nak() should follow backoff

[tamis-laan]

Proposed change

Make msg.nak() follow the set backoff strategy in the consumer.

Use case

Given the following:

# Backoff strategy
backoff = [2,4,8,16,32]

# Consumer configuration
config = ConsumerConfig(
    durable_name="action_workers",
    backoff=backoff,
    ack_wait=backoff[-1],
    max_deliver=len(backoff) + 1
)

# Subscribe
sub = await js.pull_subscribe("action.process.*", durable="action_workers", config=config)

while True:
    msgs = await sub.fetch(1)
    for msg in msgs:
        print(msg)
        msg.nak()

I would expect msg.nak() to make use of the backoff strategy. However backoff is only used for retries.

Contribution

No response

[Jarema]

nak marks message for immediate redelivery, so it does not apply ack_wait, or backoff, which is used in case of user not sending back anything.

You can however specify nak(delay) to achieve what I think would solve your use case.

[dreo]

@Jarema that seems to contradict the info in the official docs, which for backoff option says this:

A sequence of durations controlling the redelivery of messages on nak or acknowledgment timeout. Overrides ackWait. The sequence length must be less than or equal to MaxDelivery. If backoff is not set, a nak will result in immediate redelivery.

Can you clarify on how and when this option is supposed to work? I've spent some time trying to get it work without success until I've noticed this thread, so my understanding is that you are right and the docs are wrong. This however seem a little bit counterintuitive, as I can't see a reason why this should not be applied to NAKs too 🤔. Could you elaborate a bit on this? Thanks.

[aricart]

Docs are wrong - it is redelivery of acknowledgement timeouts.
I couldn't apply to NAK, the server would have to track each message NAK retries, to assign a different value. Depending on your client, you may have the ability to specify the number of seconds (or some other time unit) before the server should re-deliver it.

[dreo]

I see. Thanks for a quick reply.

For anyone interested, you can use delay param for NAK together with delivery attempt number from message metadata (message.metadata.num_delivered) to perform exponential backoff (or any other strategy) by message receiver itself - at least for this NAK use case.