Using Celery on processes and gevent in tasks at the same time
I'd like to use Celery as a queue for my tasks so my web app could enqueue a task, return a response and the task will be processed meanwhile / someday / ... I build a kind of API, so I don't know what sort of tasks will be there in advance - in future, there can be tasks dealing with HTTP requests, another IO, but also CPU-consuming tasks. In respect to that, I'd like to run Celery's workers on processes, as these are universal kind of parallelism in Python.
However, I'd like to use gevent in my tasks too, so I could have a single task spawning many HTTP requests, etc. The problem is, when I do this:
from gevent import monkey
monkey.patch_all()
Celery stops to work. It starts, but no tasks can be effectively enqueued - they seem to go to the broker, but Celery worker doesn't collect them and process them. Only starts and waits. If I delete those lines and perform the task without any gevent and parallelization, everything works.
I think it could be because gevent patches also threading. So I tried
from gevent import monkey
monkey.patch_all(thread=False)
...but then Celery doesn't even start, it crashes without giving a reason (debug level of logging turned on).
Is it possible to use Celery for enqueuing tasks and gevent for doing some stuff inside a single task? How? What do I do wrong?
Answer
I believe the recommended way to start the task is as follows.
python manage.py celery worker -P gevent --loglevel=INFO
Gevent needs to be patched as early as possible.