Result backend doesn’t work or tasks are always in PENDING state¶
All tasks are PENDING by default, so the state would’ve been better named “unknown”. Celery doesn’t update the state when a task is sent, and any task with no history is assumed to be pending (you know the task id, after all).

1. Make sure that the task doesn’t have ignore_result enabled.

Enabling this option will force the worker to skip updating states.

2. Make sure the task_ignore_result setting isn’t enabled.

3. Make sure that you don’t have any old workers still running.

It’s easy to start multiple workers by accident, so make sure that the previous worker is properly shut down before you start a new one.

An old worker that isn’t configured with the expected result backend may be running and is hijacking the tasks.

The --pidfile argument can be set to an absolute path to make sure this doesn’t happen.

4. Make sure the client is configured with the right backend.

If, for some reason, the client is configured to use a different backend than the worker, you won’t be able to receive the result. Make sure the backend is configured correctly:

>>> result = task.delay()

print(result.backend)

from: http://docs.celeryproject.org/en/latest/getting-started/first-steps-with-celery.html#result-backend-doesn-t-work-or-tasks-are-always-in-pending-state