Bookmark Subscriptions and Persisted Acknowledgments
A bookmark subscription typically also requests the persisted acknowledgment (the AMPS clients do this automatically). In this case, AMPS periodically returns an acknowledgment that includes the last bookmark written to the local transaction log and acknowledged by all replication destinations that are currently using sync acknowledgment.
This information helps with application recovery, particularly in cases where publishers are intermittent or low-volume, and the last message for a subscription is significantly older than the last message evaluated for delivery to the subscription.
This also helps to manage failover in situations where multiple instances in a given replication fabric are accepting publishes by providing a stable failover point across instances.
The bookmark provided in the persisted ack is the last persisted point, which does not need to be a message that matches the subscription. This provides the benefit of persisted acknowledgements even in cases where the subscription has a narrow filter, or subscribes to a low-velocity topic interleaved with higher-velocity topics.