What Are Retained Messages?
If you know MQTT even for just a little bit, you may already know that for each MQTT message, there is a topic name, and there is the payload. If you dig a little deeper, you’ll find that there are also message properties and flags. One of the flags is called Retain, which is what this post is about.
Upon receiving a message with the Retain flag set, the MQTT broker must store the message for the topic to which the message was published, and it must store only the latest message. So the subscribers which are interested in this topic can go offline, and reconnect at any time to receive the latest message instead of having to wait for the next message from the publisher after the subscription.