Explain the working of MQTT

Explain the working of MQTT

Answer:-

• MQTT (Message Queue Telemetry Transport) is a lightweight publish-subscribe messaging protocol designed for constrained devices and networks.
• It is efficient, payload-agnostic, and reliable even over high-latency or low-bandwidth networks.
• Operates on the client-server model, where the server is referred to as the broker, and clients act as publishers or subscribers.

Working of MQTT

Illustration (Figure)

• Devices like refrigerators, thermometers, or lights act as publishers/subscribers.
• Topics (e.g., “Appliances,” “Lights”) allow communication through the broker, ensuring decoupled and streamlined messaging.

Operational Principles

1. Hierarchical Topics:
   • MQTT uses a hierarchy of topics to organize messages.
   • Publishers send messages tagged with specific topics, and subscribers retrieve messages for topics of interest.
2. Broker’s Role:
   • The broker manages communication by receiving messages from publishers and forwarding them to subscribers of the same topic.
   • If no subscriber exists for a topic, the broker discards the message unless specified otherwise by the publisher.
3. Last Will and Testament:
   • Publishers can configure a default message (Last Will and Testament) to be delivered by the broker if the publisher disconnects unexpectedly.
4. Communication:
   • MQTT operates over TCP for reliable data transfer.
   • Messages consist of a control message and a data message.
5. Message Types:
   MQTT supports 14 message types for various operations:
   • CONNECT/CONNACK: Connection initiation and acknowledgment between publisher/subscriber and broker.
   • PUBLISH/PUBACK: Message publishing and acknowledgment between publisher and broker or broker and subscriber.
   • PUBREC/PUBREL/PUBCOMP: Assured delivery components ensuring the successful reception, release, and completion of the publish operation.
   • SUBSCRIBE/SUBACK: Topic subscription request and acknowledgment from subscriber to broker.
   • UNSUBSCRIBE/UNSUBACK: Request to unsubscribe from a topic and acknowledgment of the unsubscription.
   • PINGREQ/PINGRESP: Keep-alive request and response messages to maintain an active connection.
   • DISCONNECT: Message signaling the disconnection of publisher/subscriber from the broker.

MQTT Message Delivery QoS

1. At most once:
   • Best-effort delivery.
   • No guarantees; messages may be lost or duplicated.
2. At least once:
   • Guarantees delivery but may lead to message duplication.
3. Exactly once:
   • Ensures message delivery without duplication.

Advantages

• Lightweight, making it ideal for IoT devices.
• Minimizes network traffic with a small fixed header (2 bytes).
• Efficient message delivery tailored for constrained environments.