Understanding celery task prefetching

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  1. Prefetching can improve the performance. Workers don’t need to wait for the next message from a broker to process. Communicating with a broker once and processing a lot of messages gives a performance gain. Getting a message from a broker (even from a local one) is expensive compared to the local memory access. Workers are also allowed to acknowledge messages in batches

  2. Prefetching set to zero means “no specific limit” rather than unlimited

  3. Setting prefetching to 1 is documented to be equivalent to turning it off, but this may not always be the case (see https://stackoverflow.com/a/33357180/71522)

  4. Prefetching allows to ack messages in batches. CELERY_ACKS_LATE=True prevents acknowledging messages when they reach to a worker

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