Provided by: liblog-contextual-perl_0.008001-2_all 
NAME
Log::Contextual - Simple logging interface with a contextual log
VERSION
version 0.008001
SYNOPSIS
use Log::Contextual qw( :log :dlog set_logger with_logger );
use Log::Contextual::SimpleLogger;
use Log::Log4perl ':easy';
Log::Log4perl->easy_init($DEBUG);
my $logger = Log::Log4perl->get_logger;
set_logger $logger;
log_debug { 'program started' };
sub foo {
my $minilogger = Log::Contextual::SimpleLogger->new({
levels => [qw( trace debug )]
});
my @args = @_;
with_logger $minilogger => sub {
log_trace { 'foo entered' };
my ($foo, $bar) = Dlog_trace { "params for foo: $_" } @args;
# ...
slog_trace 'foo left';
};
}
foo();
Beginning with version 1.008 Log::Dispatchouli also works out of the box with "Log::Contextual":
use Log::Contextual qw( :log :dlog set_logger );
use Log::Dispatchouli;
my $ld = Log::Dispatchouli->new({
ident => 'slrtbrfst',
to_stderr => 1,
debug => 1,
});
set_logger $ld;
log_debug { 'program started' };
DESCRIPTION
Major benefits:
• Efficient
The default logging functions take blocks, so if a log level is disabled, the block will not run:
# the following won't run if debug is off
log_debug { "the new count in the database is " . $rs->count };
Similarly, the "D" prefixed methods only "Dumper" the input if the level is enabled.
• Handy
The logging functions return their arguments, so you can stick them in the middle of expressions:
for (log_debug { "downloading:\n" . join qq(\n), @_ } @urls) { ... }
• Generic
"Log::Contextual" is an interface for all major loggers. If you log through "Log::Contextual" you will
be able to swap underlying loggers later.
• Powerful
"Log::Contextual" chooses which logger to use based on user defined "CodeRef"s. Normally you don't
need to know this, but you can take advantage of it when you need to later.
• Scalable
If you just want to add logging to your basic application, start with Log::Contextual::SimpleLogger and
then as your needs grow you can switch to Log::Dispatchouli or Log::Dispatch or Log::Log4perl or
whatever else.
This module is a simple interface to extensible logging. It exists to abstract your logging interface so
that logging is as painless as possible, while still allowing you to switch from one logger to another.
It is bundled with a really basic logger, Log::Contextual::SimpleLogger, but in general you should use a
real logger instead. For something more serious but not overly complicated, try Log::Dispatchouli (see
"SYNOPSIS" for example.)
A WORK IN PROGRESS
This module is certainly not complete, but we will not break the interface lightly, so I would say it's
safe to use in production code. The main result from that at this point is that doing:
use Log::Contextual;
will die as we do not yet know what the defaults should be. If it turns out that nearly everyone uses
the ":log" tag and ":dlog" is really rare, we'll probably make ":log" the default. But only time and
usage will tell.
IMPORT OPTIONS
See "SETTING DEFAULT IMPORT OPTIONS" for information on setting these project wide.
-logger
When you import this module you may use "-logger" as a shortcut for "set_logger", for example:
use Log::Contextual::SimpleLogger;
use Log::Contextual qw( :dlog ),
-logger => Log::Contextual::SimpleLogger->new({ levels => [qw( debug )] });
sometimes you might want to have the logger handy for other stuff, in which case you might try something
like the following:
my $var_log;
BEGIN { $var_log = VarLogger->new }
use Log::Contextual qw( :dlog ), -logger => $var_log;
-levels
The "-levels" import option allows you to define exactly which levels your logger supports. So the
default, "[qw(debug trace warn info error fatal)]", works great for Log::Log4perl, but it doesn't support
the levels for Log::Dispatch. But supporting those levels is as easy as doing
use Log::Contextual
-levels => [qw( debug info notice warning error critical alert emergency )];
-package_logger
The "-package_logger" import option is similar to the "-logger" import option except "-package_logger"
sets the logger for the current package.
Unlike "-default_logger", "-package_logger" cannot be overridden with "set_logger" or "with_logger".
package My::Package;
use Log::Contextual::SimpleLogger;
use Log::Contextual qw( :log ),
-package_logger => Log::Contextual::WarnLogger->new({
env_prefix => 'MY_PACKAGE'
});
If you are interested in using this package for a module you are putting on CPAN we recommend
Log::Contextual::WarnLogger for your package logger.
-default_logger
The "-default_logger" import option is similar to the "-logger" import option except "-default_logger"
sets the default logger for the current package.
Basically it sets the logger to be used if "set_logger" is never called; so
package My::Package;
use Log::Contextual::SimpleLogger;
use Log::Contextual qw( :log ),
-default_logger => Log::Contextual::WarnLogger->new({
env_prefix => 'MY_PACKAGE'
});
SETTING DEFAULT IMPORT OPTIONS
Eventually you will get tired of writing the following in every single one of your packages:
use Log::Log4perl;
use Log::Log4perl ':easy';
BEGIN { Log::Log4perl->easy_init($DEBUG) }
use Log::Contextual -logger => Log::Log4perl->get_logger;
You can set any of the import options for your whole project if you define your own "Log::Contextual"
subclass as follows:
package MyApp::Log::Contextual;
use base 'Log::Contextual';
use Log::Log4perl ':easy';
Log::Log4perl->easy_init($DEBUG)
sub arg_default_logger { $_[1] || Log::Log4perl->get_logger }
sub arg_levels { [qw(debug trace warn info error fatal custom_level)] }
sub default_import { ':log' }
# or maybe instead of default_logger
sub arg_package_logger { $_[1] }
# and almost definitely not this, which is only here for completeness
sub arg_logger { $_[1] }
Note the "$_[1] ||" in "arg_default_logger". All of these methods are passed the values passed in from
the arguments to the subclass, so you can either throw them away, honor them, die on usage, etc. To be
clear, if you define your subclass, and someone uses it as follows:
use MyApp::Log::Contextual -default_logger => $foo,
-levels => [qw(bar baz biff)];
Your "arg_default_logger" method will get $foo and your "arg_levels" will get "[qw(bar baz biff)]";
Additionally, the "default_import" method is what happens if a user tries to use your subclass with no
arguments. The default just dies, but if you'd like to change the default to import a tag merely return
the tags you'd like to import. So the following will all work:
sub default_import { ':log' }
sub default_import { ':dlog' }
sub default_import { qw(:dlog :log ) }
See Log::Contextual::Easy::Default for an example of a subclass of "Log::Contextual" that makes use of
default import options.
FUNCTIONS
set_logger
my $logger = WarnLogger->new;
set_logger $logger;
Arguments: "LOGGER CODEREF"
"set_logger" will just set the current logger to whatever you pass it. It expects a "CodeRef", but if
you pass it something else it will wrap it in a "CodeRef" for you. "set_logger" is really meant only to
be called from a top-level script. To avoid foot-shooting the function will warn if you call it more
than once.
with_logger
my $logger = WarnLogger->new;
with_logger $logger => sub {
if (1 == 0) {
log_fatal { 'Non Logical Universe Detected' };
} else {
log_info { 'All is good' };
}
};
Arguments: "LOGGER CODEREF", "CodeRef $to_execute"
"with_logger" sets the logger for the scope of the "CodeRef" $to_execute. As with "set_logger",
"with_logger" will wrap $returning_logger with a "CodeRef" if needed.
has_logger
my $logger = WarnLogger->new;
set_logger $logger unless has_logger;
Arguments: none
"has_logger" will return true if a logger has been set.
log_$level
Import Tag: ":log"
Arguments: "CodeRef $returning_message, @args"
"log_$level" functions all work the same except that a different method is called on the underlying
$logger object. The basic pattern is:
sub log_$level (&@) {
if ($logger->is_$level) {
$logger->$level(shift->(@_));
}
@_
}
Note that the function returns it's arguments. This can be used in a number of ways, but often it's
convenient just for partial inspection of passthrough data
my @friends = log_trace {
'friends list being generated, data from first friend: ' .
Dumper($_[0]->TO_JSON)
} generate_friend_list();
If you want complete inspection of passthrough data, take a look at the "Dlog_$level" functions.
Which functions are exported depends on what was passed to "-levels". The default (no "-levels" option
passed) would export:
log_trace
log_debug
log_info
log_warn
log_error
log_fatal
Note: "log_fatal" does not call "die" for you, see "EXCEPTIONS AND ERROR HANDLING"
slog_$level
Mostly the same as "log_$level", but expects a string as first argument, not a block. Arguments are
passed through just the same, but since it's just a string, interpolation of arguments into it must be
done manually.
my @friends = slog_trace 'friends list being generated.', generate_friend_list();
logS_$level
Import Tag: ":log"
Arguments: "CodeRef $returning_message, Item $arg"
This is really just a special case of the "log_$level" functions. It forces scalar context when that is
what you need. Other than that it works exactly same:
my $friend = logS_trace {
'I only have one friend: ' . Dumper($_[0]->TO_JSON)
} friend();
See also: "DlogS_$level".
slogS_$level
Mostly the same as "logS_$level", but expects a string as first argument, not a block. Arguments are
passed through just the same, but since it's just a string, interpolation of arguments into it must be
done manually.
my $friend = slogS_trace 'I only have one friend.', friend();
Dlog_$level
Import Tag: ":dlog"
Arguments: "CodeRef $returning_message, @args"
All of the following six functions work the same as their "log_$level" brethren, except they return what
is passed into them and put the stringified (with Data::Dumper::Concise) version of their args into $_.
This means you can do cool things like the following:
my @nicks = Dlog_debug { "names: $_" } map $_->value, $frew->names->all;
and the output might look something like:
names: "fREW"
"fRIOUX"
"fROOH"
"fRUE"
"fiSMBoC"
Which functions are exported depends on what was passed to "-levels". The default (no "-levels" option
passed) would export:
Dlog_trace
Dlog_debug
Dlog_info
Dlog_warn
Dlog_error
Dlog_fatal
Note: "Dlog_fatal" does not call "die" for you, see "EXCEPTIONS AND ERROR HANDLING"
Dslog_$level
Mostly the same as "Dlog_$level", but expects a string as first argument, not a block. Arguments are
passed through just the same, but since it's just a string, no interpolation point can be used, instead
the Dumper output is appended.
my @nicks = Dslog_debug "names: ", map $_->value, $frew->names->all;
DlogS_$level
Import Tag: ":dlog"
Arguments: "CodeRef $returning_message, Item $arg"
Like "logS_$level", these functions are a special case of "Dlog_$level". They only take a single scalar
after the $returning_message instead of slurping up (and also setting "wantarray") all the @args
my $pals_rs = DlogS_debug { "pals resultset: $_" }
$schema->resultset('Pals')->search({ perlers => 1 });
DslogS_$level
Mostly the same as "DlogS_$level", but expects a string as first argument, not a block. Arguments are
passed through just the same, but since it's just a string, no interpolation point can be used, instead
the Dumper output is appended.
my $pals_rs = DslogS_debug "pals resultset: ",
$schema->resultset('Pals')->search({ perlers => 1 });
LOGGER CODEREF
Anywhere a logger object can be passed, a coderef is accepted. This is so that the user can use
different logger objects based on runtime information. The logger coderef is passed the package of the
caller, and the caller level the coderef needs to use if it wants more caller information. The latter is
in a hashref to allow for more options in the future.
Here is a basic example of a logger that exploits "caller" to reproduce the output of "warn" with a
logger:
my @caller_info;
my $var_log = Log::Contextual::SimpleLogger->new({
levels => [qw(trace debug info warn error fatal)],
coderef => sub { chomp($_[0]); warn "$_[0] at $caller_info[1] line $caller_info[2].\n" }
});
my $warn_faker = sub {
my ($package, $args) = @_;
@caller_info = caller($args->{caller_level});
$var_log
};
set_logger($warn_faker);
log_debug { 'test' };
The following is an example that uses the information passed to the logger coderef. It sets the global
logger to $l3, the logger for the "A1" package to $l1, except the "lol" method in "A1" which uses the $l2
logger and lastly the logger for the "A2" package to $l2.
Note that it increases the caller level as it dispatches based on where the caller of the log function,
not the log function itself.
my $complex_dispatcher = do {
my $l1 = ...;
my $l2 = ...;
my $l3 = ...;
my %registry = (
-logger => $l3,
A1 => {
-logger => $l1,
lol => $l2,
},
A2 => { -logger => $l2 },
);
sub {
my ( $package, $info ) = @_;
my $logger = $registry{'-logger'};
if (my $r = $registry{$package}) {
$logger = $r->{'-logger'} if $r->{'-logger'};
my (undef, undef, undef, $sub) = caller($info->{caller_level} + 1);
$sub =~ s/^\Q$package\E:://g;
$logger = $r->{$sub} if $r->{$sub};
}
return $logger;
}
};
set_logger $complex_dispatcher;
LOGGER INTERFACE
Because this module is ultimately pretty looking glue (glittery?) with the awesome benefit of the
Contextual part, users will often want to make their favorite logger work with it. The following are the
methods that should be implemented in the logger:
is_trace
is_debug
is_info
is_warn
is_error
is_fatal
trace
debug
info
warn
error
fatal
The first six merely need to return true if that level is enabled. The latter six take the results of
whatever the user returned from their coderef and log them. For a basic example see
Log::Contextual::SimpleLogger.
LOG ROUTING
In between the loggers and the log functions is a log router that is responsible for finding a logger to
handle the log event and passing the log information to the logger. This relationship is described in the
documentation for "Log::Contextual::Role::Router".
"Log::Contextual" and packages that extend it will by default share a router singleton that implements
the with_logger() and set_logger() functions and also respects the -logger, -package_logger, and
-default_logger import options with their associated default value functions. The router singleton is
available as the return value of the router() function. Users of Log::Contextual may overload router() to
return instances of custom log routers that could for example work with loggers that use a different
interface.
EXCEPTIONS AND ERROR HANDLING
"Log::Contextual", by design, does not intentionally invoke "die" on your behalf(*see footnote*) for
"log_fatal".
Logging events are characterized as information, not flow control, and conflating the two results in
negative design anti-patterns.
As such, "log_fatal" would at be better used to communicate information about a future failure, for
example:
if ( condition ) {
log_fatal { "Bad Condition is true" };
die My::Exception->new();
}
This has a number of benefits:
• You're more likely to want to use useful Exception Objects and flow control instead of cheating with
log messages.
• You're less likely to run a risk of losing what the actual problem was when some error occurs in your
creation of the Exception Object
• You're less likely to run the risk of losing important log context due to exceptions occurring mid
way through "die" unwinding and "exit" global destruction.
If you're still too lazy to use exceptions, then you can do what you probably want as follows:
if ( ... ) {
log_fatal { "Bad condition is true" };
die "Bad condtion is true";
}
Or for ":dlog" style:
use Data::Dumper::Consise qw( Dumper );
if ( ... ) {
# Dlog_fatal but not
my $reason = "Bad condtion is true because: " . Dumper($thing);
log_fatal { $reason };
die $reason;
}
footnote
The underlying behaviour of "log_fatal" is dependent on the backing library.
All the Loggers shipping with "Log::Contextual" behave this way, as do many of the supported loggers,
like "Log::Log4perl". However, not all loggers work this way, and one must be careful.
"Log::Dispatch" doesn't support implementing "log_fatal" at all
"Log::Dispatchouli" implements "log_fatal" using "die" ( via Carp )
CONTRIBUTORS
kentnl - Kent Fredric <kentfredric@gmail.com>
triddle - Tyler Riddle <t.riddle@shadowcat.co.uk>
voj - Jakob Voß <voss@gbv.de>
DESIGNER
mst - Matt S. Trout <mst@shadowcat.co.uk>
AUTHOR
Arthur Axel "fREW" Schmidt <frioux+cpan@gmail.com>
COPYRIGHT AND LICENSE
This software is copyright (c) 2018 by Arthur Axel "fREW" Schmidt.
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5
programming language system itself.
perl v5.34.0 2022-06-15 Log::Contextual(3pm)