Django, RabbitMQ, & Celery - why does Celery run old versions of my tasks after I update my Django code in development?

Question

So I have a Django app that occasionally sends a task to Celery for asynchronous execution. I've found that as I work on my code in development, the Django development server knows how to automatically detect when code has changed and then restart the server so I can see my changes. However, the RabbitMQ/Celery section of my app doesn't pick up on these sorts of changes in development. If I change code that will later be run in a Celery task, Celery will still keep running the old version of the code. The only way I can get it to pick up on the change is to:

1. stop the Celery worker
2. stop RabbitMQ
3. reset RabbitMQ
4. start RabbitMQ
5. add the user to RabbitMQ that my Django app is configured to use
6. set appropriate permissions for this user
7. restart the Celery worker

This seems like a far more drastic approach than I should have to take, however. Is there a more lightweight approach I can use?

Answer 1

I've found that as I work on my code in development, the Django development server knows how to automatically detect when code has changed and then restart the server so I can see my changes. However, the RabbitMQ/Celery section of my app doesn't pick up on these sorts of changes in development.

What you've described here is exactly correct and expected. Keep in mind that Python will use a module cache, so you WILL need to restart the Python interpreter before you can use the new code.

The question is "Why doesn't Celery pick up the new version", but this is how most libraries will work. The Django development server, however, is an exception. It has special code that helps it automatically reload Python code as necessary. It basically restarts the web server without you needing to restart the web server.

Note that when you run Django in production, you probably WILL have to restart/reload your server (since you won't be using the development server in production, and most production servers don't try to take on the hassle of implementing a problematic feature of detecting file changes and auto-reloading the server).

Finally, you shouldn't need to restart RabbitMQ. You should only have to restart the Celery worker to use the new version of the Python code. You might have to clear the queue if the new version of the code is changing the data in the message, however. For example, the Celery worker might be receiving version 1 of the message when it is expecting to receive version 2.