Thursday, 18 December 2008

Running Alone

John in my team at work has just pointed me at a great little CPAN module for ensuring that a script can only have one copy running at a time.

In our team, we have a lot of potentially slow running processes, which are often run as cron jobs. Now, ordinarily, you would pick cron start times
that ensure that you have finished any previous run of the script. But it is quite common for us to want to run as soon as possible, but not if the
previous run hasn't finished.

One method previously suggested was to touch a file in /tmp and then delete it when finished, wrapping the cron caller in a command not to run if
the touched file is present. However, what happens if /tmp is cleared out (which has happened to me!)

So, John found Sys::RunAlone. If the format of your script is to run a main method, and you put __END__ or __DATA__ at the bottom of the script,
then this can lock the text, and then knows no to exit if it finds a lock on the section. This means you could have your cronjob running every 30 minutes
and not worry if the script takes 5 minutes in a cycle, or 45 minutes.

A definite benefit.

A big thanks to Elizabeth Mattijsen for writing this very useful module.

(A quick example of this running is below. If you try to run the same script in another terminal before the first has finished it's sleep, you will get
the correct error).

#!/usr/bin/perl -wT
use strict;
use warnings;
use Sys::RunAlone;


sub main {
warn 'Hello ...';
warn 'world!';

sub go_to_sleep {
my $sleepytime = shift;
sleep $sleepytime;


Blog entry

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