When a publisher sends a message on a topic to a broker and no subscriber is listening on that topic, then that particular message will be discarded. Cases where the publisher publishes messages upon state changes such as temperature sensor sends value only when temperature changes then any new subscriber which subscribes to get temperature of this sensor will not get the current value until next change takes place. This can be prevented by sending setting retained flags as true, when the publisher connects for the first time to the broker.