The good people of HiveMQ are running a series of articles about MQTT on their blog.
- Part 1: Introducing MQTT
- Part 2: Publish & Subscribe
- Part 3: Client, broker, and connection establishment
- Part 4: MQTT publish, subscribe, and unsubscribe
- Part 5: MQTT topics and best practices
- Part 6: Quality of Service 0, 1 & 2
- Part 7: Persistent session and Queueing messages
- Part 8: Retained messages
- Part 9: Last Will and Testament
- Part 10: Keep Alive and Client Take-Over
Security Fundamentals
Good reading.