Message Queue Telemetry Transport (MQTT)
Introduction
MQTT (originally an initialism of MQ Telemetry Transport) is a lightweight, publish-subscribe, machine to machine network protocol for message queue/message queuing service. It is designed for connections with remote locations that have devices with resource constraints or limited network bandwidth, such as in the Internet of Things (IoT). It must run over a transport protocol that provides ordered, lossless, bi-directional connections—typically, TCP/IP, but also possibly over QUIC. It is an open OASIS standard and an ISO recommendation (ISO/IEC 20922).
Many objects, such as cars, kitchen appliances, thermostats, and watches, can be connected to the internet to provide an endless stream of data that needs to be processed. The HTTP protocol is not appropriate for handling this kind of data because it is not in a request-response format.
Basic Concepts
The MQTT protocol follows a publish-subscribe model in which a message is published to a topic and is immediately received by all of the subscribers to that topic. In other words, topics are used by the broker to send messages to all of the clients and to filter any message received by the clients.
Clients can publish data to the broker. Clients can also subscribe to topics and the broker will send data to them when the broker receives data about that topic. The broker is responsible for receiving all of the messages and determining which clients are subscribed to each message. The broker then sends the message to the subscribed clients.
Architecture
The architecture of MQTT is made of the publish and subscribe connections between the client and the broker. As you can see in the image below, the MQTT broker is at the center of the architecture, surrounded by various clients that can publish or subscribe to that broker.
graph LR
B(MQTT Broker)
P1(Client 1) -->|Publish| B
P2(Client 2) -->|Publish| B
P3(Client 3) -->|Publish| B
B -->|Subscribe| S1(Client 4)
B -->|Subscribe| S2(Client 5)Implementations of MQTT Protocol
Mosquitto
Note: See the notes on mosquitto for more details.
Eclipse Mosquitto is the broker that handles the messages passed by IoT devices to the MQTT protocol using a publish-subscribe model. See the notes on the publish-subscribe pattern for more details. Publishers send a message and subscribers receive the message via the broker. Messages are sent to message brokers, who are responsible for routing the messages to any clients that subscribed to the associated publishers.
Python
In Python the Paho library implements the MQTT protocol to connect to a MQTT broker. This includes publish and subscription connections.
Firebase
Note: For more details read about [Firebase][-firebase].
Firebase is an application platform, or PaaS that simplifies the development of backends to application services. This includes an implementation of the MQTT protocol for real time streaming of data.