NAME

Log::ger::Manual::Internals - Log::ger internals

VERSION

version 0.031.000

DESCRIPTION

When an importer package does this:

use Log::ger;

Basically all Log::ger does is construct logging routines and install them to importer's package (target), e.g. log_warn, log_debug, log_is_debug and so on. Log::ger also records the target name. When a reinit is requested (e.g. due to a change of log level or outputs), Log::ger will again construct logging routines and install them to each target, replacing the old ones.

In addition to installing routines to a package, Log::ger can also target a hash or an object (which is basically the same as installing to a package, but the routines will expect to be called as object methods instead of plain subroutines, i.e. they expect the first argument to be the object).

GLOSSARY

Composite output

An output that simply multiplexes the log message it receives to one or more other outputs.

Implemented in Log::ger::Output::Composite.

Formatter

A routine that takes arguments supplied by the user to the log routine (e.g. log_warn("blah %s", $args, ...) and converts it to the message (usually string) that is sent to the output (or the layouter, if there is one).

Hook

A code (provided by plugins) that is called in various points of time (phase). Please see "HOOKS AND PLUGINS" for more details.

Hook priority

A way by which different hooks that register at the same phase are ordered for execution. Implemented as a number between 0 to 100 where 0 means very high (executed first) and 100 means very low (executed last). Hooks that do not care about the order in which they are executed with regard to other hooks should set their priority to 50 (normal).

Init

The process of constructing logger routines and installing them to targets. See "INIT" for more details about the process.

Is routine

A type of routines that are constructed and installed to target which checks for a certain level. When logging level is at a certain level or "higher" (more severe), the associated "is routine" will return true; and vice versa. This is an efficient way to avoid doing work when unneeded.

The default "is routine" names are "log_is_<LEVEL>", e.g. "log_is_trace", "log_is_debug", and so on. An example of using an "is routine":

use Log::ger;

sub foo {
    if (log_is_trace) {
        # perform some calculation
        ...
        log_trace "Result of calculation: ...";
    }
}

The other type of routines that are constructed and installed to target is log routines.

Layouter

A routine that takes the formatted message (usually a string) and converts it to the final message string that is sent to output. Usually a layouter is used to add additional information along with the log message itself, e.g. timestamp, source file/line number, PID (process ID), etc.

Level

The main way by which log messages are filtered. Level refers to the important or "severity" of a message. Log producer chooses to log a message with a certain level, e.g.:

log_trace "This is a low importance log message";
$log->log("error", "This is a highly important log message!");

while log consumer chooses only to "see" log messages "above" certain level only.

Another way to filter log messages is by its category.

Log consumer

A log consumer is a process (Perl application/script) which configures an output. This causes all log messages that are produced (either by the script itself or the modules that the script uses) to be directed to the specified output.

Log producer

Any code (typically a Perl module) that uses Log::ger and invokes one or more log routines. For example, this tiny module is a producer:

package MyPackage;
use Log::ger;

sub foo {
    log_trace "Entering foo";
    ...
    log_trace "Leaving foo";
}

1;

When foo() is used somewhere else, it will produce two log messages at the trace level.

Log routine

Or logger routine. A type of routines that are constructed and installed to target which produces a log message.

The default "log routine" names are "log_<LEVEL>", e.g. "log_trace", "log_debug", and so on. An example of using a "log routine":

use Log::ger;

sub foo {
    log_trace "Entering foo";
    ...
    log_trace "Leaving foo";
}

Log routines is the main type of routines that you will typically use when logging. The other type of routines that are constructed and installed to target is is routines.

Output

Destination for the formatted (see "Formatter"), laid out (see "Layouter") log message. Only log messages that have the sufficient level and matching category get sent to the output. Examples of output include: null (no where), screen (terminal), file, syslog.

In Log::ger, log messages are sent to a single output. But there is a multiplexer output that (re)sends the log messages to other output(s).

Phase

The order in which hooks are executed. For more details, see "HOOKS AND PLUGINS".

Plugin

A Perl module that supplies hooks. For more details, see "HOOKS AND PLUGINS".

Target

A package that will be installed with logging routines. Aside from package, Log::ger can also install routines to a hash or an object.

Installing to a hash is usually for internal testing purposes. As a Log::ger user, you will very rarely need to target a hash.

Installing to an object is essentially the same as installing to a package: Log::ger will pick a "random" package for the object and install the routines there.

HOOKS AND PLUGINS

Hooks are how Log::ger provides its flexibility. At various times (phases), Log::ger will turn to running hooks to get some behavior or result. For example when wanting to construct a logging routine or formatting routine or before/after installing logging routines. Plugins, which are modules in the Log::ger::{Plugin,Output,Format,Layout,...} namespaces, can supply these hooks.

Hooks are stored in the %Global_Hooks variable, where the key is phase name and the value an array of hook records. There are also %Per_Package_Hooks, %Per_Object_Hooks, and %Per_Hash_Hooks to store per-target hooks that will only be used for specific targets. This way, the logging routines for each package and object/hash can be customized.

Each hook record is in the form of:

[$key, $prio, $coderef]

where $key is (plugin) package name, $prio is a number between 0-100 (the lower the number, the higher the priority and the earlier it is run), $coderef is the actual hook routine. A plugin is supposed to put only at most one hook per phase.

Expected return value of hook

A hook routine is passed a hash argument and is expected to return an array:

[$result, $flow_control, ...]

By default each hook will be executed in order of its priority. $flow_control can be set to 1 by a hook to stop immediately after this hook instead of continuing to the next. Some phases will nevertheless stop after the first hook that returns non-undef $result. A hook that returns undef is effectively declining and causing Log::ger to move to the next hook in the chain.

Some phases might return extra elements.

Arguments passed to hook

Aguments received by hook: target (str, can be package if installing to a package, or hash or object), target_arg (str, when target is package, will be the package name; when target is hash will be the hash; when target is object will be the object), init_args (hash, arguments passed to Log::ger when importing, e.g. {category => 'My::Package'}; it also serves as a per-target stash which survives reinit, by convention you can put stuffs here under keys that start with _). In some phases, hook will receive more arguments (see phase documentation below).

Phases

Available phases:

create_formatter

Used to construct formatter routine, which will be called with arguments from the user when she calls the logging routines.

It should return:
```
[\&formatter, $flow_control, $formatter_name]
```
$formatter_name is optional and defaults to default, which is the default formatter used for all logging routines.
create_layouter

Used to construct layouter routine, which will be called with arguments ($fmsg, \%init_args, $lnum, $lname) where $fmsg is formatted message from the formatter, %init_args are arguments given to Log::ger->get_logger or to Log::ger's import(), $lnum is numeric level, $lname is string level. The layouter must return the laid-out message (usually a string).
create_routine_names

Used to construct routine names. Hook must return this (all keys are optional):
```
[{
  log_subs      => [ [NAME, STR_LEVEL, FMT_NAME], ... ],
  log_methods   => [ [NAME, STR_LEVEL, FMT_NAME], ... ],

  is_subs       => [ [NAME, STR_LEVEL, FMT_NAME], ... ],
  is_methods    => [ [NAME, STR_LEVEL, FMT_NAME], ... ],

  logml_subs    => [ [NAME, undef    , FMT_NAME], ... ],
  logml_methods => [ [NAME, undef    , FMT_NAME], ... ],
}, ...]
```
Where log_subs and log_methods are names of per-level log routines, is_subs an is_methods are names of per-level level detection routines, logml_subs and logml_methods are names of multiple-level log routines.

FMT_NAME is optional and defaults to default (the default formatter routine).
create_log_routine

Used to create per-level "log_level" routines which will be called with (\%init_args, $msg) arguments. Called for each routine specified in the log_subs (or log_methods) in the routine names (see documentation on create_routine_names phase). Extra arguments received by hook: name (routine name), level (numeric level), str_level.
create_is_routine

Used to create per-level "log_is_level" routines. Called for each routine specified in the is_subs (or is_methods) in the routine names (see documentation on create_routine_names phase). Extra Arguments received by hooks: name (routine name), level (numeric level), str_level.
create_logml_routine

Used to create multiple-level "log" routines which will be called with (\%init_args, $level, $msg) arguments. Called for each routine specified in the logml_subs (or logml_methods) in the routine names (see documentation on create_routine_names phase). Extra arguments received by hooks: name (routine name).
before_install_routines

Extra arguments received by hooks: routines (array of routines to install), formatters (hashref of formatter routines, if any), layouter (layouter routine, if any).

routine is in the form of:
```
[
  [$coderef, $name, $numlevel, $type],
  ...
]
```
Where $type is either log_sub, log_method, logml_sub, logml_method, is_sub, is_method.
after_install_routines

Extra arguments received by hooks: routines.

Aside from the global hooks, there are also per-target hooks, which are stored in %Per_Package_Hooks, %Per_Hash_Hooks, %Per_Object_Hooks.

INIT

This section describes what init_target(), which is the routine used to initialize a target, does.

First, hooks in the create_formatter phase are run. This will collect one or more formatters. In the most common case, only the default formatter will be constructed. In some cases, like Log::ger::Like::LogAny, we want to use different formatters for method like warn and debug (arguments simply joined by space) and for methods like warnf and debugf (sprintf-style with data structure dumping, the default formatter used by Log::ger).

Next, hooks in the create_layouter phase are run. This will create a layouter (multiple layouters might be supported in the future, but for now we only use one layouter per target).

Next, hooks in the create_routine_names phase are run. This will produce a list of subroutine/method names to create, along with what formatter to use for each (the default is default). Plugins that want to mimic other interfaces like Log::ger::Like::LogAny or Log::ger::Like::Log4perl will want to add their hook here to provide names other than Log::ger's default. For example, Log4perl has uppercase subroutine names like WARN or DEBUG.

There are two types of routines that are created, logging routines (like log_warn, log_debug) and level detection routines, also called the "is" routines (like log_is_warn, log_is_debug). Logging routines in turn can be reguler, per-level routines or multilevel routines that accept numeric level as the first argument.

Next, the logger routines are created by running the create_log_routine and create_logml_routine hooks, for each log level. The code from the hooks (usually from output plugins) are then combined with the formatter and layouter to form the final routine ready to be installed.

Likewise, the log level detection routines are created by running the create_is_routine hooks for each log level.

Before installing the routines, we give a chance to plugins to do stuffs in the before_install_routines phase. Some plugins use this phase to, e.g. fixup prototypes.

After installing the routines, we likewise give a chance to plugins to do stuffs in the after_install_routines phase. Some plugins like Log::ger::Plugin::OptAway this phase to actually replace the routines that are not needed with a no-op.

TARGETS

Log::ger can install logger routines to a package, or an object (which is similar to installing to a package), or a hash (usually for testing). The routines that are installed into package target are of type *_subs. The routines that are installed into object target package are of type *_methods. The routines that are installed into hash target are of type *_methods but they will expect to be called as a normal subroutine (i.e. no object reference as first $self argument).

AUTHOR

perlancar <perlancar@cpan.org>

COPYRIGHT AND LICENSE

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

To install Log::ger::Manual, copy and paste the appropriate command in to your terminal.

cpanm

cpanm Log::ger::Manual

CPAN shell

perl -MCPAN -e shell
install Log::ger::Manual

For more information on module installation, please visit the detailed CPAN module installation guide.

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