######################################################################
    Log::Log4perl 1.06
######################################################################

NAME
    Log::Log4perl - Log4j implementation for Perl

SYNOPSIS
            # Easy mode if you like it simple ...

        use Log::Log4perl qw(:easy);
        Log::Log4perl->easy_init($ERROR);

        DEBUG "This doesn't go anywhere";
        ERROR "This gets logged";

            # ... or standard mode for more features:

        Log::Log4perl::init('/etc/log4perl.conf');
    
        --or--
    
            # Check config every 10 secs
        Log::Log4perl::init_and_watch('/etc/log4perl.conf',10);

        --then--
    
        $logger = Log::Log4perl->get_logger('house.bedrm.desk.topdrwr');
    
        $logger->debug('this is a debug message');
        $logger->info('this is an info message');
        $logger->warn('etc');
        $logger->error('..');
        $logger->fatal('..');
    
        #####/etc/log4perl.conf###############################
        log4perl.logger.house              = WARN,  FileAppndr1
        log4perl.logger.house.bedroom.desk = DEBUG, FileAppndr1
    
        log4perl.appender.FileAppndr1      = Log::Log4perl::Appender::File
        log4perl.appender.FileAppndr1.filename = desk.log 
        log4perl.appender.FileAppndr1.layout   = \
                                Log::Log4perl::Layout::SimpleLayout
        ######################################################
       
ABSTRACT
        Log::Log4perl provides a powerful logging API for your application

DESCRIPTION
    Log::Log4perl lets you remote-control and fine-tune the logging
    behaviour of your system from the outside. It implements the widely
    popular (Java-based) Log4j logging package in pure Perl.

    For a detailed tutorial on Log::Log4perl usage, please read

        http://www.perl.com/pub/a/2002/09/11/log4perl.html

    Logging beats a debugger if you want to know what's going on in your
    code during runtime. However, traditional logging packages are too
    static and generate a flood of log messages in your log files that won't
    help you.

    "Log::Log4perl" is different. It allows you to control the number of
    logging messages generated at three different levels:

    *   At a central location in your system (either in a configuration file
        or in the startup code) you specify *which components* (classes,
        functions) of your system should generate logs.

    *   You specify how detailed the logging of these components should be
        by specifying logging *levels*.

    *   You also specify which so-called *appenders* you want to feed your
        log messages to ("Print it to the screen and also append it to
        /tmp/my.log") and which format ("Write the date first, then the file
        name and line number, and then the log message") they should be in.

    This is a very powerful and flexible mechanism. You can turn on and off
    your logs at any time, specify the level of detail and make that
    dependent on the subsystem that's currently executed.

    Let me give you an example: You might find out that your system has a
    problem in the "MySystem::Helpers::ScanDir" component. Turning on
    detailed debugging logs all over the system would generate a flood of
    useless log messages and bog your system down beyond recognition. With
    "Log::Log4perl", however, you can tell the system: "Continue to log only
    severe errors to the log file. Open a second log file, turn on full
    debug logs in the "MySystem::Helpers::ScanDir" component and dump all
    messages originating from there into the new log file". And all this is
    possible by just changing the parameters in a configuration file, which
    your system can re-read even while it's running!

How to use it
    The "Log::Log4perl" package can be initialized in two ways: Either via
    Perl commands or via a "log4j"-style configuration file.

  Initialize via a configuration file
    This is the easiest way to prepare your system for using
    "Log::Log4perl". Use a configuration file like this:

        ############################################################
        # A simple root logger with a Log::Log4perl::Appender::File 
        # file appender in Perl.
        ############################################################
        log4perl.rootLogger=ERROR, LOGFILE
    
        log4perl.appender.LOGFILE=Log::Log4perl::Appender::File
        log4perl.appender.LOGFILE.filename=/var/log/myerrs.log
        log4perl.appender.LOGFILE.mode=append
    
        log4perl.appender.LOGFILE.layout=PatternLayout
        log4perl.appender.LOGFILE.layout.ConversionPattern=[%r] %F %L %c - %m%n

    These lines define your standard logger that's appending severe errors
    to "/var/log/myerrs.log", using the format

        [millisecs] source-filename line-number class - message newline

    Assuming that this configuration file is saved as "log.conf", you need
    to read it in in the startup section of your code, using the following
    commands:

      use Log::Log4perl;
      Log::Log4perl->init("log.conf");

    After that's done *somewhere* in the code, you can retrieve logger
    objects *anywhere* in the code. Note that there's no need to carry any
    logger references around with your functions and methods. You can get a
    logger anytime via a singleton mechanism:

        package My::MegaPackage;
        use  Log::Log4perl;

        sub some_method {
            my($param) = @_;

            my $log = Log::Log4perl->get_logger("My::MegaPackage");

            $log->debug("Debug message");
            $log->info("Info message");
            $log->error("Error message");

            ...
        }

    With the configuration file above, "Log::Log4perl" will write "Error
    message" to the specified log file, but won't do anything for the
    "debug()" and "info()" calls, because the log level has been set to
    "ERROR" for all components in the first line of configuration file shown
    above.

    Why "Log::Log4perl->get_logger" and not "Log::Log4perl->new"? We don't
    want to create a new object every time. Usually in OO-Programming, you
    create an object once and use the reference to it to call its methods.
    However, this requires that you pass around the object to all functions
    and the last thing we want is pollute each and every function/method
    we're using with a handle to the "Logger":

        sub function {  # Brrrr!!
            my($logger, $some, $other, $parameters) = @_;
        }

    Instead, if a function/method wants a reference to the logger, it just
    calls the Logger's static "get_logger($category)" method to obtain a
    reference to the *one and only* possible logger object of a certain
    category. That's called a *singleton* if you're a Gamma fan.

    How does the logger know which messages it is supposed to log and which
    ones to suppress? "Log::Log4perl" works with inheritance: The config
    file above didn't specify anything about "My::MegaPackage". And yet,
    we've defined a logger of the category "My::MegaPackage". In this case,
    "Log::Log4perl" will walk up the class hierarchy ("My" and then the
    we're at the root) to figure out if a log level is defined somewhere. In
    the case above, the log level at the root (root *always* defines a log
    level, but not necessarily an appender) defines that the log level is
    supposed to be "ERROR" -- meaning that *DEBUG* and *INFO* messages are
    suppressed.

  Log Levels
    There are five predefined log levels: "FATAL", "ERROR", "WARN", "INFO"
    and "DEBUG" (in descending priority). Your configured logging level has
    to at least match the priority of the logging message.

    If your configured logging level is "WARN", then messages logged with
    "info()" and "debug()" message will be suppressed. "fatal()", "error()"
    and "warn()" will make their way through, because their priority is
    higher or equal than the configured setting.

    Instead of calling the methods

        $logger->debug("...");  # Log a debug message
        $logger->info("...");   # Log a info message
        $logger->warn("...");   # Log a warn message
        $logger->error("...");  # Log a error message
        $logger->fatal("...");  # Log a fatal message

    you could also call the "log()" method with the appropriate level using
    the constants defined in "Log::Log4perl::Level":

        use Log::Log4perl::Level;

        $logger->log($DEBUG, "...");
        $logger->log($INFO, "...");
        $logger->log($WARN, "...");
        $logger->log($ERROR, "...");
        $logger->log($FATAL, "...");

    But nobody does that, really. Neither does anyone need more logging
    levels than these predefined ones. If you think you do, I would suggest
    you look into steering your logging behaviour via the category
    mechanism.

    If you need to find out if the currently configured logging level would
    allow a logger's logging statement to go through, use the logger's
    "is_*level*()" methods:

        $logger->is_debug()    # True if debug messages would go through
        $logger->is_info()     # True if info messages would go through
        $logger->is_warn()     # True if warn messages would go through
        $logger->is_error()    # True if error messages would go through
        $logger->is_fatal()    # True if fatal messages would go through

    Example: "$logger->is_warn()" returns true if the logger's current
    level, as derived from either the logger's category (or, in absence of
    that, one of the logger's parent's level setting) is $WARN, $ERROR or
    $FATAL.

    Also available are a series of more Java-esque functions which return
    the same values. These are of the format "is*Level*Enabled()", so
    "$logger->isDebugEnabled()" is synonymous to "$logger->is_debug()".

    These level checking functions will come in handy later, when we want to
    block unnecessary expensive parameter construction in case the logging
    level is too low to log the statement anyway, like in:

        if($logger->is_error()) {
            $logger->error("Erroneous array: @super_long_array");
        }

    If we had just written

        $logger->error("Erroneous array: @super_long_array");

    then Perl would have interpolated @super_long_array into the string via
    an expensive operation only to figure out shortly after that the string
    can be ignored entirely because the configured logging level is lower
    than $ERROR.

    The to-be-logged message passed to all of the functions described above
    can consist of an arbitrary number of arguments, which the logging
    functions just chain together to a single string. Therefore

        $logger->debug("Hello ", "World", "!");  # and
        $logger->debug("Hello World!");

    are identical.

  Log and die or warn
    Often, when you croak / carp / warn / die, you want to log those
    messages. Rather than doing the following:

        $logger->fatal($err) && die($err);

    you can use the following:

        $logger->logwarn();
        $logger->logdie();

    These print out log messages in the WARN and FATAL level, respectively,
    and then call the built-in warn() and die() functions. Since there is an
    ERROR level between WARN and FATAL, there are two additional helper
    functions in case you'd like to use ERROR for either warn() or die():

        $logger->error_warn();
        $logger->error_die();

    Finally, there's the Carp functions that do just what the Carp functions
    do, but with logging:

        $logger->logcarp();        # warn w/ 1-level stack trace
        $logger->logcluck();       # warn w/ full stack trace
        $logger->logcroak();       # die w/ 1-level stack trace
        $logger->logconfess();     # die w/ full stack trace

  Appenders
    If you don't define any appenders, nothing will happen. Appenders will
    be triggered whenever the configured logging level requires a message to
    be logged and not suppressed.

    "Log::Log4perl" doesn't define any appenders by default, not even the
    root logger has one.

    "Log::Log4perl" already comes with a standard set of appenders:

        Log::Log4perl::Appender::Screen
        Log::Log4perl::Appender::ScreenColoredLevels
        Log::Log4perl::Appender::File
        Log::Log4perl::Appender::Socket
        Log::Log4perl::Appender::DBI
        Log::Log4perl::Appender::Synchronized
        Log::Log4perl::Appender::RRDs

    to log to the screen, to files and to databases.

    On CPAN, you can find additional appenders like

        Log::Log4perl::Layout::XMLLayout

    by Guido Carls <gcarls@cpan.org>. It allows for hooking up Log::Log4perl
    with the graphical Log Analyzer Chainsaw (see "Can I use Log::Log4perl
    with log4j's Chainsaw?" in Log::Log4perl::FAQ).

  Additional Appenders via Log::Dispatch
    "Log::Log4perl" also supports *Dave Rolskys* excellent "Log::Dispatch"
    framework which implements a wide variety of different appenders.

    Here's the list of appender modules currently available via
    "Log::Dispatch":

           Log::Dispatch::ApacheLog
           Log::Dispatch::DBI (by Tatsuhiko Miyagawa)
           Log::Dispatch::Email,
           Log::Dispatch::Email::MailSend,
           Log::Dispatch::Email::MailSendmail,
           Log::Dispatch::Email::MIMELite
           Log::Dispatch::File
           Log::Dispatch::FileRotate (by Mark Pfeiffer)
           Log::Dispatch::Handle
           Log::Dispatch::Screen
           Log::Dispatch::Syslog
           Log::Dispatch::Tk (by Dominique Dumont)

    Please note that in order to use any of these additional appenders, you
    have to fetch Log::Dispatch from CPAN and install it. Also the
    particular appender you're using might require installing the particular
    module.

    For additional information on appenders, please check the
    Log::Log4perl::Appender manual page.

  Appender Example
    Now let's assume that we want to log "info()" or higher prioritized
    messages in the "Foo::Bar" category to both STDOUT and to a log file,
    say "test.log". In the initialization section of your system, just
    define two appenders using the readily available
    "Log::Log4perl::Appender::File" and "Log::Log4perl::Appender::Screen"
    modules:

      use Log::Log4perl;

         # Configuration in a string ...
      my $conf = q(
        log4perl.category.Foo.Bar          = INFO, Logfile, Screen

        log4perl.appender.Logfile          = Log::Log4perl::Appender::File
        log4perl.appender.Logfile.filename = test.log
        log4perl.appender.Logfile.layout   = Log::Log4perl::Layout::PatternLayout
        log4perl.appender.Logfile.layout.ConversionPattern = [%r] %F %L %m%n

        log4perl.appender.Screen         = Log::Log4perl::Appender::Screen
        log4perl.appender.Screen.stderr  = 0
        log4perl.appender.Screen.layout = Log::Log4perl::Layout::SimpleLayout
      );

         # ... passed as a reference to init()
      Log::Log4perl::init( \$conf );

    Once the initialization shown above has happened once, typically in the
    startup code of your system, just use the defined logger anywhere in
    your system:

      ##########################
      # ... in some function ...
      ##########################
      my $log = Log::Log4perl::get_logger("Foo::Bar");

        # Logs both to STDOUT and to the file test.log
      $log->info("Important Info!");

    The "layout" settings specified in the configuration section define the
    format in which the message is going to be logged by the specified
    appender. The format shown for the file appender is logging not only the
    message but also the number of milliseconds since the program has
    started (%r), the name of the file the call to the logger has happened
    and the line number there (%F and %L), the message itself (%m) and a
    OS-specific newline character (%n):

        [187] ./myscript.pl 27 Important Info!

    The screen appender above, on the other hand, uses a "SimpleLayout",
    which logs the debug level, a hyphen (-) and the log message:

        INFO - Important Info!

    For more detailed info on layout formats, see "Log Layouts".

    In the configuration sample above, we chose to define a *category*
    logger ("Foo::Bar"). This will cause only messages originating from this
    specific category logger to be logged in the defined format and
    locations.

  Configuration files
    As shown above, you can define "Log::Log4perl" loggers both from within
    your Perl code or from configuration files. The latter have the
    unbeatable advantage that you can modify your system's logging behaviour
    without interfering with the code at all. So even if your code is being
    run by somebody who's totally oblivious to Perl, they still can adapt
    the module's logging behaviour to their needs.

    "Log::Log4perl" has been designed to understand "Log4j" configuration
    files -- as used by the original Java implementation. Instead of
    reiterating the format description in [2], let me just list three
    examples (also derived from [2]), which should also illustrate how it
    works:

        log4j.rootLogger=DEBUG, A1
        log4j.appender.A1=org.apache.log4j.ConsoleAppender
        log4j.appender.A1.layout=org.apache.log4j.PatternLayout
        log4j.appender.A1.layout.ConversionPattern=%-4r %-5p %c %x - %m%n

    This enables messages of priority "DEBUG" or higher in the root
    hierarchy and has the system write them to the console.
    "ConsoleAppender" is a Java appender, but "Log::Log4perl" jumps through
    a significant number of hoops internally to map these to their
    corresponding Perl classes, "Log::Log4perl::Appender::Screen" in this
    case.

    Second example:

        log4perl.rootLogger=DEBUG, A1
        log4perl.appender.A1=Log::Log4perl::Appender::Screen
        log4perl.appender.A1.layout=PatternLayout
        log4perl.appender.A1.layout.ConversionPattern=%d %-5p %c - %m%n
        log4perl.logger.com.foo=WARN

    This defines two loggers: The root logger and the "com.foo" logger. The
    root logger is easily triggered by debug-messages, but the "com.foo"
    logger makes sure that messages issued within the "Com::Foo" component
    and below are only forwarded to the appender if they're of priority
    *warning* or higher.

    Note that the "com.foo" logger doesn't define an appender. Therefore, it
    will just propagate the message up the hierarchy until the root logger
    picks it up and forwards it to the one and only appender of the root
    category, using the format defined for it.

    Third example:

        log4j.rootLogger=debug, stdout, R
        log4j.appender.stdout=org.apache.log4j.ConsoleAppender
        log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
        log4j.appender.stdout.layout.ConversionPattern=%5p (%F:%L) - %m%n
        log4j.appender.R=org.apache.log4j.RollingFileAppender
        log4j.appender.R.File=example.log
        log4j.appender.R.layout=org.apache.log4j.PatternLayout
        log4j.appender.R.layout.ConversionPattern=%p %c - %m%n

    The root logger defines two appenders here: "stdout", which uses
    "org.apache.log4j.ConsoleAppender" (ultimately mapped by "Log::Log4perl"
    to "Log::Log4perl::Appender::Screen") to write to the screen. And "R", a
    "org.apache.log4j.RollingFileAppender" (mapped by "Log::Log4perl" to
    "Log::Dispatch::FileRotate" with the "File" attribute specifying the log
    file.

    See Log::Log4perl::Config for more examples and syntax explanations.

  Log Layouts
    If the logging engine passes a message to an appender, because it thinks
    it should be logged, the appender doesn't just write it out haphazardly.
    There's ways to tell the appender how to format the message and add all
    sorts of interesting data to it: The date and time when the event
    happened, the file, the line number, the debug level of the logger and
    others.

    There's currently two layouts defined in "Log::Log4perl":
    "Log::Log4perl::Layout::SimpleLayout" and
    "Log::Log4perl::Layout::PatternLayout":

    "Log::Log4perl::SimpleLayout"
        formats a message in a simple way and just prepends it by the debug
        level and a hyphen: ""$level - $message", for example "FATAL - Can't
        open password file".

    "Log::Log4perl::Layout::PatternLayout"
        on the other hand is very powerful and allows for a very flexible
        format in "printf"-style. The format string can contain a number of
        placeholders which will be replaced by the logging engine when it's
        time to log the message:

            %c Category of the logging event.
            %C Fully qualified package (or class) name of the caller
            %d Current date in yyyy/MM/dd hh:mm:ss format
            %F File where the logging event occurred
            %H Hostname
            %l Fully qualified name of the calling method followed by the
               callers source the file name and line number between 
               parentheses.
            %L Line number within the file where the log statement was issued
            %m The message to be logged
            %M Method or function where the logging request was issued
            %n Newline (OS-independent)
            %p Priority of the logging event
            %P pid of the current process
            %r Number of milliseconds elapsed from program start to logging 
               event
            %x The elements of the NDC stack (see below)
            %X{key} The entry 'key' of the MDC (see below)
            %% A literal percent (%) sign

        NDC and MDC are explained in "Nested Diagnostic Context (NDC)" and
        "Mapped Diagnostic Context (MDC)".

        Also, %d can be fine-tuned to display only certain characteristics
        of a date, according to the SimpleDateFormat in the Java World
        (http://java.sun.com/j2se/1.3/docs/api/java/text/SimpleDateFormat.ht
        ml)

        In this way, %d{HH:mm} displays only hours and minutes of the
        current date, while %d{yy, EEEE} displays a two-digit year, followed
        by a spelled-out (like "Wednesday").

        Similar options are available for shrinking the displayed category
        or limit file/path components, %F{1} only displays the source file
        *name* without any path components while %F logs the full path.
        %c{2} only logs the last two components of the current category,
        "Foo::Bar::Baz" becomes "Bar::Baz" and saves space.

        If those placeholders aren't enough, then you can define your own
        right in the config file like this:

            log4perl.PatternLayout.cspec.U = sub { return "UID $<" }

        See Log::Log4perl::Layout::PatternLayout for further details on
        customized specifiers.

        Please note that the subroutines you're defining in this way are
        going to be run in the "main" namespace, so be sure to fully qualify
        functions and variables if they're located in different packages.

        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

            Log::Log4perl::Config->allow_code(0);

        before you call init(). Alternatively you can supply a restricted
        set of Perl opcodes that can be embedded in the config file as
        described in "Restricting what Opcodes can be in a Perl Hook".

    All placeholders are quantifiable, just like in *printf*. Following this
    tradition, "%-20c" will reserve 20 chars for the category and
    left-justify it.

    For more details on logging and how to use the flexible and the simple
    format, check out the original "log4j" website under

        http://jakarta.apache.org/log4j/docs/api/org/apache/log4j/SimpleLayout.html
        http://jakarta.apache.org/log4j/docs/api/org/apache/log4j/PatternLayout.html

  Penalties
    Logging comes with a price tag. "Log::Log4perl" has been optimized to
    allow for maximum performance, both with logging enabled and disabled.

    But you need to be aware that there's a small hit every time your code
    encounters a log statement -- no matter if logging is enabled or not.
    "Log::Log4perl" has been designed to keep this so low that it will be
    unnoticable to most applications.

    Here's a couple of tricks which help "Log::Log4perl" to avoid
    unnecessary delays:

    You can save serious time if you're logging something like

            # Expensive in non-debug mode!
        for (@super_long_array) {
            $logger->debug("Element: $_\n");
        }

    and @super_long_array is fairly big, so looping through it is pretty
    expensive. Only you, the programmer, knows that going through that "for"
    loop can be skipped entirely if the current logging level for the actual
    component is higher than "debug". In this case, use this instead:

            # Cheap in non-debug mode!
        if($logger->is_debug()) {
            for (@super_long_array) {
                $logger->debug("Element: $_\n");
            }
        }

    If you're afraid that the way you're generating the parameters to the of
    the logging function is fairly expensive, use closures:

            # Passed as subroutine ref
        use Data::Dumper;
        $logger->debug(sub { Dumper($data) } );

    This won't unravel $data via Dumper() unless it's actually needed
    because it's logged.

    Also, Log::Log4perl lets you specify arguments to logger functions in
    *message output filter syntax*:

        $logger->debug("Structure: ",
                       { filter => \&Dumper,
                         value  => $someref });

    In this way, shortly before Log::Log4perl sending the message out to any
    appenders, it will be searching all arguments for hash references and
    treat them in a special way:

    It will invoke the function given as a reference with the "filter" key
    ("Data::Dumper::Dumper()") and pass it the value that came with the key
    named "value" as an argument. The anonymous hash in the call above will
    be replaced by the return value of the filter function.

Categories
    "Log::Log4perl" uses *categories* to determine if a log statement in a
    component should be executed or suppressed at the current logging level.
    Most of the time, these categories are just the classes the log
    statements are located in:

        package Candy::Twix;

        sub new { 
            my $logger = Log::Log4perl->new("Candy::Twix");
            $logger->debug("Creating a new Twix bar");
            bless {}, shift;
        }
 
        # ...

        package Candy::Snickers;

        sub new { 
            my $logger = Log::Log4perl->new("Candy.Snickers");
            $logger->debug("Creating a new Snickers bar");
            bless {}, shift;
        }

        # ...

        package main;
        Log::Log4perl->init("mylogdefs.conf") or 
            die "Whoa, cannot read mylogdefs.conf!";

            # => "LOG> Creating a new Snickers bar"
        my $first = Candy::Snickers->new();
            # => "LOG> Creating a new Twix bar"
        my $second = Candy::Twix->new();

    Note that you can separate your category hierarchy levels using either
    dots like in Java (.) or double-colons (::) like in Perl. Both notations
    are equivalent and are handled the same way internally.

    However, categories are just there to make use of inheritance: if you
    invoke a logger in a sub-category, it will bubble up the hierarchy and
    call the appropriate appenders. Internally, categories are not related
    to the class hierarchy of the program at all -- they're purely virtual.
    You can use arbitrary categories -- for example in the following
    program, which isn't oo-style, but procedural:

        sub print_portfolio {

            my $log = Log::Log4perl->new("user.portfolio");
            $log->debug("Quotes requested: @_");

            for(@_) {
                print "$_: ", get_quote($_), "\n";
            }
        }

        sub get_quote {

            my $log = Log::Log4perl->new("internet.quotesystem");
            $log->debug("Fetching quote: $_[0]");

            return yahoo_quote($_[0]);
        }

    The logger in first function, "print_portfolio", is assigned the
    (virtual) "user.portfolio" category. Depending on the "Log4perl"
    configuration, this will either call a "user.portfolio" appender, a
    "user" appender, or an appender assigned to root -- without
    "user.portfolio" having any relevance to the class system used in the
    program. The logger in the second function adheres to the
    "internet.quotesystem" category -- again, maybe because it's bundled
    with other Internet functions, but not because there would be a class of
    this name somewhere.

    However, be careful, don't go overboard: if you're developing a system
    in object-oriented style, using the class hierarchy is usually your best
    choice. Think about the people taking over your code one day: The class
    hierarchy is probably what they know right up front, so it's easy for
    them to tune the logging to their needs.

  Turn off a component
    "Log4perl" doesn't only allow you to selectively switch *on* a category
    of log messages, you can also use the mechanism to selectively *disable*
    logging in certain components whereas logging is kept turned on in
    higher-level categories. This mechanism comes in handy if you find that
    while bumping up the logging level of a high-level (i. e. close to root)
    category, that one component logs more than it should,

    Here's how it works:

        ############################################################
        # Turn off logging in a lower-level category while keeping
        # it active in higher-level categories.
        ############################################################
        log4perl.rootLogger=DEBUG, LOGFILE
        log4perl.logger.deep.down.the.hierarchy = ERROR, LOGFILE

        # ... Define appenders ...

    This way, log messages issued from within "Deep::Down::The::Hierarchy"
    and below will be logged only if they're "ERROR" or worse, while in all
    other system components even "DEBUG" messages will be logged.

  Return Values
    All logging methods return values indicating if their message actually
    reached one or more appenders. If the message has been suppressed
    because of level constraints, "undef" is returned.

    For example,

        my $ret = $logger->info("Message");

    will return "undef" if the system debug level for the current category
    is not "INFO" or more permissive. If Log::Log4perl forwarded the message
    to one or more appenders, the number of appenders is returned.

    If appenders decide to veto on the message with an appender threshold,
    the log method's return value will have them excluded. This means that
    if you've got one appender holding an appender threshold and you're
    logging a message which passes the system's log level hurdle but not the
    appender threshold, 0 will be returned by the log function.

    The bottom line is: Logging functions will return a *true* value if the
    message made it through to one or more appenders and a *false* value if
    it didn't. This allows for constructs like

        $logger->fatal("@_") or print STDERR "@_\n";

    which will ensure that the fatal message isn't lost if the current level
    is lower than FATAL or printed twice if the level is acceptable but an
    appender already points to STDERR.

  Pitfalls with Categories
    Be careful with just blindly reusing the system's packages as
    categories. If you do, you'll get into trouble with inherited methods.
    Imagine the following class setup:

        use Log::Log4perl;

        ###########################################
        package Bar;
        ###########################################
        sub new {
            my($class) = @_;
            my $logger = Log::Log4perl::get_logger(__PACKAGE__);
            $logger->debug("Creating instance");
            bless {}, $class;
        }
        ###########################################
        package Bar::Twix;
        ###########################################
        our @ISA = qw(Bar);

        ###########################################
        package main;
        ###########################################
        Log::Log4perl->init(\ qq{
        log4perl.category.Bar.Twix = DEBUG, Screen
        log4perl.appender.Screen = Log::Log4perl::Appender::Screen
        log4perl.appender.Screen.layout = SimpleLayout
        });

        my $bar = Bar::Twix->new();

    "Bar::Twix" just inherits everything from "Bar", including the
    constructor "new()". Contrary to what you might be thinking at first,
    this won't log anything. Reason for this is the "get_logger()" call in
    package "Bar", which will always get a logger of the "Bar" category,
    even if we call "new()" via the "Bar::Twix" package, which will make
    perl go up the inheritance tree to actually execute "Bar::new()". Since
    we've only defined logging behaviour for "Bar::Twix" in the
    configuration file, nothing will happen.

    This can be fixed by changing the "get_logger()" method in "Bar::new()"
    to obtain a logger of the category matching the *actual* class of the
    object, like in

            # ... in Bar::new() ...
        my $logger = Log::Log4perl::get_logger($class);

    This way, you'll make sure the logger logs appropriately, no matter if
    the method is inherited or called directly. "new()" always gets the real
    class name as an argument and all other methods can determine it via
    "ref($self)"), so it shouldn't be a problem to get the right class every
    time.

  Initialize once and only once
    It's important to realize that Log::Log4perl gets initialized once and
    only once, typically at the start of a program or system. Calling
    "init()" more than once will cause it to clobber the existing
    configuration and *replace* it by the new one.

    If you're in a traditional CGI environment, where every request is
    handeled by a new process, calling "init()" every time is fine. In
    persistent environments like "mod_perl", however, Log::Log4perl should
    be initialized either at system startup time (Apache offers startup
    handlers for that) or via

            # Init or skip if already done
        Log::Log4perl->init_once($conf_file);

    "init_once()" is identical to "init()", just with the exception that it
    will leave a potentially existing configuration alone and will only call
    "init()" if Log::Log4perl hasn't been initialized yet.

    If you're just curious if Log::Log4perl has been initialized yet, the
    check

        if(Log::Log4perl->initialized()) {
            # Yes, Log::Log4perl has already been initialized
        } else {
            # No, not initialized yet ...
        }

    can be used.

    If you're afraid that the components of your system are stepping on each
    other's toes or if you are thinking that different components should
    initialize Log::Log4perl seperately, try to consolidate your system to
    use a centralized Log4perl configuration file and use Log4perl's
    *categories* to separate your components.

  Custom Filters
    Log4perl allows the use of customized filters in its appenders to
    control the output of messages. These filters might grep for certain
    text chunks in a message, verify that its priority matches or exceeds a
    certain level or that this is the 10th time the same message has been
    submitted -- and come to a log/no log decision based upon these
    circumstantial facts.

    Check out Log::Log4perl::Filter for detailed instructions on how to use
    them.

  Performance
    The performance of Log::Log4perl calls obviously depends on a lot of
    things. But to give you a general idea, here's some rough numbers:

    On a Pentium 4 Linux box at 2.4 GHz, you'll get through

    *   500,000 suppressed log statements per second

    *   30,000 logged messages per second (using an in-memory appender)

    *   init_and_watch delay mode: 300,000 suppressed, 30,000 logged.
        init_and_watch signal mode: 450,000 suppressed, 30,000 logged.

    Numbers depend on the complexity of the Log::Log4perl configuration. For
    a more detailed benchmark test, check the "docs/benchmark.results.txt"
    document in the Log::Log4perl distribution.

Cool Tricks
    Here's a collection of useful tricks for the advanced "Log::Log4perl"
    user. For more, check the the FAQ, either in the distribution
    (Log::Log4perl::FAQ) or on http://log4perl.sourceforge.net.

  Shortcuts
    When getting an instance of a logger, instead of saying

        use Log::Log4perl;
        my $logger = Log::Log4perl->get_logger();

    it's often more convenient to import the "get_logger" method from
    "Log::Log4perl" into the current namespace:

        use Log::Log4perl qw(get_logger);
        my $logger = get_logger();

    Please note this difference: To obtain the root logger, please use
    "get_logger("")", call it without parameters ("get_logger()"), you'll
    get the logger of a category named after the current package.
    "get_logger()" is equivalent to "get_logger(__PACKAGE__)".

  Alternative initialization
    Instead of having "init()" read in a configuration file by specifying a
    file name or passing it a reference to an open filehandle
    ("Log::Log4perl->init( \*FILE )"), you can also pass in a reference to a
    string, containing the content of the file:

        Log::Log4perl->init( \$config_text );

    Also, if you've got the "name=value" pairs of the configuration in a
    hash, you can just as well initialize "Log::Log4perl" with a reference
    to it:

        my %key_value_pairs = (
            "log4perl.rootLogger"       => "ERROR, LOGFILE",
            "log4perl.appender.LOGFILE" => "Log::Log4perl::Appender::File",
            ...
        );

        Log::Log4perl->init( \%key_value_pairs );

    Or also you can use a URL, see below:

  Using LWP to parse URLs
    (This section borrowed from XML::DOM::Parser by T.J. Mather).

    The init() function now also supports URLs, e.g.
    *http://www.erols.com/enno/xsa.xml*. It uses LWP to download the file
    and then calls parse() on the resulting string. By default it will use a
    LWP::UserAgent that is created as follows:

     use LWP::UserAgent;
     $LWP_USER_AGENT = LWP::UserAgent->new;
     $LWP_USER_AGENT->env_proxy;

    Note that env_proxy reads proxy settings from environment variables,
    which is what I need to do to get thru our firewall. If you want to use
    a different LWP::UserAgent, you can set it with

        Log::Log4perl::Config::set_LWP_UserAgent($my_agent);

    Currently, LWP is used when the filename (passed to parsefile) starts
    with one of the following URL schemes: http, https, ftp, wais, gopher,
    or file (followed by a colon.)

    Don't use this feature with init_and_watch().

  Automatic reloading of changed configuration files
    Instead of just statically initializing Log::Log4perl via

        Log::Log4perl->init($conf_file);

    there's a way to have Log::Log4perl periodically check for changes in
    the configuration and reload it if necessary:

        Log::Log4perl->init_and_watch($conf_file, $delay);

    In this mode, Log::Log4perl will examine the configuration file
    $conf_file every $delay seconds for changes via the file's last
    modification timestamp. If the file has been updated, it will be
    reloaded and replace the current Log::Log4perl configuration.

    The way this works is that with every logger function called (debug(),
    is_debug(), etc.), Log::Log4perl will check if the delay interval has
    expired. If so, it will run a -M file check on the configuration file.
    If its timestamp has been modified, the current configuration will be
    dumped and new content of the file will be loaded.

    This convenience comes at a price, though: Calling time() with every
    logging function call, especially the ones that are "suppressed" (!),
    will slow down these Log4perl calls by about 40%.

    To alleviate this performance hit a bit, "init_and_watch()" can be
    configured to listen for a Unix signal to reload the configuration
    instead:

        Log::Log4perl->init_and_watch($conf_file, 'HUP');

    This will set up a signal handler for SIGHUP and reload the
    configuration if the application receives this signal, e.g. via the
    "kill" command:

        kill -HUP pid

    where "pid" is the process ID of the application. This will bring you
    back to about 85% of Log::Log4perl's normal execution speed for
    suppressed statements. For details, check out "Performance". For more
    info on the signal handler, look for "SIGNAL MODE" in
    Log::Log4perl::Config::Watch.

    One thing to watch out for: If the configuration file contains a syntax
    or other fatal error, a running application will stop with "die" if this
    damaged configuration will be loaded during runtime, triggered either by
    a signal or if the delay period expired and the change is detected. This
    behaviour might change in the future.

  Variable Substitution
    To avoid having to retype the same expressions over and over again,
    Log::Log4perl's configuration files support simple variable
    substitution. New variables are defined simply by adding

        varname = value

    lines to the configuration file before using

        ${varname}

    afterwards to recall the assigned values. Here's an example:

        layout_class   = Log::Log4perl::Layout::PatternLayout
        layout_pattern = %d %F{1} %L> %m %n
    
        log4perl.category.Bar.Twix = WARN, Logfile, Screen

        log4perl.appender.Logfile  = Log::Log4perl::Appender::File
        log4perl.appender.Logfile.filename = test.log
        log4perl.appender.Logfile.layout = ${layout_class}
        log4perl.appender.Logfile.layout.ConversionPattern = ${layout_pattern}

        log4perl.appender.Screen  = Log::Log4perl::Appender::Screen
        log4perl.appender.Screen.layout = ${layout_class}
        log4perl.appender.Screen.layout.ConversionPattern = ${layout_pattern}

    This is a convenient way to define two appenders with the same layout
    without having to retype the pattern definitions.

    Variable substitution via "${varname}" will first try to find an
    explicitely defined variable. If that fails, it will check your shell's
    environment for a variable of that name. If that also fails, the program
    will "die()".

  Perl Hooks in the Configuration File
    If some of the values used in the Log4perl configuration file need to be
    dynamically modified by the program, use Perl hooks:

        log4perl.appender.File.filename = \
            sub { return getLogfileName(); }

    Each value starting with the string "sub {..." is interpreted as Perl
    code to be executed at the time the application parses the configuration
    via "Log::Log4perl::init()". The return value of the subroutine is used
    by Log::Log4perl as the configuration value.

    The Perl code is executed in the "main" package, functions in other
    packages have to be called in fully-qualified notation.

    Here's another example, utilizing an environment variable as a username
    for a DBI appender:

        log4perl.appender.DB.username = \
            sub { $ENV{DB_USER_NAME } }

    However, please note the difference between these code snippets and
    those used for user-defined conversion specifiers as discussed in
    Log::Log4perl::Layout::PatternLayout: While the snippets above are run
    *once* when "Log::Log4perl::init()" is called, the conversion specifier
    snippets are executed *each time* a message is rendered according to the
    PatternLayout.

    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 set

        Log::Log4perl::Config->allow_code(0);

    before you call init(). Alternatively you can supply a restricted set of
    Perl opcodes that can be embedded in the config file as described in
    "Restricting what Opcodes can be in a Perl Hook".

  Restricting what Opcodes can be in a Perl Hook
    The value you pass to Log::Log4perl::Config->allow_code() determines
    whether the code that is embedded in the config file is eval'd
    unrestricted, or eval'd in a Safe compartment. By default, a value of
    '1' is assumed, which does a normal 'eval' without any restrictions. A
    value of '0' however prevents any embedded code from being evaluated.

    If you would like fine-grained control over what can and cannot be
    included in embedded code, then please utilize the following methods:

     Log::Log4perl::Config->allow_code( $allow );
     Log::Log4perl::Config->allowed_code_ops($op1, $op2, ... );
     Log::Log4perl::Config->vars_shared_with_safe_compartment( [ \%vars | $package, \@vars ] );
     Log::Log4perl::Config->allowed_code_ops_convenience_map( [ \%map | $name, \@mask ] );

    Log::Log4perl::Config->allowed_code_ops() takes a list of opcode masks
    that are allowed to run in the compartment. The opcode masks must be
    specified as described in Opcode:

     Log::Log4perl::Config->allowed_code_ops(':subprocess');
 
    This example would allow Perl operations like backticks, system, fork,
    and waitpid to be executed in the compartment. Of course, you probably
    don't want to use this mask -- it would allow exactly what the Safe
    compartment is designed to prevent.

    Log::Log4perl::Config->vars_shared_with_safe_compartment() takes the
    symbols which should be exported into the Safe compartment before the
    code is evaluated. The keys of this hash are the package names that the
    symbols are in, and the values are array references to the literal
    symbol names. For convenience, the default settings export the '%ENV'
    hash from the 'main' package into the compartment:

     Log::Log4perl::Config->vars_shared_with_safe_compartment(
       main => [ '%ENV' ],
     );

    Log::Log4perl::Config->allowed_code_ops_convenience_map() is an accessor
    method to a map of convenience names to opcode masks. At present, the
    following convenience names are defined:

     safe        = [ ':browse' ]
     restrictive = [ ':default' ]
 
    For convenience, if Log::Log4perl::Config->allow_code() is called with a
    value which is a key of the map previously defined with
    Log::Log4perl::Config->allowed_code_ops_convenience_map(), then the
    allowed opcodes are set according to the value defined in the map. If
    this is confusing, consider the following:

     use Log::Log4perl;
 
     my $config = <<'END';
      log4perl.logger = INFO, Main
      log4perl.appender.Main = Log::Log4perl::Appender::File
      log4perl.appender.Main.filename = \
          sub { "example" . getpwuid($<) . ".log" }
      log4perl.appender.Main.layout = Log::Log4perl::Layout::SimpleLayout
     END
 
     $Log::Log4perl::Config->allow_code('restrictive');
     Log::Log4perl->init( \$config );       # will fail
     $Log::Log4perl::Config->allow_code('safe');
     Log::Log4perl->init( \$config );       # will succeed

    The reason that the first call to ->init() fails is because the
    'restrictive' name maps to an opcode mask of ':default'. getpwuid() is
    not part of ':default', so ->init() fails. The 'safe' name maps to an
    opcode mask of ':browse', which allows getpwuid() to run, so ->init()
    succeeds.

    allowed_code_ops_convenience_map() can be invoked in several ways:

    allowed_code_ops_convenience_map()
        Returns the entire convenience name map as a hash reference in
        scalar context or a hash in list context.

    allowed_code_ops_convenience_map( \%map )
        Replaces the entire conveniece name map with the supplied hash
        reference.

    allowed_code_ops_convenience_map( $name )
        Returns the opcode mask for the given convenience name, or undef if
        no such name is defined in the map.

    allowed_code_ops_convenience_map( $name, \@mask )
        Adds the given name/mask pair to the convenience name map. If the
        name already exists in the map, it's value is replaced with the new
        mask.

    as can vars_shared_with_safe_compartment():

    vars_shared_with_safe_compartment()
        Return the entire map of packages to variables as a hash reference
        in scalar context or a hash in list context.

    vars_shared_with_safe_compartment( \%packages )
        Replaces the entire map of packages to variables with the supplied
        hash reference.

    vars_shared_with_safe_compartment( $package )
        Returns the arrayref of variables to be shared for a specific
        package.

    vars_shared_with_safe_compartment( $package, \@vars )
        Adds the given package / varlist pair to the map. If the package
        already exists in the map, it's value is replaced with the new
        arrayref of variable names.

    For more information on opcodes and Safe Compartments, see Opcode and
    Safe.

  Changing the Log Level on a Logger
    Log4perl provides some internal functions for quickly adjusting the log
    level from within a running Perl program.

    Now, some people might argue that you should adjust your levels from
    within an external Log4perl configuration file, but Log4perl is
    everybody's darling.

    Typically run-time adjusting of levels is done at the beginning, or in
    response to some external input (like a "more logging" runtime command
    for diagnostics).

    You get the log level from a logger object with:

        $current_level = $logger->level();

    and you may set it with the same method, provided you first imported the
    log level constants, with:

        use Log::Log4perl::Level;

    Then you can set the level on a logger to one of the constants,

        $logger->level($ERROR); # one of DEBUG, INFO, WARN, ERROR, FATAL

    To increase the level of logging currently being done, use:

        $logger->more_logging($delta);

    and to decrease it, use:

        $logger->less_logging($delta);

    $delta must be a positive integer (for now, we may fix this later ;).

    There are also two equivalent functions:

        $logger->inc_level($delta);
        $logger->dec_level($delta);

    They're included to allow you a choice in readability. Some folks will
    prefer more/less_logging, as they're fairly clear in what they do, and
    allow the programmer not to worry too much about what a Level is and
    whether a higher Level means more or less logging. However, other folks
    who do understand and have lots of code that deals with levels will
    probably prefer the inc_level() and dec_level() methods as they want to
    work with Levels and not worry about whether that means more or less
    logging. :)

    That diatribe aside, typically you'll use more_logging() or inc_level()
    as such:

        my $v = 0; # default level of verbosity.
    
        GetOptions("v+" => \$v, ...);

        $logger->more_logging($v);  # inc logging level once for each -v in ARGV

  Custom Log Levels
    First off, let me tell you that creating custom levels is heavily
    deprecated by the log4j folks. Indeed, instead of creating additional
    levels on top of the predefined DEBUG, INFO, WARN, ERROR and FATAL, you
    should use categories to control the amount of logging smartly, based on
    the location of the log-active code in the system.

    Nevertheless, Log4perl provides a nice way to create custom levels via
    the create_custom_level() routine function. However, this must be done
    before the first call to init() or get_logger(). Say you want to create
    a NOTIFY logging level that comes after WARN (and thus before INFO).
    You'd do such as follows:

        use Log::Log4perl;
        use Log::Log4perl::Level;

        Log::Log4perl::Logger::create_custom_level("NOTIFY", "WARN");

    And that's it! create_custom_level() creates the following functions /
    variables for level FOO:

        $FOO_INT        # integer to use in toLevel()
        $logger->foo()  # log function to log if level = FOO
        $logger->is_foo()   # true if current level is >= FOO

    These levels can also be used in your config file, but note that your
    config file probably won't be portable to another log4perl or log4j
    environment unless you've made the appropriate mods there too.

  System-wide log levels
    As a fairly drastic measure to decrease (or increase) the logging level
    all over the system with one single configuration option, use the
    "threshold" keyword in the Log4perl configuration file:

        log4perl.threshold = ERROR

    sets the system-wide (or hierarchy-wide according to the log4j
    documentation) to ERROR and therefore deprives every logger in the
    system of the right to log lower-prio messages.

  Easy Mode
    For teaching purposes (especially for [1]), I've put ":easy" mode into
    "Log::Log4perl", which just initializes a single root logger with a
    defined priority and a screen appender including some nice standard
    layout:

        ### Initialization Section
        use Log::Log4perl qw(:easy);
        Log::Log4perl->easy_init($ERROR);  # Set priority of root logger to ERROR

        ### Application Section
        my $logger = get_logger();
        $logger->fatal("This will get logged.");
        $logger->debug("This won't.");

    This will dump something like

        2002/08/04 11:43:09 ERROR> script.pl:16 main::function - This will get logged.

    to the screen. While this has been proven to work well familiarizing
    people with "Log::Logperl" slowly, effectively avoiding to clobber them
    over the head with a plethora of different knobs to fiddle with
    (categories, appenders, levels, layout), the overall mission of
    "Log::Log4perl" is to let people use categories right from the start to
    get used to the concept. So, let's keep this one fairly hidden in the
    man page (congrats on reading this far :).

  Stealth loggers
    Sometimes, people are lazy. If you're whipping up a 50-line script and
    want the comfort of Log::Log4perl without having the burden of carrying
    a separate log4perl.conf file or a 5-liner defining that you want to
    append your log statements to a file, you can use the following
    features:

        use Log::Log4perl qw(:easy);

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

            # Logs to test.log via stealth logger
        DEBUG("Debug this!");
        INFO("Info this!");
        WARN("Warn this!");
        ERROR("Error this!");

        some_function();

        sub some_function {
                # Same here
            FATAL("Fatal this!");
        }

    In ":easy" mode, "Log::Log4perl" will instantiate a *stealth logger*
    named $_default_logger and import it into the current package. Also, it
    will introduce the convenience functions "DEBUG()", "INFO()", "WARN()",
    "ERROR()" and "FATAL()" into the package namespace, which take arguments
    and forward them to "_default_logger->debug()",
    "_default_logger->info()" and so on.

    The "easy_init" method can be called with a single level value to create
    a STDERR appender and a root logger as in

        Log::Log4perl->easy_init($DEBUG);

    or, as shown below (and in the example above) with a reference to a
    hash, specifying values for "level" (the logger's priority), "file" (the
    appender's data sink), "category" (the logger's category> and "layout"
    for the appender's pattern layout specification. All key-value pairs are
    optional, they default to $DEBUG for "level", "STDERR" for "file", ""
    (root category) for "category" and "%d %m%n" for "layout":

        Log::Log4perl->easy_init( { level    => $DEBUG,
                                    file     => ">test.log",
                                    category => "Bar::Twix",
                                    layout   => '%F{1}-%L-%M: %m%n' } );

    The "file" parameter takes file names preceded by ">" (overwrite) and
    ">>" (append) as arguments. This will cause
    "Log::Log4perl::Appender::File" appenders to be created behind the
    scenes. Also the keywords "STDOUT" and "STDERR" (no ">" or ">>") are
    recognized, which will utilize and configure
    "Log::Log4perl::Appender::Screen" appropriately.

    If a file appender receives Unicode strings, use

        file => ":utf8> test.log"

    to establish a utf8 line discpline on the file, otherwise you'll get a
    'wide character in print' warning message and probably not what you'd
    expect as output.

    The stealth loggers can be used in different packages, you just need to
    make sure you're calling the "use" function in every package you're
    using "Log::Log4perl"'s easy services:

        package Bar::Twix;
        use Log::Log4perl qw(:easy);
        sub eat { DEBUG("Twix mjam"); }

        package Bar::Mars;
        use Log::Log4perl qw(:easy);
        sub eat { INFO("Mars mjam"); }

        package main;

        use Log::Log4perl qw(:easy);

        Log::Log4perl->easy_init( { level    => $DEBUG,
                                    file     => ">>test.log",
                                    category => "Bar::Twix",
                                    layout   => '%F{1}-%L-%M: %m%n' },
                                  { level    => $DEBUG,
                                    file     => "STDOUT",
                                    category => "Bar::Mars",
                                    layout   => '%m%n' },
                                );
        Bar::Twix::eat();
        Bar::Mars::eat();

    As shown above, "easy_init()" will take any number of different logger
    definitions as hash references.

    Also, stealth loggers feature the functions "LOGWARN()", "LOGDIE()", and
    "LOGEXIT()", combining a logging request with a subsequent Perl warn()
    or die() or exit() statement. So, for example

        if($all_is_lost) {
            LOGDIE("Terrible Problem");
        }

    will log the message if the package's logger is at least "FATAL" but
    "die()" (including the traditional output to STDERR) in any case
    afterwards.

    See "Log and die or warn" for the similar "logdie()" and "logwarn()"
    functions of regular (i.e non-stealth) loggers.

    Similarily, "LOGCARP()", "LOGCLUCK()", "LOGCROAK()", and "LOGCONFESS()"
    are provided in ":easy" mode, facilitating the use of "logcarp()",
    "logcluck()", "logcroak()", and "logconfess()" with stealth loggers.

    When using Log::Log4perl in easy mode, please make sure you understand
    the implications of "Pitfalls with Categories".

    By the way, these convenience functions perform exactly as fast as the
    standard Log::Log4perl logger methods, there's *no* performance penalty
    whatsoever.

  Nested Diagnostic Context (NDC)
    If you find that your application could use a global (thread-specific)
    data stack which your loggers throughout the system have easy access to,
    use Nested Diagnostic Contexts (NDCs). Also check out "Mapped Diagnostic
    Context (MDC)", this might turn out to be even more useful.

    For example, when handling a request of a web client, it's probably
    useful to have the user's IP address available in all log statements
    within code dealing with this particular request. Instead of passing
    this piece of data around between your application functions, you can
    just use the global (but thread-specific) NDC mechanism. It allows you
    to push data pieces (scalars usually) onto its stack via

        Log::Log4perl::NDC->push("San");
        Log::Log4perl::NDC->push("Francisco");

    and have your loggers retrieve them again via the "%x" placeholder in
    the PatternLayout. With the stack values above and a PatternLayout
    format like "%x %m%n", the call

        $logger->debug("rocks");

    will end up as

        San Francisco rocks

    in the log appender.

    The stack mechanism allows for nested structures. Just make sure that at
    the end of the request, you either decrease the stack one by one by
    calling

        Log::Log4perl::NDC->pop();
        Log::Log4perl::NDC->pop();

    or clear out the entire NDC stack by calling

        Log::Log4perl::NDC->remove();

    Even if you should forget to do that, "Log::Log4perl" won't grow the
    stack indefinitely, but limit it to a maximum, defined in
    "Log::Log4perl::NDC" (currently 5). A call to "push()" on a full stack
    will just replace the topmost element by the new value.

    Again, the stack is always available via the "%x" placeholder in the
    Log::Log4perl::Layout::PatternLayout class whenever a logger fires. It
    will replace "%x" by the blank-separated list of the values on the
    stack. It does that by just calling

        Log::Log4perl::NDC->get();

    internally. See details on how this standard log4j feature is
    implemented in Log::Log4perl::NDC.

  Mapped Diagnostic Context (MDC)
    Just like the previously discussed NDC stores thread-specific
    information in a stack structure, the MDC implements a hash table to
    store key/value pairs in.

    The static method

        Log::Log4perl::MDC->put($key, $value);

    stores $value under a key $key, with which it can be retrieved later
    (possibly in a totally different part of the system) by calling the
    "get" method:

        my $value = Log::Log4perl::MDC->get($key);

    If no value has been stored previously under $key, the "get" method will
    return the string "[undef]" to allow for easy string interpolation later
    on.

    Typically, MDC values are retrieved later on via the "%X{...}"
    placeholder in "Log::Log4perl::Layout::PatternLayout". For example, an
    application taking a web request might store the remote host like

        Log::Log4perl::MDC->put("remote_host", $r->headers("HOST"));

    at its beginning and if the appender's layout looks something like

        log4perl.appender.Logfile.layout.ConversionPattern = %X{remote_host}: %m%n

    then a log statement like

       DEBUG("Content delivered");

    will log something like

       adsl-63.dsl.snf.pacbell.net: Content delivered 

    later on in the program.

    For details, please check Log::Log4perl::MDC.

  Resurrecting hidden Log4perl Statements
    Sometimes scripts need to be deployed in environments without having
    Log::Log4perl installed yet. On the other hand, you dont't want to live
    without your Log4perl statements -- they're gonna come in handy later.

    So, just deploy your script with Log4perl statements commented out with
    the pattern "###l4p", like in

        ###l4p DEBUG "It works!";
        # ...
        ###l4p INFO "Really!";

    If Log::Log4perl is available, use the ":resurrect" tag to have Log4perl
    resurrect those burried statements before the script starts running:

        use Log::Log4perl qw(:resurrect :easy);

        ###l4p Log::Log4perl->easy_init($DEBUG);
        ###l4p DEBUG "It works!";
        # ...
        ###l4p INFO "Really!";

    This will have a source filter kick in and indeed print

        2004/11/18 22:08:46 It works!
        2004/11/18 22:08:46 Really!

    In environments lacking Log::Log4perl, just comment out the first line
    and the script will run nevertheless (but of course without logging):

        # use Log::Log4perl qw(:resurrect :easy);

        ###l4p Log::Log4perl->easy_init($DEBUG);
        ###l4p DEBUG "It works!";
        # ...
        ###l4p INFO "Really!";

    because everything's a regular comment now. Alternatively, put the magic
    Log::Log4perl comment resurrection line into your shell's PERL5OPT
    environment variable, e.g. for bash:

        set PERL5OPT=-MLog::Log4perl=:resurrect,:easy
        export PERL5OPT

    This will awaken the giant within an otherwise silent script like the
    following:

        #!/usr/bin/perl

        ###l4p Log::Log4perl->easy_init($DEBUG);
        ###l4p DEBUG "It works!";

  Access defined appenders
    All appenders defined in the configuration file or via Perl code can be
    retrieved by the "appender_by_name()" class method. This comes in handy
    if you want to manipulate or query appender properties after the
    Log4perl configuration has been loaded via "init()".

    Note that internally, Log::Log4perl uses the "Log::Log4perl::Appender"
    wrapper class to control the real appenders (like
    "Log::Log4perl::Appender::File" or "Log::Dispatch::FileRotate"). The
    "Log::Log4perl::Appender" class has an "appender" attribute, pointing to
    the real appender.

    The reason for this is that external appenders like
    "Log::Dispatch::FileRotate" don't support all of Log::Log4perl's
    appender control mechanisms (like appender thresholds).

    The previously mentioned method "appender_by_name()" returns a
    referrence to the *real* appender object. If you want access to the
    wrapper class (e.g. if you want to modify the appender's threshold), use
    the hash $Log::Log4perl::Logger::APPENDER_BY_NAME{...} instead, which
    holds references all appender wrapper objects.

  Modify appender thresholds
    To conveniently adjust appender thresholds (e.g. because a script uses
    more_logging()), use

           # decrease thresholds of all appenders
        Log::Log4perl->appender_thresholds_adjust(-1);

    This will decrease the thresholds of all appenders in the system by one
    level, i.e. WARN becomes INFO, INFO becomes DEBUG, etc. To only modify
    selected ones, use

           # decrease thresholds of all appenders
        Log::Log4perl->appender_thresholds_adjust(-1, ['AppName1', ...]);

    and pass the names of affected appenders in a ref to an array.

Advanced configuration within Perl
    Initializing Log::Log4perl can certainly also be done from within Perl.
    At last, this is what "Log::Log4perl::Config" does behind the scenes.
    Log::Log4perl's configuration file parsers are using a publically
    available API to set up Log::Log4perl's categories, appenders and
    layouts.

    Here's an example on how to configure two appenders with the same layout
    in Perl, without using a configuration file at all:

      ########################
      # Initialization section
      ########################
      use Log::Log4perl;
      use Log::Log4perl::Layout;
      use Log::Log4perl::Level;

         # Define a category logger
      my $log = Log::Log4perl->get_logger("Foo::Bar");

         # Define a layout
      my $layout = Log::Log4perl::Layout::PatternLayout->new("[%r] %F %L %m%n");

         # Define a file appender
      my $file_appender = Log::Log4perl::Appender->new(
                              "Log::Log4perl::Appender::File",
                              name      => "filelog",
                              filename  => "/tmp/my.log");

         # Define a stdout appender
      my $stdout_appender =  Log::Log4perl::Appender->new(
                              "Log::Log4perl::Appender::Screen",
                              name      => "screenlog",
                              stderr    => 0);

         # Have both appenders use the same layout (could be different)
      $stdout_appender->layout($layout);
      $file_appender->layout($layout);

      $log->add_appender($stdout_appender);
      $log->add_appender($file_appender);
      $log->level($INFO);

    Please note the class of the appender object is passed as a *string* to
    "Log::Log4perl::Appender" in the *first* argument. Behind the scenes,
    "Log::Log4perl::Appender" will create the necessary
    "Log::Log4perl::Appender::*" (or "Log::Dispatch::*") object and pass
    along the name value pairs we provided to
    "Log::Log4perl::Appender->new()" after the first argument.

    The "name" value is optional and if you don't provide one,
    "Log::Log4perl::Appender->new()" will create a unique one for you. The
    names and values of additional parameters are dependent on the
    requirements of the particular appender class and can be looked up in
    their manual pages.

    A side note: In case you're wondering if
    "Log::Log4perl::Appender->new()" will also take care of the "min_level"
    argument to the "Log::Dispatch::*" constructors called behind the scenes
    -- yes, it does. This is because we want the "Log::Dispatch" objects to
    blindly log everything we send them ("debug" is their lowest setting)
    because *we* in "Log::Log4perl" want to call the shots and decide on
    when and what to log.

    The call to the appender's *layout()* method specifies the format (as a
    previously created "Log::Log4perl::Layout::PatternLayout" object) in
    which the message is being logged in the specified appender. If you
    don't specify a layout, the logger will fall back to
    "Log::Log4perl::SimpleLayout", which logs the debug level, a hyphen (-)
    and the log message.

    Layouts are objects, here's how you create them:

            # Create a simple layout
        my $simple = Log::Log4perl::SimpleLayout();

            # create a flexible layout:
            # ("yyyy/MM/dd hh:mm:ss (file:lineno)> message\n")
        my $pattern = Log::Log4perl::Layout::PatternLayout("%d (%F:%L)> %m%n");

    Every appender has exactly one layout assigned to it. You assign the
    layout to the appender using the appender's "layout()" object:

        my $app =  Log::Log4perl::Appender->new(
                      "Log::Log4perl::Appender::Screen",
                      name      => "screenlog",
                      stderr    => 0);

            # Assign the previously defined flexible layout
        $app->layout($pattern);

            # Add the appender to a previously defined logger
        $logger->add_appender($app);

            # ... and you're good to go!
        $logger->debug("Blah");
            # => "2002/07/10 23:55:35 (test.pl:207)> Blah\n"

    It's also possible to remove appenders from a logger:

        $logger->remove_appender($appender_name);

    will remove an appender, specified by name, from a given logger. Please
    note that this does *not* remove an appender from the system.

    To eradicate an appender from the system, you need to call
    "Log::Log4perl->eradicate_appender($appender_name)" which will first
    remove the appender from every logger in the system and then will delete
    all references Log4perl holds to it.

How about Log::Dispatch::Config?
    Tatsuhiko Miyagawa's "Log::Dispatch::Config" is a very clever simplified
    logger implementation, covering some of the *log4j* functionality. Among
    the things that "Log::Log4perl" can but "Log::Dispatch::Config" can't
    are:

    *   You can't assign categories to loggers. For small systems that's
        fine, but if you can't turn off and on detailed logging in only a
        tiny subsystem of your environment, you're missing out on a majorly
        useful log4j feature.

    *   Defining appender thresholds. Important if you want to solve
        problems like "log all messages of level FATAL to STDERR, plus log
        all DEBUG messages in "Foo::Bar" to a log file". If you don't have
        appenders thresholds, there's no way to prevent cluttering STDERR
        with DEBUG messages.

    *   PatternLayout specifications in accordance with the standard (e.g.
        "%d{HH:mm}").

    Bottom line: Log::Dispatch::Config is fine for small systems with simple
    logging requirements. However, if you're designing a system with lots of
    subsystems which you need to control independantly, you'll love the
    features of "Log::Log4perl", which is equally easy to use.

Using Log::Log4perl from wrapper classes
    If you don't use "Log::Log4perl" as described above, but from a wrapper
    class (like your own Logging class which in turn uses "Log::Log4perl"),
    the pattern layout will generate wrong data for %F, %C, %L and the like.
    Reason for this is that "Log::Log4perl"'s loggers assume a static caller
    depth to the application that's using them. If you're using one (or
    more) wrapper classes, "Log::Log4perl" will indicate where your logger
    classes called the loggers, not where your application called your
    wrapper, which is probably what you want in this case. But don't
    dispair, there's a solution: Just increase the value of
    $Log::Log4perl::caller_depth (defaults to 0) by one for every wrapper
    that's in between your application and "Log::Log4perl", then
    "Log::Log4perl" will compensate for the difference.

Access to Internals
    The following methods are only of use if you want to peek/poke in the
    internals of Log::Log4perl. Be careful not to disrupt its inner
    workings.

    "Log::Log4perl->appenders()"
        To find out which appenders are currently defined (not only for a
        particular logger, but overall), a "appenders()" method is available
        to return a reference to a hash mapping appender names to their
        Log::Log4perl::Appender object references.

Dirty Tricks
    infiltrate_lwp()
        The famous LWP::UserAgent module isn't Log::Log4perl-enabled. Often,
        though, especially when tracing Web-related problems, it would be
        helpful to get some insight on what's happening inside
        LWP::UserAgent. Ideally, LWP::UserAgent would even play along in the
        Log::Log4perl framework.

        A call to "Log::Log4perl->infiltrate_lwp()" does exactly this. In a
        very rude way, it pulls the rug from under LWP::UserAgent and
        transforms its "debug/conn" messages into "debug()" calls of loggers
        of the category "LWP::UserAgent". Similarily, "LWP::UserAgent"'s
        "trace" messages are turned into "Log::Log4perl"'s "info()" method
        calls.

    Suppressing 'duplicate' LOGDIE messages
        If a script with a simple Log4perl configuration uses logdie() to
        catch errors and stop processing, as in

            use Log::Log4perl qw(:easy) ;
            Log::Log4perl->easy_init($DEBUG);
    
            shaky_function() or LOGDIE "It failed!";

        there's a cosmetic problem: The message gets printed twice:

            2005/07/10 18:37:14 It failed!
            It failed! at ./t line 12

        The obvious solution is to use LOGEXIT() instead of LOGDIE(), but
        there's also a special tag for Log4perl that suppresses the second
        message:

            use Log::Log4perl qw(:no_extra_logdie_message);

        This causes logdie() and logcroak() to call exit() instead of die().
        To modify the script exit code in these occasions, set the variable
        $Log::Log4perl::LOGEXIT_CODE to the desired value, the default is 1.

EXAMPLE
    A simple example to cut-and-paste and get started:

        use Log::Log4perl qw(get_logger);
    
        my $conf = q(
        log4perl.category.Bar.Twix         = WARN, Logfile
        log4perl.appender.Logfile          = Log::Log4perl::Appender::File
        log4perl.appender.Logfile.filename = test.log
        log4perl.appender.Logfile.layout = \
            Log::Log4perl::Layout::PatternLayout
        log4perl.appender.Logfile.layout.ConversionPattern = %d %F{1} %L> %m %n
        );
    
        Log::Log4perl::init(\$conf);
    
        my $logger = get_logger("Bar::Twix");
        $logger->error("Blah");

    This will log something like

        2002/09/19 23:48:15 t1 25> Blah 

    to the log file "test.log", which Log4perl will append to or create it
    if it doesn't exist already.

INSTALLATION
    If you want to use external appenders provided with "Log::Dispatch", you
    need to install "Log::Dispatch" (2.00 or better) from CPAN, which itself
    depends on "Attribute-Handlers" and "Params-Validate". And a lot of
    other modules, that's the reason why we're now shipping Log::Log4perl
    with its own standard appenders and only if you wish to use additional
    ones, you'll have to go through the "Log::Dispatch" installation
    process.

    Log::Log4perl needs "Test::More", "Test::Harness" and "File::Spec", but
    they already come with fairly recent versions of perl. If not,
    everything's automatically fetched from CPAN if you're using the CPAN
    shell (CPAN.pm), because they're listed as dependencies.

    "Time::HiRes" (1.20 or better) is required only if you need the
    fine-grained time stamps of the %r parameter in
    "Log::Log4perl::Layout::PatternLayout".

    Manual installation works as usual with

        perl Makefile.PL
        make
        make test
        make install

    If you're running Windows (98, 2000, NT, XP etc.), and you're too lazy
    to rummage through all of Log-Log4perl's dependencies, don't despair:
    We're providing a PPM package which installs easily with your
    Activestate Perl. Check
    "how_can_i_install_log__log4perl_on_microsoft_windows" in
    Log::Log4perl::FAQ for details.

DEVELOPMENT
    Log::Log4perl is still being actively developed. We will always make
    sure the test suite (approx. 500 cases) will pass, but there might still
    be bugs. please check http://log4perl.sourceforge.net for the latest
    release. The api has reached a mature state, we will not change it
    unless for a good reason.

    Bug reports and feedback are always welcome, just email them to our
    mailing list shown in the AUTHORS section. We're usually addressing them
    immediately.

REFERENCES
    [1] Michael Schilli, "Retire your debugger, log smartly with
        Log::Log4perl!", Tutorial on perl.com, 09/2002,
        http://www.perl.com/pub/a/2002/09/11/log4perl.html

    [2] Ceki Gülcü, "Short introduction to log4j",
        http://jakarta.apache.org/log4j/docs/manual.html

    [3] Vipan Singla, "Don't Use System.out.println! Use Log4j.",
        http://www.vipan.com/htdocs/log4jhelp.html

    [4] The Log::Log4perl project home page: http://log4perl.sourceforge.net

SEE ALSO
    Log::Log4perl::Config, Log::Log4perl::Appender,
    Log::Log4perl::Layout::PatternLayout,
    Log::Log4perl::Layout::SimpleLayout, Log::Log4perl::Level,
    Log::Log4perl::JavaMap Log::Log4perl::NDC,

AUTHORS
    Please send bug reports or requests for enhancements to the authors via
    our

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

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

        Contributors (in alphabetical order):
        Hutton Davidson, Chris R. Donnelly, Matisse Enzer, Hugh Esco,
        James FitzGibbon, Carl Franks, Dennis Gregorovic, Paul
        Harrington, David Hull, Robert Jacobson, Jeff Macdonald, Markus
        Peter, Brett Rann, Erik Selberg, Aaron Straup Cope, Lars Thegler,
        David Viner, Mac Yang.

COPYRIGHT AND LICENSE
    Copyright 2002-2004 by Mike Schilli <m@perlmeister.com> and Kevin Goess
    <cpan@goess.org>.

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

