NAME¶
Log::Agent - logging agent
SYNOPSIS¶
use Log::Agent; # in all reusable components
logerr "error";
logtrc "notice:12", "notice that" if ...;
logdie "log and die";
use Log::Agent; # in application's main
logconfig(-prefix => $0); # simplest, uses default driver
use Log::Agent; # another more complex example
require Log::Agent::Driver::File; # logging made to file
logconfig(-driver =>
Log::Agent::Driver::File->make(
-prefix => $0,
-showpid => 1,
-channels => {
'error' => "$0.err",
'output' => "$0.out",
'debug' => "$0.dbg",
},
)
);
DESCRIPTION¶
The "Log::Agent" module provides an abstract layer for logging and
tracing, which is independent from the actual method used to physically
perform those activities. It acts as an agent (hence the name) that collects
the requests and delegates processing to a sublayer: the logging driver.
The "Log::Agent" module is meant to be used in all reusable
components, since they cannot know in advance how the application which ends
up using them will perform its logging activities: either by emitting messages
on stdout and errors on stderr, or by directing messages to logfiles, or by
using
syslog(3).
The logging interface is common for all the logging drivers, and is therefore
the result of a compromise between many logging schemes: any information given
at this level must be either handled by all drivers, or may be ignored
depending on the application's final choice.
PRIORITIES AND LEVEL¶
The "Log::Agent" module can use both priorities (as defined by
syslog(3)) or logging levels, or either, in which case there is an
implicit computation of the missing item (i.e. the level 4, for instance,
corresponds to the "warning" priority, and vice-versa). See
Log::Agent::Priorities for more details.
A logging level is defined as being a threshold: any level lesser than or equal
to that threshold will be logged.
At the "Log::Agent" level, it is possible to define a trace level and
a debug level. Only the messages below those levels (inclusive) will be handed
out to the underlying driver for logging. They are used by the
logtrc()
and
logdbg() routines, respectively.
CHANNELS¶
The "Log::Agent" class defines three logging channels, which are
"error", "output" and "debug". Depending on the
driver used for logging, those channels are ignored (typically with
syslog()) or may be implicitely defined (default logging, i.e. the one
achieved by the "Log::Agent::Driver::Default" driver, remaps
"error" and "debug" to stderr, "output" to
stdout).
INTERFACE¶
Anywhere a
message is expected, it can be a single string, or a
printf()-like format string followed by the required arguments. The
special macro %m is handled directly by "Log::Agent" and is replaced
by the string version of $!, which is the last error message returned by the
last failing system call.
NOTE: There should not be any trailing "\n" in the
message strings, nor any embededed one, although this is not enforced.
Remember that the main purpose of "Log::Agent" is to specify logging
messages in a standard way! Therefore, most of the time, a "should"
should be read as "must" and "should not" as "must
not", which is the strongest interdiction form available in English, as
far as I know.
Here are valid
message examples:
"started since $time"
"started since %s", $time
"fork: %m"
The follwing logging interface is made available to modules:
- logdbg priority, message
- Debug logging of message to the "debug" channel.
You may specify any priority you want, i.e. a "debug" priority is
not enforced here. You may even specify "notice:4" if you wish,
to have the message logged if the debug level is set to 4 or less. If
handed over to syslog(3), the message will nonetheless be logged at
the "notice" priority.
- logtrc priority, message
- Trace logging of message to the "output" channel.
Like logdbg() above, you are not restricted to the "info"
priority. This routine checks the logging level (either explicit as in
"info:14" or implicit as in "notice") against the
trace level.
- logsay message
- Log the message at the "notice" priority to the
"output" channel. The logging always takes place under the
default "-trace" settings, but only if the routine is called,
naturally. This means you can still say:
logsay "some trace message" if $verbose;
and control whether the message is emitted by using some external
configuration for your module (e.g. by adding a -verbose flag to the
creation routine of your class).
- logwarn message
- Log a warning message at the "warning" priority to the
"error" channel.
- logcarp message
- Same as logwarn(), but issues a Carp::carp(3) call instead,
which will warn from the perspective of the routine's caller.
- logerr message
- Log an error message at the "error" priority to the
"error" channel.
- logdie message
- Log a fatal message at the "critical" priority to the
"error" channel, and then dies.
- logconfess message
- Same as logdie(), but issues a Carp::confess(3) call
instead. It is possible to configure the "Log::Agent" module via
the "-confess" switch to automatically redirect a
logdie() to logconfess(), which is invaluable during unit
testing.
- logcroak message
- Same as logdie(), but issues a Carp::croak(3) call instead.
It is possible to configure the "Log::Agent" module via the
"-confess" switch to automatically redirect a logcroak()
to logconfess(), which is invaluable during unit testing.
- Log::Agent::inited
- Returns true when "Log::Agent" was initialized, either
explicitly via a logconfig() or implicitely via any logxxx()
call.
Modules sometimes wish to report errors from the perspective of their caller's
caller, not really their caller. The following interface is therefore
provided:
- logxcarp offset, message
- Same a logcarp(), but with an additional offset to be applied on
the stack. To warn one level above your caller, set it to 1.
- logxcroak offset, message
- Same a logcroak(), but with an additional offset to be applied on
the stack. To report an error one level above your caller, set it to
1.
For applications that wish to implement a debug layer on top of
"Log::Agent", the following routine is provided. Note that it is not
imported by default, i.e. it needs to be explicitly mentionned at
"use" time, since it is not meant to be used directly under regular
usage.
- logwrite channel, priority, message
- Unconditionally write the message at the given priority on
channel. The channel can be one of "debug",
"error" or "output".
At the application level, one needs to commit once and for all about the logging
scheme to be used. This is done thanks to the
logconfig() routine which
takes the following switches, in alphabetical order:
- "-caller" => [ parameters ]
- Request that caller information (relative to the logxxx() call) be
part of the log message. The given parameters are handed off to the
creation routine of "Log::Agent::Tag::Caller" and are documented
there.
I usually say something like:
-caller => [ -display => '($sub/$line)', -postfix => 1 ]
which I find informative enough. On occasion, I found myself using more
complex sequences. See Log::Agent::Tag::Caller.
- "-confess" => flag
- When true, all logdie() calls will be automatically masqueraded as
logconfess().
- "-debug" => priority or level
- Sets the priority threshold (can be expressed as a string or a number, the
string being mapped to a logging level as described above in PRIORITIES
AND LEVEL) for logdbg() calls.
Calls tagged with a level less than or equal to the given threshold will
pass through, others will return prematurely without logging
anything.
- "-driver" => driver_object
- This switch defines the driver object to be used, which must be an heir of
the "Log::Agent::Driver" class. See Log::Agent::Driver(3)
for a list of the available drivers.
- "-level" => priority or level
- Specifies both "-debug" and "-trace" levels at the
same time, to a common value.
- "-prefix" => name
- Defines the application name which will be pre-pended to all messages,
followed by ": " (a colon and a space). Using this switch alone
will configure the default driver to use that prefix (stripped down to its
basename component).
When a driver object is used, the "-prefix" switch is kept at the
"Log::Agent" level only and is not passed to the driver: it is
up to the driver's creation routine to request the "-prefix".
Having this information in Log::Agent enables the module to die on
critical errors with that error prefix, since it cannot rely on the
logging driver for that, obviously.
- "-priority" => [ parameters ]
- Request that message priority information be part of the log message. The
given parameters are handed off to the creation routine of
"Log::Agent::Tag::Priority" and are documented there.
I usually say something like:
-priority => [ -display => '[$priority]' ]
which will display the whole priority name at the beginning of the messages,
e.g. "[warning]" for a logwarn() or "[error]"
for logerr(). See Log::Agent::Tag::Priority and
Log::Agent::Priorities.
NOTE: Using "-priority" does not prevent the
"-duperr" flag of the file driver to also add its own hardwired
prefixing in front of duplicated error messages. The two options act at a
different level.
- "-tags" => [ list of
"Log::Agent::Tag" objects ]
- Specifies user-defined tags to be added to each message. The objects given
here must inherit from "Log::Agent::Tag" and conform to its
interface. See Log::Agent::Tag for details.
At runtime, well after logconfig() was issued, it may be desirable to
add (or remove) a user tag. Use the "logtags()" routine for this
purpose, and iteract directly with the tag list object.
For instance, a web module might wish to tag all the messages with a session
ID, information that might not have been available by the time
logconfig() was issued.
- "-trace" => priority or level
- Same a "-debug" but applies to logsay(),
logwarn(), logerr() and logtrc().
When unspecified, "Log::Agent" runs at the "notice"
level.
Additional routines, not exported by default, are:
- logtags
- Returns a "Log::Agent::Tag_List" object, which holds all
user-defined tags that are to be added to each log message.
The initial list of tags is normally supplied by the application at
logconfig() time, via the "-tags" argument. To add or
remove tags after configuration time, one needs direct access to the tag
list, obtained via this routine. See Log::Agent::Tag_List for the
operations that can be performed.
KNOWN LIMITATIONS¶
The following limitations exist in this early version. They might be addressed
in future versions if they are perceived as annoying limitatons instead of
being just documented ones. :-)
- •
- A module which calls logdie() may have its die trapped if called
from within an eval(), but unfortunately, the value of $@ is
unpredictable: it may be prefixed or not depending on the driver used.
This is harder to fix as one might think of at first glance.
- •
- Some drivers lack customization and hardwire a few things that come from
my personal taste, like the prefixing done when duperr is set in
Log::Agent::Driver::File, or the fact that the "debug" and
"stderr" channels are merged as one in the
Log::Agent::Driver::Default driver.
- •
- When using logcroak() or logconfess(), the place where the
call was made can still be visible when -caller is used, since the
addition of the caller information to the message is done before calling
the logging driver. Is this a problem?
AUTHOR¶
Log::Agent was originally authored by Raphael Manfredi
<Raphael_Manfredi@pobox.com> and is currently maintained by Mark
Rogaski
<mrogaski@cpan.org>.
LICENSE¶
Copyright (c) 1999-2000 Raphael Manfredi.
Copyright (c) 2002-2003, 2005, 2013 Mark Rogaski; all rights reserved.
This module is free software. You can redistribute it and/or modify it under the
terms of the Artistic License 2.0.
This program is distributed in the hope that it will be useful, but without any
warranty; without even the implied warranty of merchantability or fitness for
a particular purpose.
SEE ALSO¶
Log::Agent::Driver(3),
Carp(3).