QoS - Retain

Besides the QoS level a message can also have a retain flag set. By default the flag is set to false, but if true the server will store the message as the last good value for the topic.

Later on if a client subscribes to the same topic the last known good value will be sent back to the client. This way new subscribers will get the last message published even if the client was offlinewhen the publish took place.