A queue can be configured to remain within a given size by removing the oldest messages when needed to make space for newer messages. However this took no account of message priority.
In some cases messages that were more valuable would be removed while there were less valuable messages left on the queue.
If such a queue has also been configured to recognise message priority (i.e. qpid.priorities is specified) then it will discard lower priority messages first.
The application can configure the queue (and mark the messages) such that less useful/less important messages are discarded first.
Release Note Entry:
When the Messaging component runs out of space, it must remove older messages to make space for new incoming messages. The messages to be deleted first are selected using an algorithm that takes into account both the priority and the age of the message. This algorithm allows the oldest of the low priority messages to be considered expendable while the high priority messages are preserved.