How it works?

MQTT (Message Queuing Telemetry Transport) is a publish-subscribe messaging protocol that operates on the client-server model. It’s designed for efficient and lightweight communication, making it ideal for scenarios where low bandwidth, high latency, or unreliable network connections are common. Here’s a detailed explanation of how MQTT works:

1. MQTT Clients:

• MQTT communication involves two types of entities: MQTT clients and an MQTT broker.
• Clients can be devices (e.g., sensors, actuators, IoT devices) or applications that want to exchange messages.
• Each client has a unique client ID, which helps the broker identify and manage clients.

2. MQTT Broker:

• The MQTT broker is a server responsible for facilitating communication between clients.
• It receives messages from clients and ensures they are delivered to the appropriate destinations (subscribed clients).
• The broker maintains a list of active client connections and manages topics and subscriptions.

3. Topics:

• MQTT communication is organized around topics, which are strings used to categorize messages.
• Clients can publish messages to specific topics, and other clients can subscribe to these topics to receive messages.
• Topics are hierarchical and can have multiple levels separated by slashes, such as “home/living-room/temperature.”

4. Publishing Messages:

• A client that wants to send a message to one or more recipients publishes it to a specific topic on the MQTT broker.
• The message payload can be any data, such as sensor readings, status updates, or commands.
• When a message is published, the broker forwards it to all clients that have subscribed to the same topic.

5. Subscribing to Topics:

• Clients can subscribe to one or more topics of interest.
• When a client subscribes to a topic, it informs the broker about its interest in receiving messages published to that topic.
• The broker keeps track of subscriptions and delivers matching messages to subscribed clients.

6. Quality of Service (QoS):

• MQTT offers three QoS levels to control message delivery reliability:
  • QoS 0 (At Most Once): The message is sent once without acknowledgment. It might be delivered multiple times or not at all.
  • QoS 1 (At Least Once): The message is sent at least once and acknowledged by the recipient.
  • QoS 2 (Exactly Once): The message is sent exactly once and acknowledged by both sender and recipient.

7. Retained Messages:

• MQTT supports retained messages, where the broker holds the last message published on a topic.
• When a client subscribes to a topic with retained messages, it receives the most recent retained message immediately after subscribing.

8. Last Will and Testament (LWT):

• Clients can specify a “last will” message and a topic when connecting to the broker.
• If a client disconnects unexpectedly (e.g., due to a network outage), the broker publishes the “last will” message to the specified topic, allowing other clients to be informed of the client’s disconnection.

In summary, MQTT provides a flexible and efficient way for devices and applications to communicate in a decoupled and asynchronous manner. Clients publish messages to topics of interest, and the broker ensures the delivery of these messages to subscribed clients, all while offering various levels of message delivery assurance and support for retained messages and last will messages. MQTT’s lightweight design and versatility have made it a popular choice for IoT, M2M, and other applications with constrained resources or challenging network conditions.