x-delay-type topic does not work when khepri is enabled in RabbitMQ

[gomoripeti]

Describe the bug

Messages are not routed to any destination from a delayed exchange of type "topic" when Khepri is enabled in RabbitMQ.

When Khepri is enabled in RabbitMQ, routing a topic binding utilises a khepri projection (rabbit_khepri_topic_trie_*) (eg https://github.com/rabbitmq/rabbitmq-server/blob/v4.2.3/deps/rabbit/src/rabbit_db_topic_exchange.erl#L532) However that projection is only applied to a khepri path which has a hardcoded topic exchange type hardcoded https://github.com/rabbitmq/rabbitmq-server/blob/v4.2.3/deps/rabbit/src/rabbit_khepri.erl#L1615

I opened this issue for tracking purposes. I intend to work on addressing it.

Reproduction steps

...

Expected behavior

The delayed exchange should work the same way no matter if khepri feature flag is enabled or not.

Additional context

No response

[michaelklishin]

@gomoripeti or we can finally admit that this plugin is dead.

Mnesia, which it is based on, is about to be removed from RabbitMQ (except for the bits necessary for migration to Khepri).

Tanzu RabbitMQ now has a replicated Delayed Queue type capable of handling backlogs into tens of millions of delayed messages efficiently, all while offering the same replication characteristics as quorum queues.

It's too difficult to justify working on this plugin in any shape or form. Modifying RabbitMQ core to accommodate such a plugin is particularly difficult.

[gomoripeti]

I agree that modifying RabbitMQ core just for the sake of this plugin is out of question.

If anyone stumbles upon this issue, a workaround could be to define the delayed exchange with x-delayed-type = fanout and bind it to a single normal topic exchange. Then bind your queues to the helper topic exchange instead.

[michaelklishin]

Yes, exchange-to-exchange bindings should be an easy enough workaround.

As is using Deadlettering for basic delays, some messaging frameworks offer it as a feature and set up the topology for retries (NServiceBus did it first, IIRC).