consumer_utilisation metric is always zero for quorum queues on RabbitMQ 4.3.1

[XA21X]

Describe the bug

On RabbitMQ 4.3.1, quorum queues with active consumers reports consumer_utilisation / consumer_capacity 0. This was noticed after upgrading a cluster to 4.3.1 and our alerts for consumer utilisation never recovered.

Reproduction steps

1. Create a one-node RabbitMQ 4.3.1 broker.
2. Create a durable quorum queue and consume from it.
3. Query management API and check consumer_utilisation metric for the queue is zero.

# TYPE rabbitmq_detailed_queue_consumer_capacity gauge
# HELP rabbitmq_detailed_queue_consumer_capacity Consumer capacity
rabbitmq_detailed_queue_consumer_capacity{vhost="/",queue="demo-qq"} 0.0
# TYPE rabbitmq_detailed_queue_consumer_utilisation gauge
# HELP rabbitmq_detailed_queue_consumer_utilisation Same as consumer capacity
rabbitmq_detailed_queue_consumer_utilisation{vhost="/",queue="demo-qq"} 0.0

Expected behavior

Quorum queues with active consumers should report a non-zero utilisation value (and 1.0 when idle).

Attempting the same test on RabbitMQ 4.2.4 succeeds, and we observe a value of 1.0.

# TYPE rabbitmq_detailed_queue_consumer_capacity gauge
# HELP rabbitmq_detailed_queue_consumer_capacity Consumer capacity
rabbitmq_detailed_queue_consumer_capacity{vhost="/",queue="demo-qq"} 1.0
# TYPE rabbitmq_detailed_queue_consumer_utilisation gauge
# HELP rabbitmq_detailed_queue_consumer_utilisation Same as consumer capacity
rabbitmq_detailed_queue_consumer_utilisation{vhost="/",queue="demo-qq"} 1.0

Additional context

I don't have the Erlang experience to quickly assess its accuracy, but here's a root cause analysis from AI: In 4.3.1, quorum queues use rabbit_fifo version 8 (Ra v3, merged in #13885). Consumer capacity tracking was accidentally dropped during that rewrite. The tick handler no longer writes to the rabbit_fifo_usage ETS table, but rabbit_quorum_queue:handle_tick/3 still reads it via rabbit_fifo:usage/1, which returns 0.0 when empty. Older fifo versions (v7 and below, used in 4.2.x) still populate the table on tick.

[michaelklishin]

@XA21X there's meaningfully more than just one ETS write missing in Ra machine v8, so this may or may not be an accidental omission.

I have asked the rest of the team about the intent.

[kjnilsson]

@XA21X consumer utilisation never worked correctly with QQs, it is a legacy feature from classic queues that we removed in this version. Apologies it should have been called out in the release notes.

[XA21X]

All good, thank you for the clarification. 👍 Is there a recommended alternative other than monitoring the consumer count and number of messages in the queue? (I know it's use case dependent.) And is there any intention to eventually add support for quorum queues? Otherwise I think the documentation should be updated to mention it being deprecated for QQs.

Consumer capacity will be 0% for queues that have no consumers. For queues that have online consumers but no message flow, the value will be 100%: the idea is that any number of consumers can sustain this kind of delivery rate.

EDIT: It's also odd to report it as zero instead of removing it entirely - is that the plan? 🤔

[kjnilsson]

There is no plan to support it. I don't see how it brings any benefit over simply monitoring ready messages.