MQTT (Message Queuing Telemetry Transport)

MQTT is a lightweight publish/subscribe messaging protocol designed for constrained devices and low-bandwidth, high-latency networks. It's the backbone of many IoT applications.

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Core Concepts

1. Publish/Subscribe Model

Unlike client-server (REST), MQTT uses an event-driven architecture:

Publisher → Broker → Subscriber

• Publisher sends messages to a topic
• Broker routes messages to subscribers
• Subscriber receives messages from topics they subscribed to

2. Topics

Topics are hierarchical strings separated by /:

home/livingroom/temperature
home/bedroom/light
sensors/+/motion     # + is a single-level wildcard
sensors/#            # # is multi-level wildcard

3. QoS (Quality of Service)

Level	Guarantee	Use Case
QoS 0	At most once	Weather sensors (occasional loss OK)
QoS 1	At least once	Billing data (no loss allowed)
QoS 2	Exactly once	Critical commands

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Architecture

flowchart LR
    subgraph "IoT Devices"
    A[Sensor] --> B[MQTT Broker]
    B --> C[Actuator]
    D[Mobile App] --> B
    E[Cloud Backend] --> B
    end
    
    B -.-> F[(Message<br/>Persistence)]

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Key Features

Clean Sessions

• Clean Session = true: Broker doesn't store subscriptions/messages for disconnected clients
• Clean Session = false: Broker stores messages until delivery

Retained Messages

Last message on a topic is kept by the broker. New subscribers immediately receive it.

Last Will & Testament

Client can set a message the broker sends if it disconnects unexpectedly.

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Common Use Cases

Industry	Application
Smart Home	Light, thermostat, door sensors
Industrial	Machine monitoring, predictive maintenance
Healthcare	Patient vital signs, equipment tracking
Agriculture	Soil moisture, greenhouse climate

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Example (JavaScript with MQTT.js)

const mqtt = require('mqtt');
const client = mqtt.connect('mqtt://broker.hivemq.com:1883');

// Subscribe to topic
client.subscribe('home/livingroom/temperature');

// Receive messages
client.on('message', (topic, message) => {
  console.log(`Temperature: ${message.toString()}°C`);
});

// Publish message
client.publish('home/livingroom/temperature', '24.5');

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Why MQTT for IoT?

Advantage	Why It Matters
Small overhead	Works on limited bandwidth
Three QoS levels	Flexible reliability
Pub/Sub decouples	Devices don't need to know each other
Last Will	Know when devices go offline
Retained messages	New subscribers get immediate state

It competes with AMQP (enterprise, complex), CoAP (constrained HTTP-like), and WebSockets (web integration).