Durable Subscriptions
Durable subscriptions allow subscribers to receive all the messages published on a topic, including those published while the subscriber is inactive. When the subscribing applications are not running, the messaging provider holds the messages in nonvolatile storage. It retains the messages until one of the following occurs:
The subscribing application becomes active, identifies itself to the provider, and sends an acknowledgment of receipt of the message.
The expiration time for the messages is reached.