basically this demonstrates that gevent & eventlet will mess up badly when you use non-thread safe code

Warning: while this is written for now EOLed stack, the results can be replicated for the recent most versions of python and/or celery as well

Installation

1. have dependecies installed using requirements.txt
2. run redis on localhost: docker run -d -p 6379:6379 redis
3. init db: python2 manage.py migrate

Trials

Run with pool = prefork

expectation : No errors, everything runs fine. (ok there might be some database locked errors, but that's not what we're looking for)

1. run python2 manage.py fill to prefill the queue with tasks
2. run celery worker -A demo -c 8 --pool prefork -l INFO to run tasks

Run with pool = gevent

expectation : few "IntegrityError: NOT NULL constraint failed: core_store.val", proving that shared state was mutated while the other thread was running concurrently.

1. run python2 manage.py fill to prefill the queue with tasks
2. run celery worker -A demo -c 8 --pool gevent -l INFO to run tasks

Run with pool = eventlet

expectation : few "IntegrityError: NOT NULL constraint failed: core_store.val", proving that shared state was mutated while the other thread was running concurrently.

1. run python2 manage.py fill to prefill the queue with tasks
2. run celery worker -A demo -c 8 --pool eventlet -l INFO to run tasks

conclusion

Stay the fuck away from gevent and eventlet if you have thread unsafe code, or you'll end up with inconsistent behaviour which can go catastrophically wrong.