Should we replace Celery with Heroku scheduler?

[bfirsh]

Celery is only used to run scheduled tasks (sending emails, etc) and is not used to run any background tasks.

Now we are moving to Heroku, we could use the Heroku Scheduler to run these tasks instead of Celery.

Advantages:

• Cheaper (permanent celery dyno is $25/mo, scheduler will cost pennies)
• We don't have to upgrade Celery to 4.x (currently uses Celery 3 which doesn't work with Django 1.11)
• Less complexity (Celery has a lot of moving parts and breaks)

Disadvantages:

• Means we would have to add back Celery in the future if we want to use it for anything (but it's not even working at the moment so that doesn't seem like much extra work...)
• A little bit of work to migrate the Celery tasks to management commands
• Ties us to Heroku?
• Scheduler commands are configured in the web UI instead of being committed to repo, which is annoying

[jonathan-s]

I think I'd vote for using the heroku scheduler. Right now the advantages seem to weigh over.

[jessykate]

Since the tasks themselves would still be in code, i don't mind too much that the schedule is defined outside the repo, and makes it easier to move to a different scheduler in the future if we want.

We could also choose to document the scheduling in the repo in a config directory of some kind, but it might get stale if it's not actually used programmatically.

[jonathan-s]

@bfirsh @jessykate, I looked up the heroku scheduler. It's very coarse; every 10 minutes, every hour, daily. That's it. Perhaps it'd be better to use this method instead -> https://devcenter.heroku.com/articles/clock-processes-python

Though that still requires one worker... so it's not saving any money (or perhaps it's saving some? Right now it looks like there is one worker for celeryd). But it would reduce any complexity as it would be more like a normal cronjob.

[jessykate]

We actually have pretty light weight scheduling needs. I haven't looked at the various options but the heroku scheduler seems like it would save us a good $25 or $50/mo 😮 ! I'm kind of keen on that since MN doesn't make very much and i'd rather spend it on humans doing dev work if we have the option?? :)

[bfirsh]

Yeah we don't need fine grained scheduing. The only thing that is run is sending out emails in the morning, and a weekly one too (which can be done with if day == sunday in the daily one).

The clock process they describe is effectively what Celery is.