How does MQTT's publish/subscribe pattern work?

MQTT's publish/subscribe pattern is a messaging architecture that decouples message producers and consumers, enabling flexible one-to-many communication.

Core Concepts

1. Topic

• Definition: A topic is the routing address for messages, using a hierarchical structure
• Format: String separated by slashes (/), such as home/livingroom/temperature
• Features:
  • Clear hierarchy for easy organization and management
  • Supports wildcard subscriptions
  • Case-sensitive
  • Length limit: maximum 65535 bytes

2. Publisher

• Role: Message producer
• Function: Sends messages to specific topics
• Features:
  • Doesn't need to know about subscribers
  • Can publish to multiple topics simultaneously
  • Returns immediately after publishing, doesn't wait for subscriber response

3. Subscriber

• Role: Message consumer
• Function: Subscribes to topics of interest and receives relevant messages
• Features:
  • Can subscribe to multiple topics
  • Can use wildcards to subscribe to a category of topics
  • Only receives messages published after subscription

4. Broker

• Role: Message relay and router
• Functions:
  • Receives messages from publishers
  • Distributes messages to subscribers based on subscription relationships
  • Manages client connections and sessions
  • Handles QoS guarantees for messages

Workflow

1. Connection Establishment: Clients (publishers/subscribers) connect to the Broker
2. Topic Subscription: Subscribers send subscription requests to the Broker
3. Message Publishing: Publishers send messages to specific topics
4. Message Routing: Broker receives messages and finds clients subscribed to that topic
5. Message Distribution: Broker forwards messages to all subscribers
6. Message Reception: Subscribers receive and process messages

Wildcard Subscriptions

Single-level Wildcard (+)

• Matches a single level
• Example: home/+/temperature matches home/livingroom/temperature, but not home/livingroom/kitchen/temperature

Multi-level Wildcard (#)

• Matches multiple levels, must be at the end of the topic
• Example: home/# matches all topics under home/

Advantages

1. Decoupling: Publishers and subscribers are completely decoupled, independent of each other
2. Flexibility: Supports one-to-many, many-to-one, and many-to-many communication patterns
3. Scalability: Easy to add new publishers and subscribers
4. Asynchrony: Publishers don't need to wait for subscriber responses
5. Efficiency: Broker handles message routing, reducing network overhead

Comparison with Point-to-Point Pattern

Feature	Publish/Subscribe	Point-to-Point
Coupling	Low	High
Message Receivers	Multiple	One
Message Persistence	Optional	Usually required
Complexity	Medium	Simple
Use Cases	Broadcasting, Notifications	Direct Communication

MQTT's publish/subscribe pattern makes it an ideal choice for IoT, real-time communication, and message push scenarios.