Multiple processes, one log file?

splinky has asked for the wisdom of the Perl Monks concerning the following question:

My fellow monks...

My system is comprised of multiple Perl processes under Linux. If it came down to it, I could have each process log to its own file and then merge them together after the fact, but I would really like for all these processes to log to the same file, with all the logs interleaved and ordered by time.

I've tried a few simple experiments with Log::Log4perl using Log::Log4perl::Appender::File, and while it appears to work, I haven't seen anything in the documentation to assure me that it's meant to work.

Is there a simple solution?

Thanks.

Comment on Multiple processes, one log file?

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Re: Multiple processes, one log file? by davidrw (Prior) on May 19, 2005 at 19:20 UTC
I would image that any of the Syslog modules would handle it correctly, since syslogd does. Though I think a popular opinion would be to use Log::Log4perl -- from the docs, it lists Log::Dispatch::Syslog as an available appender via Log::Dispatch.	[reply]
Re: Multiple processes, one log file? by sgifford (Prior) on May 20, 2005 at 01:30 UTC
I implemented something similar (though not using `log4perl`) using `flock`. You just get an exclusive lock on the logfile before writing, then write, then release the lock. Before that, my experience has been that if you write less than the system block size or `MAX_PIPE` (4K on many systems), you can get away without locking. I'm not sure if that's guaranteed; I remember reading it once, but I've been looking for the reference for two years without finding it, so I can't say for sure. :)	[reply] [d/l] [select]
Re^2: Multiple processes, one log file? by cosimo (Hermit) on May 20, 2005 at 15:19 UTC
In fact it is not guaranteed. We reinvented our log wheel and 95% of log activity is on the same log file, and we find it very useful. From perl docs, various web examples, this merlyn's webcolumn (written when I didn't even know what perl was) and extensive™ testing, I learnt that using `flock()` with `LOCK_EX` flag is the only way to have consistent updates/writes by many processes onto a single log file (or page-counters, for that matter), though this method is not reliable across platforms. Not using `flock` resulted in mangled log files, with different processes log buffers being mixed and truncated at random.	[reply] [d/l] [select]
Re: Multiple processes, one log file? by astroboy (Chaplain) on May 20, 2005 at 02:27 UTC
Do your processes share the same parent or are you in a persistent environment like mod_perl? The issues are covered in the Log4perl FAQ. Anyway, have a look at Log::Log4perl::Appender::Synchronized	[reply]
Re: Multiple processes, one log file? by Vautrin (Hermit) on May 19, 2005 at 19:42 UTC
Just as a thought, what about creating a daemon that would run in the background and listen on a socket? It would only need one thread and it could just log as information came into the hopper... .	[reply]
Re^2: Multiple processes, one log file? by sgifford (Prior) on May 20, 2005 at 01:31 UTC
You could call it "`syslog`". :-)	[reply]
Re^3: Multiple processes, one log file? by Vautrin (Hermit) on May 20, 2005 at 02:01 UTC
It's not fun if you don't roll it yourself. :-D .	[reply]


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