should I put my logfiles?
       Your log files may go anywhere you want them, but the effective user id
       of the calling process must have write access.

       If the log file doesn't exist at program start, Log4perl's file

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       appender will create it. For this, it needs write access to the
       directory where the new file will be located in. If the log file
       already exists at startup, the process simply needs write access to the
       file. Note that it will need write access to the file's directory if
       you're encountering situations where the logfile gets recreated, e.g.
       during log rotation.

       If Log::Log4perl is used by a web server application (e.g. in a CGI
       script or mod_perl), then the webserver's user (usually "nobody" or
       "www") must have the permissions mentioned above.

       To prepare your web server to use log4perl, we'd recommend:

           webserver:~$ su -
           webserver:~# mkdir /var/log/cgiapps
           webserver:~# chown nobody:root /var/log/cgiapps/
           webserver:~# chown nobody:root -R /var/log/cgiapps/
           webserver:~# chmod 02755 -R /var/log/cgiapps/

       Then set your /etc/log4perl.conf file to include:

           log4perl.appender.FileAppndr1.filename =

   How can my file appender deal with disappearing log files?
       The file appender that comes with Log4perl,
       Log::Log4perl::Appender::File, will open a specified log file at
       initialization time and will keep writing to it via a file handle.

       In case the associated file goes way, messages written by a long-
       running process will still be written to the file handle. In case the
       file has been moved to a different location on the same file system,
       the writer will keep writing to it under the new filename. In case the
       file has been removed from the file system, the log messages will end
       up in nowhere land. This is not a bug in Log4perl, this is how Unix
       works. There is no error message in this case, because the writer has
       no idea that the file handle is not associated with a visible file.

       To prevent the loss of log messages when log files disappear, the file
       appender's "recreate" option needs to be set to a true value:

           log4perl.appender.Logfile.recreate = 1

       This will instruct the file appender to check in regular intervals
       (default: 30 seconds) if the log file is still there. If it finds out
       that the file is missing, it will recreate it.

       Continuously checking if the log file still exists is fairly expensive.
       For this reason it is only performed every 30 seconds. To change this
       interval, the option "recreate_check_interval" can be set to the number
       of seconds between checks. In the extreme case where the check should
       be performed before every write, it can even be set to 0:

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           log4perl.appender.Logfile.recreate = 1
           log4perl.appender.Logfile.recreate_check_interval = 0

       To avoid having to check the file system so frequently, a signal
       handler can be set up:

           log4perl.appender.Logfile.recreate = 1
           log4perl.appender.Logfile.recreate_check_signal = USR1

       This will install a signal handler which will recreate a missing log
       file immediatly when it receives the defined signal.

       Note that the init_and_watch() method for Log4perl's initialization can
       also be instructed to install a signal handler, usually using the HUP
       signal. Make sure to use a different signal if you're using both of
       them at the same time.

   How can I rotate a logfile with newsyslog?
       Here's a few things that need to be taken care of when using the
       popular log file rotating utilty "newsyslog"
       ( with Log4perl's file appender in
       long-running processes.

       For example, with a newsyslog configuration like

           # newsyslog.conf
           /tmp/test.log 666  12  5  *  B

       and a call to

           # newsyslog -f /path/to/newsyslog.conf

       "newsyslog" will take action if "/tmp/test.log" is larger than the
       specified 5K in size. It will move the current log file "/tmp/test.log"
       to "/tmp/test.log.0" and create a new and empty "/tmp/test.log" with
       the specified permissions (this is why "newsyslog" needs to run as
       root).  An already existing "/tmp/test.log.0" would be moved to
       "/tmp/test.log.1", "/tmp/test.log.1" to "/tmp/test.log.2", and so
       forth, for every one of a max number of 12 archived logfiles that have
       been configured in "newsyslog.conf".

       Although a new file has been created, from Log4perl's appender's point
       of view, this situation is identical to the one described in the
       previous FAQ entry, labeled "How can my file appender deal with
       disappearing log files".

       To make sure that log messages are written to the new log file and not
       to an archived one or end up in nowhere land, the appender's "recreate"
       and "recreate_check_interval" have to be configured to deal with the
       'disappearing' log file.

       The situation gets interesting when "newsyslog"'s option to compress
       archived log files is enabled. This causes the original log file not to
       be moved, but to disappear. If the file appender isn't configured to

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       recreate the logfile in this situation, log messages will actually be
       lost without warning. This also applies for the short time frame of
       "recreate_check_interval" seconds in between the recreator's file

       To make sure that no messages get lost, one option is to set the
       interval to

           log4perl.appender.Logfile.recreate_check_interval = 0

       However, this is fairly expensive. A better approach is to define a
       signal handler:

           log4perl.appender.Logfile.recreate = 1
           log4perl.appender.Logfile.recreate_check_signal  = USR1
           log4perl.appender.Logfile.recreate_pid_write = /tmp/myappid

       As a service for "newsyslog" users, Log4perl's file appender writes the
       current process ID to a PID file specified by the "recreate_pid_write"
       option.  "newsyslog" then needs to be configured as in

           # newsyslog.conf configuration for compressing archive files and
           # sending a signal to the Log4perl-enabled application
           /tmp/test.log 666  12  5  *  B /tmp/myappid 30

       to send the defined signal (30, which is USR1 on FreeBSD) to the
       application process at rotation time. Note that the signal number is
       different on Linux, where USR1 denotes as 10. Check "man signal" for

   How can a process under user id A log to a file under user id B?
       This scenario often occurs in configurations where processes run under
       various user IDs but need to write to a log file under a fixed, but
       different user id.

       With a traditional file appender, the log file will probably be created
       under one user's id and appended to under a different user's id. With a
       typical umask of 0002, the file will be created with -rw-rw-r--
       permissions. If a user who's not in the first user's group subsequently
       appends to the log file, it will fail because of a permission problem.

       Two potential solutions come to mind:

       o   Creating the file with a umask of 0000 will allow all users to
           append to the log file. Log4perl's file appender
           "Log::Log4perl::Appender::File" has an "umask" option that can be
           set to support this:

               log4perl.appender.File = Log::Log4perl::Appender::File
               log4perl.appender.File.umask = sub { 0000 };

           This way, the log file will be created with -rw-rw-rw- permissions
           and therefore has world write permissions. This might open up the
           logfile for unwanted manipulations by arbitrary users, though.

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       o   Running the process under an effective user id of "root" will allow
           it to write to the log file, no matter who started the process.
           However, this is not a good idea, because of security concerns.

       Luckily, under Unix, there's the syslog daemon which runs as root and
       takes log requests from user processes over a socket and writes them to
       log files as configured in "/etc/syslog.conf".

       By modifying "/etc/syslog.conf" and HUPing the syslog daemon, you can
       configure new log files:

           # /etc/syslog.conf
           user.* /some/path/file.log

       Using the "Log::Dispatch::Syslog" appender, which comes with the
       "Log::Log4perl" distribution, you can then send messages via syslog:

           use Log::Log4perl qw(:easy);

               log4perl.logger = DEBUG, app

               # Writes to /some/path/file.log
           ERROR "Message!";

       This way, the syslog daemon will solve the permission problem.

       Note that while it is possible to use syslog() without Log4perl (syslog
       supports log levels, too), traditional syslog setups have a significant

       Without Log4perl's ability to activate logging in only specific parts
       of a system, complex systems will trigger log events all over the place
       and slow down execution to a crawl at high debug levels.

       Remote-controlling logging in the hierarchical parts of an application
       via Log4perl's categories is one of its most distinguished features.
       It allows for enabling high debug levels in specified areas without
       noticable performance impact.

   I want to use UTC instead of the local time!
       If a layout defines a date, Log::Log4perl uses local time to populate
       it.  If you want UTC instead, set

           $Log::Log4perl::DateFormat::GMTIME = 1;

       in your program before the first log statement.

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   Can Log4perl intercept messages written to a filehandle?
       You have a function that prints to a filehandle. You want to tie into
       that filehandle and forward all arriving messages to a Log4perl logger.

       First, let's write a package that ties a file handle and forwards it to
       a Log4perl logger:

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

           sub TIEHANDLE {
              my($class, %options) = @_;

              my $self = {
                  level    => $DEBUG,
                  category => '',

              $self->{logger} = get_logger($self->{category}),
              bless $self, $class;

           sub PRINT {
               my($self, @rest) = @_;
               $self->{logger}->log($self->{level}, @rest);

           sub PRINTF {
               my($self, $fmt, @rest) = @_;
               $self->PRINT(sprintf($fmt, @rest));


       Now, if you have a function like

           sub function_printing_to_fh {
               my($fh) = @_;
               printf $fh "Hi there!\n";

       which takes a filehandle and prints something to it, it can be used
       with Log4perl:

           use Log::Log4perl qw(:easy);
           usa FileHandleLogger;


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           tie *SOMEHANDLE, 'FileHandleLogger' or
               die "tie failed ($!)";

               # prints "2007/03/22 21:43:30 Hi there!"

       If you want, you can even specify a different log level or category:

           tie *SOMEHANDLE, 'FileHandleLogger',
               level => $INFO, category => "Foo::Bar" or die "tie failed ($!)";

   I want multiline messages rendered line-by-line!
       With the standard "PatternLayout", if you send a multiline message to
       an appender as in

           use Log::Log4perl qw(:easy);

       it gets rendered this way:

           2007/04/04 23:23:39 multi

       If you want each line to be rendered separately according to the layout
       use "Log::Log4perl::Layout::PatternLayout::Multiline":

           use Log::Log4perl qw(:easy);

             log4perl.category         = DEBUG, Screen
             log4perl.appender.Screen = Log::Log4perl::Appender::Screen
             log4perl.appender.Screen.layout = \\
             log4perl.appender.Screen.layout.ConversionPattern = %d %m %n

           DEBUG "some\nmultiline\nmessage";

       and you'll get

           2007/04/04 23:23:39 some
           2007/04/04 23:23:39 multiline
           2007/04/04 23:23:39 message


   I'm on Windows and I'm getting all these 'redefined' messages!
       If you're on Windows and are getting warning messages like

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         Constant subroutine Log::Log4perl::_INTERNAL_DEBUG redefined at
           C:/Programme/Perl/lib/ line 103.
         Subroutine import redefined at
           C:/Programme/Perl/site/lib/Log/ line 69.
         Subroutine initialized redefined at
           C:/Programme/Perl/site/lib/Log/ line 207.

       then chances are that you're using 'Log::Log4Perl' (wrong uppercase P)
       instead of the correct 'Log::Log4perl'. Perl on Windows doesn't handle
       this error well and spits out a slew of confusing warning messages. But
       now you know, just use the correct module name and you'll be fine.

   Log4perl complains that no initialization happened during shutdown!
       If you're using Log4perl log commands in DESTROY methods of your
       objects, you might see confusing messages like

           Log4perl: Seems like no initialization happened. Forgot to call init()?
           Use of uninitialized value in subroutine entry at
           /home/y/lib/perl5/site_perl/5.6.1/Log/ line 134 during global
           destruction. (in cleanup) Undefined subroutine &main:: called at
           /home/y/lib/perl5/site_perl/5.6.1/Log/ line 134 during global

       when the program shuts down. What's going on?

       This phenomenon happens if you have circular references in your
       objects, which perl can't clean up when an object goes out of scope but
       waits until global destruction instead. At this time, however, Log4perl
       has already shut down, so you can't use it anymore.

       For example, here's a simple class which uses a logger in its DESTROY

           package A;
           use Log::Log4perl qw(:easy);
           sub new { bless {}, shift }
           sub DESTROY { DEBUG "Waaah!"; }

       Now, if the main program creates a self-referencing object, like in

           package main;
           use Log::Log4perl qw(:easy);

           my $a = A->new();
           $a->{selfref} = $a;

       then you'll see the error message shown above during global
       destruction.  How to tackle this problem?

       First, you should clean up your circular references before global
       destruction. They will not only cause objects to be destroyed in an
       order that's hard to predict, but also eat up memory until the program
       shuts down.

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       So, the program above could easily be fixed by putting

           $a->{selfref} = undef;

       at the end or in an END handler. If that's hard to do, use weak

           package main;
           use Scalar::Util qw(weaken);
           use Log::Log4perl qw(:easy);

           my $a = A->new();
           $a->{selfref} = weaken $a;

       This allows perl to clean up the circular reference when the object
       goes out of scope, and doesn't wait until global destruction.

   How can I access POE heap values from Log4perl's layout?
       POE is a framework for creating multitasked applications running in a
       single process and a single thread. POE's threads equivalents are
       'sessions' and since they run quasi-simultaneously, you can't use
       Log4perl's global NDC/MDC to hold session-specific data.

       However, POE already maintains a data store for every session. It is
       called 'heap' and is just a hash storing session-specific data in key-
       value pairs.  To access this per-session heap data from a Log4perl
       layout, define a custom cspec and reference it with the newly defined
       pattern in the layout:

           use strict;
           use POE;
           use Log::Log4perl qw(:easy);

           Log::Log4perl->init( \ q{
               log4perl.logger = DEBUG, Screen
               log4perl.appender.Screen = Log::Log4perl::Appender::Screen
               log4perl.appender.Screen.layout = PatternLayout
               log4perl.appender.Screen.layout.ConversionPattern = %U %m%n
               log4perl.PatternLayout.cspec.U = \
                   sub { POE::Kernel->get_active_session->get_heap()->{ user } }
           } );

           for (qw( Huey Lewey Dewey )) {
                   inline_states => {
                       _start    => sub {
                           $_[HEAP]->{user} = $_;
                       hello     => sub {
                           DEBUG "I'm here now";

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       The code snippet above defines a new layout placeholder (called 'cspec'
       in Log4perl) %U which calls a subroutine, retrieves the active session,
       gets its heap and looks up the entry specified ('user').

       Starting with Log::Log4perl 1.20, cspecs also support parameters in
       curly braces, so you can say

           log4perl.appender.Screen.layout.ConversionPattern = %U{user} %U{id} %m%n
           log4perl.PatternLayout.cspec.U = \
                   sub { POE::Kernel->get_active_session-> \
                         get_heap()->{ $_[0]->{curlies} } }

       and print the POE session heap entries 'user' and 'id' with every
       logged message. For more details on cpecs, read the PatternLayout

   I want to print something unconditionally!
       Sometimes it's a script that's supposed to log messages regardless if
       Log4perl has been initialized or not. Or there's a logging statement
       that's not going to be suppressed under any circumstances -- many
       people want to have the final word, make the executive decision,
       because it seems like the only logical choice.

       But think about it: First off, if a messages is supposed to be printed,
       where is it supposed to end up at? STDOUT? STDERR? And are you sure you
       want to set in stone that this message needs to be printed, while
       someone else might find it annoying and wants to get rid of it?

       The truth is, there's always going to be someone who wants to log a
       messages at all cost, but also another person who wants to suppress it
       with equal vigilance. There's no good way to serve these two
       conflicting desires, someone will always want to win at the cost of
       leaving the other party dissappointed.

       So, the best Log4perl offers is the ALWAYS level for a message that
       even fires if the system log level is set to $OFF:

           use Log::Log4perl qw(:easy);

           Log::Log4perl->easy_init( $OFF );
           ALWAYS "This gets logged always. Well, almost always";

       The logger won't fire, though, if Log4perl hasn't been initialized or
       if someone defines a custom log hurdle that's higher than $OFF.

       Bottom line: Leave the setting of the logging level to the initial Perl
       script -- let their owners decided what they want, no matter how
       tempting it may be to decide it for them.

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       Mike Schilli, <>

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