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A durable subscriber, as shown in Figure 18.6, is a consumer that wants to receive all of the messages sent over a particular publish-subscribe channel, including messages sent while the consumer is disconnected from the messaging system. This requires the messaging system to store messages for later replay to the disconnected consumer. There also has to be a mechanism for a consumer to indicate that it wants to establish a durable subscription. Generally, a publish-subscribe channel (or topic) can have both durable and non-durable subscribers, which behave as follows:

  • non-durable subscriber—Can have two states: connected and disconnected. While a non-durable subscriber is connected to a topic, it receives all of the topic's messages in real time. However, a non-durable subscriber never receives messages sent to the topic while the subscriber is disconnected.

  • durable subscriber—Can have two states: connected and inactive. The inactive state means that the durable subscriber is disconnected from the topic, but wants to receive the messages that arrive in the interim. When the durable subscriber reconnects to the topic, it receives a replay of all the messages sent while it was inactive.

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