Celery tasks stuck in PENDING forever: result.state always 'PENDING' and result.get() hangs (result backend missing or misconfigured)

Cause: In Celery, PENDING is the default state for any task id with no recorded history; the docs say it effectively means "unknown". The state is only recorded if a result backend is configured and both the client and the worker use the same one. So tasks that ran fine can still look PENDING when no result_backend is set, when the worker and client use different backends, when results are ignored, or when the task has not yet reached any worker. Fix (best first): 1. Configure a result backend on the app, for example Celery("proj", broker="redis://localhost:6379/0", backend="redis://localhost:6379/1") or the result_backend setting. Options include Redis, a database via SQLAlchemy, the Django ORM, and RPC. Restart both the worker and every client process after changing it. 2. Check the client and worker agree: print(result.backend) in the client and compare with the worker's startup banner, which lists the results backend. A stale worker started earlier with a different config can steal tasks; stop old workers and use an absolute --pidfile path. 3. Make sure results are not ignored: no ignore_result=True on the task or decorator, and task_ignore_result not set to True in config. 4. Make sure the task actually reaches a worker: broker URL correct, worker consuming the right queue (-Q), and the task name registered (it appears in the worker's [tasks] list). A task sent to a queue nobody consumes also stays PENDING. 5. If you want to see STARTED, set task_track_started = True; it is off by default, so PENDING then SUCCESS is normal otherwise. 6. Call get() or forget() on every AsyncResult you keep so backend storage does not grow without bound, and set result_expires if needed. Verify: result = add.delay(2, 2); result.get(timeout=10) returns 4 and result.state is SUCCESS. Sources: https://docs.celeryq.dev/en/stable/getting-started/first-steps-with-celery.html https://docs.celeryq.dev/en/stable/userguide/tasks.html