Difference between signals and celery

[Robin Lery]

This may be a lame question, but I am really confused with these two. I know signals are used to do some task when something has happened. But what about celery? In the documentation it says:

Celery is an asynchronous task queue/job queue based on distributed message passing.

Will someone please explain to me of what celery is? What's the difference between these two and when to use them? Will be much appreciated! Thank you.

[Nevio Vesic]

The easiest answer I could give is that django signals are like hooks. Something you wish to preform AFTER or BEFORE some model action. Like you wish to adjust account balance on model save. Than you would use post_save builtin django signal.

Celery is here to handle task in background. Like you wish to process some sort of upload but you want to queue it up against some broker such as rabbitmq, redis, etc. On this way you can optimize stuff and return back user faster response saying "hey, process is backgrounded. Will notify you once it's completed". Than when task finishes you can call callback and let user know as example. Consider this. You have 100000 users uploading video. That video would go into celery task so that you can scale it up properly and distribute against network + make UX appropriate. 

Hope this makes sense.

All best,

Nevio

[Carlos Daniel Ruvalcaba Valenzuela]

Signals are like events, for example, when the ORM has done an update. Celery is more like a Task queue, you define code to execute certain task, load it on celery task server which is running separately from your django process, then when you need to execute it in the background you tell it, which usually means sending a message through a broker such as redis or rabbitmq to the task server, then it gets executed.

Celery is for background tasks, such as processing images, doing updates without blocking the main function return, etc. Also celery task can run in another machine, while signals are executed locally.

Regards,

Carlos Ruvalcaba

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[Robin Lery]

Thank all so much for you replies. Just one more question though. Does this means Celery needs a different server?

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[Avraham Serour]

No, you can run celery from the same server, the problem is you won't be able to do that on many cheap shared hostings

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[Robin Lery]

oh..ok. Thank you so much! :)



Its really great to learn something new.

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[fchow]

Question:  If a view saves a model that has a post-save hook, does the view return after the post-save hook has completed? Or does the view return and the hook runs in the background like a celery task would?