Ensure stable routing for x-modulus-hash exchange#15849
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## What? This commit makes routing stable for the `x-modulus-hash` exchange: If the same destination queues stay bound to the exchange (i.e. do not bind or unbind queues after the "initial setup"), messages with the same domain entity (routing key) will always end up in the same destination queue, even across node restarts. ## How? With Mnesia this was guaranteed due to order_set tables. Khepri introduced a regression since it uses a bag ETS projection table. This commit simply sorts the destinations before picking the Nth destination. ## Why? Uses cases where message order matters are common. This commit allows to for example bind N quorum queues to an `x-modulus-hash` exchange instance (the binding key doesn't matter) and use the Single Active Consumer (SAC) feature on each quorum queue. This will provide * message ordering thanks to stable routing and SAC * concurrent consumption: N app instances can process messages in parallel * fault tolerance since the broker will deliver messages to another consumer when the active consumer crashes Using the consistent hash exchange is an alternative, but unnecessarily complex for this use case. Yet another alternative for this use case is using the murmur3 exchange type (#8319).
Move the x-modulus-hash exchange type from rabbitmq_sharding to rabbit since this exchange type is useful even without the sharding plugin.
This commit adds a test case to explicitly verify that the `x-modulus-hash` exchange supports weighted routing. Because the exchange uses `lists:sort/1` instead of `lists:usort/1` when fetching the bound destinations, a user can bind the exact same queue multiple times to the exchange (using different dummy binding keys) to increase its weight. If a queue is bound N times, it will appear N times in the destination list, giving it a proportionally higher chance of receiving messages.
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This commit adds documentation for the built-in `x-modulus-hash` exchange. It adds a dedicated page explaining its core features: stable routing, weighted routing, and how it compares to the consistent hash exchange. It also adds a bullet point to the message ordering section in `queues.md` to highlight the common use case of using `x-modulus-hash` with Single Active Consumer (SAC) to process messages concurrently while preserving order for each domain entity. Docs for rabbitmq/rabbitmq-server#15849
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This commit adds documentation for the built-in `x-modulus-hash` exchange. It adds a dedicated page explaining its core features: stable routing, weighted routing, and how it compares to the consistent hash exchange. It also adds a bullet point to the message ordering section in `queues.md` to highlight the common use case of using `x-modulus-hash` with Single Active Consumer (SAC) to process messages concurrently while preserving order for each domain entity. Docs for rabbitmq/rabbitmq-server#15849
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Ensure stable routing for `x-modulus-hash` exchange (backport #15849)
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…-2026 Add #15849 to `4.3.0` release notes
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RabbitMQ 4.3+. References rabbitmq/rabbitmq-server#15849 References #1918
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RabbitMQ 4.3+. References rabbitmq/rabbitmq-server#15849 References #1918 (cherry picked from commit 9c41e2c)
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What?
This PR makes routing stable for the
x-modulus-hashexchange: If the same destination queues stay bound to the exchange (i.e. do not bind or unbind queues after the "initial setup"), messages with the same domain entity (routing key) will always end up in the same destination queue, even across node restarts.Move the x-modulus-hash exchange type from
rabbitmq_shardingtorabbitsince this exchange type is useful even without the sharding plugin.How?
With Mnesia this was guaranteed due to order_set tables. Khepri introduced a regression since it uses a bag ETS projection table.
This PR simply sorts the destinations before picking the Nth destination.
Why?
Uses cases where message order matters are common. This PR allows to for example bind N quorum queues to an
x-modulus-hashexchange instance (the binding keys do not matter) and use the Single Active Consumer (SAC) feature on each quorum queue. This will provideUsing the consistent hash exchange is an alternative, but unnecessarily complex for this use case.
Yet another alternative for this use case is using the murmur3 exchange type (#8319).
Docs:
rabbitmq/rabbitmq-website#2494