What about a queue that is the ingress point for a fan-in between O(N) active continuous producers to a single active continuous consumer? In such a case, you'

What about a queue that is the ingress point for a fan-in between O(N) active continuous producers to a single active continuous consumer? In such a case, you'd expect at least N messages in the queue at all times — and often a small multiple of N. Not because the consumer can't consume those messages in a timely manner, but because, given a reliable queue, messages can't be consumed until they've been 2PCed into the queue, and that means that there are always going to be some messages sitting around in the "acknowledged to the producer, but not yet available to the consumer" state. Which still takes up resources of the queueing system, and so should still be modelled as the messages being in the queue.