[This is part of the Large Problems in Django Series, see previous entries about: Documentation, APIs, Search, and Database Migrations]

A lot of Django applications have tasks that they need to perform out of process. When you are executing a web request, if you try to do all the work that you need before returning to the user, your site will be increasingly slow. The answer to this problem is to fire off a request to do those tasks, while returning to the user in a reasonable amount of time. Celery refers to itself as a “Distributed Task Queue”, and is the current best of breed in the Python realm.

Why Use Celery¶

Easy¶ For the most basic functionality, all you need to do is: move your function into your tasks.py

wrap it with a @task decorator

call it with task.delay(*args) just like before. Now, your task is magically running out of process and you can get on with whatever it is your code is meant to be doing.

Network Effects¶ This is currently the best and most complete application in Python that does these things. A lot of people are using it, which means that features will be added consistently. There is also pretty good support in the #celery IRC channel, which usually has around 40-50 people in it. It is being actively developed and all other things being equal, using a tool with a community around it is much better.

Concurrency¶ The celeryd daemon supports multiprocessing, which allows it to run multiple tasks at once. You can get “cheap concurrency” this way, by loading it up with tasks and having it execute them. You can also run multiple instances of celeryd across multiple servers, you can get your tasks that run concurrently across servers. Running multiple instances is also a good way of insuring redundancy in case one of your daemons goes down.

Monitoring¶ One of the scary things about having remote execution of tasks is that if your daemon goes away, your site will appear not to function. Celery has an accompanying project called celerymon which provides monitoring services for Celery.

No more hacky cron jobs¶ I don’t know about you, but most of the time when I want something to be run in the background, cron is my go to choice. I’m ashamed to admit that I’ve written code that is meant to run in a cron job every minute checking for something to have happened. However, celery has most of the features that cron has, while giving you real support for deamonizing and delaying tasks. Being able to retry tasks is a great benefit is has over cron, so when something fails, you can run it again later.

Great documentation¶ The celery docs are great, including everything from basic setup and example instructions to howtos. We put it into production at work, and the docs for using redis as a “ghetto queue” were great and worked the first try.