Topics are destinations in Pub/Sub messaging. When messages are published, they are delivered to all active subscribers. Some subscribers register an interest in receiving messages sent while they were inactive. These are durable subscriptions. The broker notes the durable subscription and ensures that all messages from the topic's publishers are retained until they either are acknowledged by the durable subscriber or have expired.
Durable subscriptions provide a mechanism to save messages for an unavailable client. Whenever a subscriber reconnects to the topic under the name it registered for its durable subscription, all undelivered messages to that topic that have not expired are delivered in order. The administrator can terminate durable subscriptions or a client can use the
cancelDurableSubscription procedure or the
subscribeprocedure to close the durable subscription.
