Thanks to the lightweight Publish/subscribe method MQTT is interesting for use in mobile applications, where energy-efficient and reliable data transfer is key. Publish/subscribe is an alternative to the traditional HTTP client-server model, where the client communicates directly with the endpoint.
The difference is that in the publish/subscribe model, the client does not extract data from the server, but he receives the messages relevant to the topic he’s subscribed to from the server. A reliable TCP connection is required for communication. If this is interrupted, the server can store the data in a data buffer and send it to clients when the connection is re-established.