MQTT

The Message Queuing Telemetry Transport (MQTT) is a lightweight, publish-subscribe network protocol that transports messages between devices. The protocol usually runs over TCP/IP; however, any network protocol that provides ordered, lossless, bi-directional connections can support MQTT. It is designed for connections with remote locations where a "small code footprint" is required or the network bandwidth is limited. The protocol is an open OASIS standard and an ISO recommendation (ISO/IEC 20922).
History.
Andy Stanford-Clark (IBM) and Arlen Nipper (then working for Eurotech, Inc.) authored the first version of the protocol in 1999. It was used to monitor oil pipelines within the SCADA framework. The goal was to have a protocol that is bandwidth-efficient, lightweight and uses little battery power, because the devices were connected via satellite link which, at that time, was extremely expensive.
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 (MQTT for Sensor Networks) is a variation of the main protocol aimed at battery-powered embedded devices on non-TCP/IP networks, such as Zigbee. After taking over maintenance of the standard from IBM version 3.1.1 with minor changes was released as an OASIS standard on October 29, 2014. A more substantial upgrade to MQTT version 5, adding several new features, was released on March 7, 2019.
Historically, the "MQ" in "MQTT" came from the IBM MQ (then 'MQSeries') MQ product line. However, the protocol provides publish-and-subscribe messaging (no queues, in spite of the name) and was specifically designed for resource-constrained devices and low bandwidth, high latency networks such as dial up lines and satellite links, for example.
In the specification opened by IBM as version 3.1 the formal name was "MQ Telemetry Protocol (MQTT)" but the abbreviation MQTT was used extensively in the text of the specification. In all documents released by OASIS the protocol is strictly referred to as MQTT, although the technical committee itself is named "OASIS Message Queuing Telemetry Transport Technical Committee".
Overview.
The MQTT protocol defines two types of network entities: a message broker and a number of clients. An MQTT broker is a server that receives all messages from the clients and then routes the messages to the appropriate destination clients. An MQTT client is any device (from a micro controller up to a fully-fledged server) that runs an MQTT library and connects to an MQTT broker over a network.
Information is organized in a hierarchy of topics. When a publisher has a new item of data to distribute, it sends a control message with the data to the connected broker. The broker then distributes the information to any clients that have subscribed to that topic. The publisher does not need to have any data on the number or locations of subscribers, and subscribers, in turn, do not have to be configured with any data about the publishers. 
If a broker receives a message on a topic for which there are no current subscribers, the broker discards the message unless the publisher of the message designated the message as a retained message. A retained message is a normal MQTT message with the retained flag set to true. The broker stores the last retained message and the corresponding QoS for the selected topic. Each client that subscribes to a topic pattern that matches the topic of the retained message receives the retained message immediately after they subscribe. The broker stores only one retained message per topic. This allows new subscribers to a topic to receive the most current value rather than waiting for the next update from a publisher. 
When a publishing client first connects to the broker, it can set up a default message to be sent to subscribers if the broker detects that the publishing client has unexpectedly disconnected from the broker. 
Clients only interact with a broker, but a system may contain several broker servers that exchange data based on their current subscribers' topics. 
A minimal MQTT control message can be as little as two bytes of data. A control message can carry nearly 256 megabytes of data if needed. There are fourteen defined message types used to connect and disconnect a client from a broker, to publish data, to acknowledge receipt of data, and to supervise the connection between client and server. 
MQTT relies on the TCP protocol for data transmission. A variant, MQTT-SN, is used over other transports such as UDP or Bluetooth. 
MQTT sends connection credentials in plain text format and does not include any measures for security or authentication. This can be provided by using TLS to encrypt and protect the transferred information against interception, modification or forgery.
The default unencrypted MQTT port is 1883. The encrypted port is 8883.
MQTT broker.
The MQTT broker is software running on a computer (running on-premises or in the cloud), and could be self-built or hosted by a third party. It is available in both open source and proprietary implementations.
The broker acts as a post office, MQTT doesn't use the address of the intended recipient but uses the subject line called “Topic”, and anyone who wants a copy of that message will subscribe to that topic. 
Multiple clients can receive the message from a single broker (one to many capability). Similarly, multiple publishers can publish topics to a single subscriber (many to one).
Each client can both produce and receive data by both publishing and subscribing, i.e. the devices can publish sensor data and still be able to receive the configuration information or control commands (MQTT is a bi-directional communication protocol). This helps in both sharing data, managing and controlling devices.
With MQTT broker architecture, the devices and application becomes decoupled and more secure. MQTT uses Transport Layer Security (TLS) encryption with user name, password protected connections, and optional certifications that requires clients to provide a certificate file that matches with the server’s. The clients are unaware of each other's IP address.
In case of a single source of failure, broker software and clients have an automatic handover to Redundant/automatic backup broker. The backup broker can also be set up to share the load of clients across multiple servers onsite, cloud, or the combination of both. 
The broker can support both standard MQTT and MQTT for compliant specifications such as Sparkplug, can be done with same server, same time and with same levels of security.
The broker can store the data in the form of retained messages (need to subscribe with database client) so that new subscribers to the topic can get the last value straight away.
The broker also keeps track of all the session’s information as the devices goes on and off called “persistent sessions”.
The main advantages of MQTT broker are:
Message types.
Connect.
Waits for a connection to be established with the server and creates a link between the nodes.
Disconnect.
Waits for the MQTT client to finish any work it must do, and for the TCP/IP session to disconnect.
Publish.
Returns immediately to the application thread after passing the request to the MQTT client.
MQTT v5.0.
In 2019, OASIS released the official MQTT 5.0 standard. Version 5.0 includes the following major new features:
Quality of service (QoS).
Each connection to the broker can specify a "quality of service" measure. These are classified in increasing order of overhead:
This field does not affect handling of the underlying TCP data transmissions; it is only used between MQTT senders and receivers.
Applications.
Several projects implement MQTT, for example: