Replacing Queue Subscriptions

Queues support the replace option for subscriptions. As with subscriptions to other topics, queue subscriptions can replace the content filter, the topic, the options, or all of the above. Replacement is atomic. The queue consumer is guaranteed that, after the replace occurs, only messages that match the new subscription will be delivered.

Replacing queue subscriptions differs from unsubscribing and resubscribing with new parameters in two ways:

1. AMPS does not break message leases or adjust the number of currently-unacknowledged messages for the subscription, even if the messages no longer match the current subscription. AMPS makes no assumptions about the state of the messages, and requires the subscriber to acknowledge them or allow the lease to expire.

2. AMPS may change the maximum backlog for the subscription if either the max_backlog option or the topic for the subscription has changed. AMPS adjusts the backlog using the same logic as when the subscription was entered: the maximum backlog will be the smaller of the option set by the consumer or the limit on the queue. This can result in a situation where the consumer has more messages leased than the current maximum for the subscription, and no new messages will be delivered until that number drops below the current maximum.

   For example, if the consumer has a requested max_backlog of 10 and updates a subscription from a queue with a configured maximum of 10 to a queue with a configured maximum of 5, the new backlog for the subscription will be 5. However, the consumer may still have 10 messages outstanding.

In all other ways, AMPS behaves as though the replaced subscription was a new subscription to the queue.