I find the distinction between queue and pub sub system quite poor. A pub sub system is just a persistent queue at its core, the only distinction is you have multiple queues for each subscriber, hence multiple readers. everything else stays the same. Ordering is expected to be strict in both cases. The Durability factor is also baked in both systems. On the question of bounded and unbounded queue: does not message queues also spill to disk in order to prevent OOM scenarios?