Topics are used in the publish-subscribe paradigm. When a client publishes a message to a topic, he expects that a copy of the message will be delivered to each client that has subscribed to the topic. Topic messages are delivered in the same manner a television show is delivered. Unless you have the TV on and are watching the show, you will miss it. Similarly, if the client is not up, running and receiving messages from the topics, it will miss messages published to the topic. To get around this problem of missing messages, clients can start a durable subscription. This is like having a VCR record a show you cannot watch at its scheduled time so that you can see what you missed when you turn your TV back on.