6.7.3. Considerations when using Priority Queues

Priority Queues deliver messages to acquiring consumers in order of priority, rather than the usual First-In-First-Out (FIFO) order of a queue. The delivery order for browsing consumers is "undefined". At the time of writing, browsing consumers receive messages from a priority queue in FIFO order; however, you should not rely on this behavior in your applications, as it may change in the future.

If the message enqueue rate sufficient outpaces the dequeue rate in a priority queue, it is possible that lower priority messages may never be removed from the queue. To avoid this situation the Fairshare feature allows a consumer to take a specified block of message from each priority level in turn.