[log4perl-devel] Log::Log4perl Recipe of the Week (#17)

[<log...@pe...>]

Welcome to the Log::Log4perl recipe of the week. Today:

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Log::Log4perl Recipe of the Week (#17):

I want to log ERROR and WARN messages to different files!
How can I do that?
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Let's assume you wanted to have each logging statement
written to a different file, based on the statement's
priority. Messages with priority "WARN" are supposed to go
to "/tmp/app.warn", events prioritized as "ERROR" should end
up in "/tmp/app.error".

Now, if you define two appenders "AppWarn" and "AppError"
and assign them both to the root logger, messages bubbling
up from any loggers below will be logged by both appenders
because of Log4perl's message propagation feature. If you
limit their exposure via the appender threshold mechanism
and set "AppWarn"'s threshold to "WARN" and "AppError"'s to
"ERROR", you'll still get "ERROR" messages in "AppWarn",
because "AppWarn"'s "WARN" setting will just filter out
messages with a *lower* priority than "WARN" -- "ERROR" is
higher and will be allowed to pass through.

What we need for this is a Log4perl *Custom Filter*, newly
available with Log::Log4perl 0.30, which, at the time of
this writing, is available as a development release.

Both appenders need to verify that the priority of the
oncoming messages exactly *matches* the priority the
appender is supposed to log messages of. To accomplish this
task, let's define two custom filters, "MatchError" and
"MatchWarn", which, when attached to their appenders, will
limit messages passed on to them to those matching a given
priority:

    log4perl.logger = WARN, AppWarn, AppError

        # Filter to match level ERROR
    log4perl.filter.MatchError = Log::Log4perl::Filter::LevelMatch
    log4perl.filter.MatchError.LevelToMatch  = ERROR
    log4perl.filter.MatchError.AcceptOnMatch = true

        # Filter to match level WARN
    log4perl.filter.MatchWarn  = Log::Log4perl::Filter::LevelMatch
    log4perl.filter.MatchWarn.LevelToMatch  = WARN
    log4perl.filter.MatchWarn.AcceptOnMatch = true

        # Error appender
    log4perl.appender.AppError = Log::Dispatch::File
    log4perl.appender.AppError.filename = /tmp/app.err
    log4perl.appender.AppError.layout   = SimpleLayout
    log4perl.appender.AppError.Filter   = MatchError

        # Warning appender
    log4perl.appender.AppWarn = Log::Dispatch::File
    log4perl.appender.AppWarn.filename = /tmp/app.warn
    log4perl.appender.AppWarn.layout   = SimpleLayout
    log4perl.appender.AppWarn.Filter   = MatchWarn

The appenders "AppWarn" and "AppError" defined above are
logging to "/tmp/app.warn" and "/tmp/app.err" respectively
and have the custom filters "MatchWarn" and "MatchError"
attached. This setup will direct all WARN messages, issued
anywhere in the system, to /tmp/app.warn (and ERROR messages
to /tmp/app.error) -- without any overlaps.


Have fun! Until next week.

-- Mike

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# Mike Schilli                    #
# log...@pe...        #
# http://perlmeister.com          #
# http://log4perl.sourceforge.net #
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