General developer forum

Certainly yes. On our moodle cluster, all the webservers try and run the cronjobs, and the first one to get the "lock" gets to run it. This ensures a pretty fair distribution of load, as all servers are on NTP, so the least loaded one has the most chance to be early to the lock.

Initially, I had messed up the commandline args for dotlockfile, and more than one server was running cron.php -- we immediately got reports of users getting duplicate forum posts mailouts about 50% of the time. I didn't read the Moodle code, as it was my locking that was buggy, but I think there's a race condition with a pretty wide window.

Now, I don't think it's particularly problematic, and I think it'd be extremely hard to prevent race conditions across all the modules. cron.php could help prevent this problem by incorporating a locking mechanism. There are some tricky points over stale locks if cron.php dies -- I still think it's better to do it from a shell wrapper to cron.php.

In the end, I think whoever is running a cluster should know how to manage their cronjobs to avoid duplication/overlaps.