It could be solved by Celery, but it would also require a database and Celery doesn't scale that well unless runs on top of Redis which is not really fault tole

It could be solved by Celery, but it would also require a database and Celery doesn't scale that well unless runs on top of Redis which is not really fault tolerant. Also actions invocation with guarantees and exponential retries is not trivial with Celery. Actually at Uber large number of services are being migrated from Python/Celery to Go/Cadence.