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18 PAUSE user(s)
6 non-PAUSE user(s).

Jonathan Swartz

NAME

Log::Any -- Bringing loggers and listeners together

VERSION

version 0.15

SYNOPSIS

In a CPAN or other module:

    package Foo;
    use Log::Any qw($log);

    $log->error("an error occurred");
    $log->debugf("arguments are: %s", \@_)
        if $log->is_debug();

    my $log2 = Log::Any->get_logger(category => 'My::Class');

In your application:

    use Log::Any::Adapter;
    
    # Send all logs to Log::Log4perl
    Log::Any::Adapter->set('Log4perl');

    # Send all logs to Log::Dispatch
    my $log = Log::Dispatch->new(outputs => [[ ... ]]);
    Log::Any::Adapter->set( 'Dispatch', dispatcher => $log );

    # See Log::Any::Adapter documentation for more options

DESCRIPTION

Log::Any allows CPAN modules to safely and efficiently log messages, while letting the application choose (or decline to choose) a logging mechanism such as Log::Dispatch or Log::Log4perl.

Log::Any has a very tiny footprint and no dependencies beyond Perl 5.6, which makes it appropriate for even small CPAN modules to use. It defaults to 'null' logging activity, so a module can safely log without worrying about whether the application has chosen (or will ever choose) a logging mechanism.

The application, in turn, may choose one or more logging mechanisms via Log::Any::Adapter.

LOG LEVELS

Log::Any supports the following log levels and aliases, which is meant to be inclusive of the major logging packages:

     trace
     debug
     info (inform)
     notice
     warning (warn)
     error (err)
     critical (crit, fatal)
     alert
     emergency

Levels are translated as appropriate to the underlying logging mechanism. For example, log4perl only has six levels, so we translate 'notice' to 'info' and the top three levels to 'fatal'.

CATEGORIES

Every logger has a category, generally the name of the class that asked for the logger. Some logging mechanisms, like log4perl, can direct logs to different places depending on category.

PRODUCING LOGS (FOR MODULES)

Getting a logger

The most convenient way to get a logger in your module is:

    use Log::Any qw($log);

This creates a package variable $log and assigns it to the logger for the current package. It is equivalent to

    our $log = Log::Any->get_logger(category => __PACKAGE__);

In general, to get a logger for a specified category:

    my $log = Log::Any->get_logger(category => $category)

If no category is specified, the caller package is used.

Logging

To log a message, use any of the log levels or aliases. e.g.

    $log->error("this is an error");
    $log->warn("this is a warning");
    $log->warning("this is also a warning");

You should not include a newline in your message; that is the responsibility of the logging mechanism, which may or may not want the newline.

There are also printf-style versions of each of these methods:

    $log->errorf("an error occurred: %s", $@);
    $log->debugf("called with %d params: %s", $param_count, \@params);

The printf-style methods have a few advantages, besides being arguably more readable:

  • Any complex references (like \@params above) are automatically converted to single-line strings with Data::Dumper.

  • Any undefined values are automatically converted to the string "<undef>".

  • A logging mechanism could potentially use the unchanging format string (or a digest thereof) to group related log messages together.

Log level detection

To detect whether a log level is on, use "is_" followed by any of the log levels or aliases. e.g.

    if ($log->is_info()) { ... }
    $log->debug("arguments are: " . Dumper(\@_))
        if $log->is_debug();

This is important for efficiency, as you can avoid the work of putting together the logging message (in the above case, stringifying @_) if the log level is not active.

Some logging mechanisms don't support detection of log levels. In these cases the detection methods will always return 1.

In contrast, the default logging mechanism - Null - will return 0 for all detection methods.

Testing

Log::Any::Test provides a mechanism to test code that uses Log::Any.

CONSUMING LOGS (FOR APPLICATIONS)

To direct logs somewhere - a file, the screen, etc. - you must use Log::Any::Adapter. This is intentionally kept in a separate distributions to keep Log::Any as simple and unchanging as possible.

MOTIVATION

Many modules have something interesting to say. Unfortunately there is no standard way for them to say it - some output to STDERR, others to warn, others to custom file logs. And there is no standard way to get a module to start talking - sometimes you must call a uniquely named method, other times set a package variable.

This being Perl, there are many logging mechanisms available on CPAN. Each has their pros and cons. Unfortunately, the existence of so many mechanisms makes it difficult for a CPAN author to commit his/her users to one of them. This may be why many CPAN modules invent their own logging or choose not to log at all.

To untangle this situation, we must separate the two parts of a logging API. The first, log production, includes methods to output logs (like $log->debug) and methods to inspect whether a log level is activated (like $log->is_debug). This is generally all that CPAN modules care about. The second, log consumption, includes a way to configure where logging goes (a file, the screen, etc.) and the code to send it there. This choice generally belongs to the application.

Log::Any provides a standard log production API for modules. Log::Any::Adapter allows applications to choose the mechanism for log consumption.

See http://www.openswartz.com/2007/09/06/standard-logging-api/ for the original post proposing this module.

Q & A

Isn't Log::Any just yet another logging mechanism?

No. Log::Any does not, and never will, include code that knows how to log to a particular place (file, screen, etc.) It can only forward logging requests to another logging mechanism.

Why don't you just pick the best logging mechanism, and use and promote it?

Each of the logging mechanisms have their pros and cons, particularly in terms of how they are configured. For example, log4perl offers a great deal of power and flexibility but uses a global and potentially heavy configuration, whereas Log::Dispatch is extremely configuration-light but doesn't handle categories. There is also the unnamed future logger that may have advantages over either of these two, and all the custom in-house loggers people have created and cannot (for whatever reason) stop using.

Is it safe for my critical module to depend on Log::Any?

Our intent is to keep Log::Any minimal, and change it only when absolutely necessary. Most of the "innovation", if any, is expected to occur in Log::Any::Adapter, which your module should not have to depend on (unless it wants to direct logs somewhere specific). Log::Any has no module dependencies other than Test::Simple for testing.

Why doesn't Log::Any use insert modern Perl technique?

To encourage CPAN module authors to adopt and use Log::Any, we aim to have as few dependencies and chances of breakage as possible. Thus, no Moose or other niceties.

SEE ALSO

Log::Any::Adapter, the many Log:: modules on CPAN

COPYRIGHT AND LICENSE

This software is copyright (c) 2011 by Jonathan Swartz.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.




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