Fig.1 MQTT Work flow representation
The above Fig.1 represents how the MQTT broker works. The MQTT client connects to the MQTT broker with the subscribe command. All the information is organized in a hierarchy of topics. The client is subscribed to the particular topic name from where the messages have to be published. For instance, if user needs the temperature sensor value under the topic temp, then the client first subscribes to the topic temp. Then the broker fetches the value from the topic name and publishes it to the client.
MQTT relies on TCP/IP protocol for data transmission protocol. The client always interacts with the broker, but a system can have multiple broker servers that exchange data based on their current subscriber’s topic.