MQTT (MQ Telemetry Transport or Message Queuing Telemetry Transport) is an ISO standard (ISO/IEC PRF 20922)[ publish-subscribe-based “lightweight” messaging protocol for use on top of the TCP/IP protocol. It is designed for connections with remote locations where a “small code footprint” is required or the network bandwidth is limited. The publish-subscribe messaging pattern requires a message broker. The broker is responsible for distributing messages to interested clients based on the topic of a message. Andy Stanford-Clark and Arlen Nipper of Cirrus Link Solutions authored the first version of the protocol in 1999.
The specification does not specify the meaning of “small code footprint” or the meaning of “limited network bandwidth”. Thus, the protocol’s availability for use depends on the context. In 2013, IBM submitted MQTT v3.1 to the OASIS specification body with a charter that ensured only minor changes to the specification could be accepted. MQTT-SN is a variation of the main protocol aimed at embedded devices on non-TCP/IP networks, such as ZigBee
MQTT defines methods (sometimes referred to as verbs) to indicate the desired action to be performed on the identified resource. What this resource represents, whether pre-existing data or data that is generated dynamically, depends on the implementation of the server. Often, the resource corresponds to a file or the output of an executable residing on the server.
Connect: Waits for a connection to be established with the server.
Disconnect: Waits for the MQTT client to finish any work it must do, and for the TCP/IP session to disconnect.
Subscribe: Waits for completion of the Subscribe or Unsubscribe method.
Unsubscribe: Requests the server unsubscribe the client from one or more topics.
Publish: Returns immediately to the application thread after passing the request to the MQTT client.