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Comment by justinator | Hacker News Reader

Huh, does python not have a way to flock a file itself?

A simple flock doesn't always work all that well - the devil is in the details. And there is the issue that you miss a run of the script and have to wait another half hour for the next run.

I've used the Highlander, "There can be only one!" code pattern for forever, and it works really well:

https://www.perlmonks.org/?node_id=14260

You could change things to check on the status of your lock every minute or so for 15 minutes or whatever.

Also in module form,

https://metacpan.org/pod/App::Highlander

And much more updated:

https://medium.com/booking-com-development/highlander-b67172...

No need to attempt to stuff all this in a cronjob IMHO.

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> Huh, does python not have a way to flock a file itself?

It does. (https://docs.python.org/3/library/fcntl.html#fcntl.flock)

Python has SQLite. This means you can lock a db file on all platforms and synchronize processes using SQLite.

That seems a little inefficient to pull in SQLite just so that you can have it lock a file, when you can just lock your own file, don't you think?

glances around nervously is flocking one of those "what's old is new again" concept that people are just now relearning? Surely a blog post on how not to have two concurrent cronjobs running is kind of a weird thing to read when some Linux grey beard figured this out in the 90's on their lunch break.

SQLite comes for free in Python distribution. The only reason why I mentioned it. Of course, just having a nice cross-platform file locking interface would be nice (not the case for Python).

I meant in a complexity/memory way. Exclusive locks themselves aren't actually cross platform - it would vary on your fs. That's why you don't see a nice cross-platform file locking interface. I'm not sure if you can guarantee an exclusive lock in SQLite on something like a nfs?

This is from SQLite docs:

> SQLite uses POSIX advisory locks to implement locking on Unix. On Windows it uses the LockFile(), LockFileEx(), and UnlockFile() system calls. SQLite assumes that these system calls all work as advertised. If that is not the case, then database corruption can result. One should note that POSIX advisory locking is known to be buggy or even unimplemented on many NFS implementations (including recent versions of Mac OS X) and that there are reports of locking problems for network filesystems under Windows. Your best defense is to not use SQLite for files on a network filesystem.

I guess cross-platform for naive me is POSIX + Windows.

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