Emit event if configured queue length is reached

[ansd]

Is your feature request related to a problem? Please describe.

An operator would like to receive a notification if a queue limit, e.g. a configured max-length or max-length-bytes is reached, and possibly another notification if the queue gets emptied again.

Describe the solution you'd like

The queue emits an event to rabbit_event such that these events can be consumed by the event exchange.
Events should not be emitted per message being rejected / dropped / dead-lettered to avoid excessive event creations. Instead the event should only be emitted when the queue limit is reached, and another event if the queue limit falls below some threshold (e.g. 90% of the limit).

Describe alternatives you've considered

An alternative is to dead letter messages with reason maxlen, consume from the dead letter queue and have the client app create an alert. However this solution doesn't work for overflow behaviour reject-publish.

Additional context

It's already possible today to create alerts if a queue depth reaches a specific limit, see for example

• https://github.com/rabbitmq/cluster-operator/blob/main/observability/prometheus/rules/rabbitmq-per-object/queue-is-growing.yml
• https://www.rabbitmq.com/blog/2021/05/03/alerting

However, this issue is specifically about alerts for the configured max-length or max-length-bytes being reached.

[SimonUnge]

@ansd

We have been thinking about something like this too, and would be happy to look into it. We have some more alerts we would like to get in, other limits the user nears/exceeds/leaves etc. Will create a little discussion and see what you think!