What is the expected behavior of a quorum queue if it has a policy that configures a dead-letter-strategy but no dead-letter-exchange?

[rsperl]

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4.2.4

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28.3.x

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rhel

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Steps to reproduce the behavior in question

Create two policies for quorum queues. Both should specify dead-letter-strategy: "at-least-once", but one should also configure dead-letter-exchange: "dlx-exchange".

Create two queues and apply one policy to each.

Publish a message to each, then get the message from the UI with the option nack, requeue=false.

The qq with the dlx configured will have the message stuck in dead-lettered. The qq with only the dead-letter-strategy will be dropped.

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What problem are you trying to solve?

I'm trying to understand the intended behavior of a message in a quorum queue that has a policy that only defines the dead-letter-strategy but no dead-letter-exchange. We had a situation where both were defined, but the dlx did not get created, so messages piled up in the source queue.

We thought that to solve this we should remove both dead-letter-* options, but in practice, we see that it was only required to remove the dead-letter-exchange option to not have messages accumulate in the source queue.

Could you clarify what the intended behavior is? As I read https://www.rabbitmq.com/blog/2022/03/29/at-least-once-dead-lettering , it sounds like I should expect an accumulation of messages, but in practice it sounds like that option is silently ignored.

[cookesan]

dead-letter-strategy by itself does not create a dead-letter target. It only changes how quorum queues perform dead-lettering once a DLX is actually configured.

For a quorum queue, the DLX destination still comes from dead-letter-exchange and optionally a routing key. Without dead-letter-exchange, there is nowhere for the message to be republished, so you should not expect the at-least-once dead-letter worker behavior just because dead-letter-strategy is set.

The practical policy should include both pieces if you want quorum queue at-least-once dead lettering:

rabbitmqctl set_policy qq-dlx '^qq\.' \
  '{"dead-letter-exchange":"my-dlx","dead-letter-strategy":"at-least-once"}' \
  --apply-to quorum_queues

If this came up because of delivery-limit, the safe reading is: with no DLX configured, messages that would otherwise be dead-lettered have no DLX destination and are discarded rather than retained for at-least-once dead-letter transfer.

[rsperl]

Thank you!

[michaelklishin]

If you do not configure the exchange to dead letter to, no dead lettering will take place.