Django: what’s new in 6.0

Why is celery awful?

Because it’s a seducer. It does what you need to do and you two are happy together. So you shower more tasks on Celery and it becomes cold and non-responsive at random times.

And debugging is a pain in the ass. Most places I’ve been that have it, I’ve tried to sell them on adding Flower to give better insight and everyone thinks that’s a very good idea but there isn’t time because we need to debug these inscrutable Celery issues.

https://flower.readthedocs.io/en/latest/

Celery is great and awful at the same time. In particular, because it is many Python folks' first introduction to distributed task processing and all the things that can go wrong with it. Not to mention, debugging can be a nightmare. Some examples:

- your function arguments aren't serializable - your side effects (e.g. database writes) aren't idempotent - discovering what backpressure is and that you need it - losing queued tasks during deployment / non-compatible code changes

There's also some stuff particular to celery's runtime model that makes it incredibly prone to memory leaks and other fun stuff.

Honestly, it's a great education.

It's okay till it's not. Everyone I know who had Celery in production was looking for a substitution (custom or third-party) on a regular basis. Too many moving pieces and nuances (config × logic × backend), too many unresolved problems deep in its core (we've seen some ghosts you can't debug), too much of a codebase to understand or hack. At some point we were able to stabilize it (a bunch of magic tricks and patches) and froze every related piece; it worked well under pressure (thanks, RabbitMQ).

From your experience, what is a better alternative guys?

> The Many Problems with Celery:

— https://steve.dignam.xyz/2023/05/20/many-problems-with-celer...

> The problems with (Python’s) Celery:

— https://docs.hatchet.run/blog/problems-with-celery

> Dramatiq motivation:

— https://dramatiq.io/motivation.html

Here are some alternatives:

Dramatiq: https://github.com/Bogdanp/dramatiq

RQ: https://github.com/rq/rq

Huey: https://github.com/coleifer/huey

Hatchet: https://github.com/hatchet-dev/hatchet

> your side effects (e.g. database writes) aren't idempotent

What does idempotent mean in this context, or did you mean atomic/rollback on error?

I'm confused because how could a database write be idempotent in Django? Maybe if it introduced a version on each entity and used that for crdt on writes? But that'd be a significant performance impact, as it couldn't just be a single write anymore, instead they'd have to do it via multiple round trips

There’s no alternative (while prototyping), and anything else is better (when you properly defined your case).

django-q2: https://github.com/django-q2/django-q2

DjangoQ2 is a fine alternative during early development

Not the comment that you replied to but I use my own Urd. It's a fancier Cron that you can stop fast. Which is imo what you normally want.

Task queues are like email. It's what everyone is used to so people ask for more of it, but it's not actually good/the right tool.

In my experience async job idempotency is implemented as upserts. Insert all job outputs on the first run. Do (mostly) nothing on subsequent runs. Maybe increment a counter or timestamp.

In the context of background jobs idempotent means that if your job gets run for a second time (and it will get run for a second time at some point, they all do at-least-once delivery) there aren't any unfortunate side effects to that. Often that's just a case of checking if the relevant database updates have already been done, maybe not firing a push notification in cases of a repeated job.

Here is a nice guide from AWS https://docs.aws.amazon.com/wellarchitected/latest/framework...

Would you consider tools like Temporal, DBOS, Absurd Workflows, PGQueuer as alternatives?

https://temporal.io/

https://docs.dbos.dev/

https://news.ycombinator.com/item?id=45797228

https://python-absurd-client.readthedocs.io/en/latest/quicks...

https://pgqueuer.readthedocs.io/en/latest/

If you need idempotent db writes, then use something like Temporal. You can't really blame Celery for not having that because that is not what Celery aims to be.

Temporal requires a lot more setup than setting up a Redis instance though. That's the only problem with it. And I find the Python API a bit more difficult to grasp. But otherwise a solid piece of technology.

With Temporal, your activity logic still needs to ensure idempotency e.g. by checking if an event id / idempotency key exists in a table. It's still at-least-once delivery. Temporal does make it easy to mint an idempotency key by concatenating workflow run id and activity id, if you don't have a one provided client-side.

Although we could say the same thing about Kafka, couldn't we? It's made for much higher throughput and has usually other use cases, but it's also great until it's not great.

Temporal is an AMAZING piece of software, however I don't believe it's a replacement for something more simple like Celery. Even if you write helpers, the overhead to setting up workflows, invoking them, etc. is just too much for simple jobs like sending an email (imo). I would love to work in a codebase that had access to both, depending on the complexity of what you're trying to background.

At least the last time I used Kafka (which was several years ago so things might have changed) it wasn't at all easy to get started. It was a downright asshole in fact. If you pursue a relationship with an asshole, you shouldn't be surprised when they become cold to you

Yes, absolutely. It's still pretty much that way. Especially if you want to make changes to a running installation, add nodes etc.