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Requesting information from the broker
A subscriber can request a retained publication on a specified topic from the broker. To do this, it uses the Request Update command message, which is described in Request Update. Applications usually do this if, when they registered with the broker, they asked to be sent publications on request only. If the broker has a retained publication for the topic specified, it is sent to the subscriber.
This command message can also be sent by a subscriber that did not register in this way, to request that the latest copy of a publication be sent to it. This might be necessary if a subscriber has already seen a publication, but has failed without saving it, and on restart wants to see it again.
This command message can be satisfied only by a retained publication at the broker (see State and event information). If the broker to which this message is sent has no retained publication for the topic specified, the request fails.
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