MQTT
Message Queuing Telemetry Transport
MQTT Compared to HTTP
Both HTTP and MQTT are based on TCP/IP
HTTP uses Request/Response (1 to 1)
MQTT uses Publish/Subscribe pattern (1-to-1 or 1-to-many)
HTTP is document centric, MQTT is data centric
HTTP is more complex than MQTT which is simple
MQTT message size is smaller, with only a 2 byte header
MQTT offers 3 Quality of Service (QoS) settings, with HTTP all messages get same level of service.
Quality of service (QoS)
- 0 = 最多一次傳送（只負責傳送，發送過後就不管數據的傳送情況）
- 1 = 至少一次傳送（確認數據交付）
- 2 = 正好一次傳送（保證數據交付成功）
MQTT Message Type
| 名稱 | 代表值 | 說明 |
|---|---|---|
| Reserved | 0 | 保留之後使用 |
| CONNECT | 1 | 發出連線請求 |
| CONNACK | 2 | 連線請求的ACK（確認符） |
| PUBLISH | 3 | 發佈訊息 |
| PUBACK | 4 | 發佈訊息ACK (QoS 1) |
| PUBREC | 5 | 已收到發佈訊息 (QoS 2) |
| PUBREL | 6 | 釋放發布訊息 (QoS 2) |
| PUBCOMP | 7 | 訊息發佈完成 (QoS 2) |
| SUBSCRIBE | 8 | 發出訂閱請求 |
| SUBACK | 9 | 請求訂閱的ACK |
| UNSUBSCRIBE | 10 | 發出取消訂閱請求 |
| UNSUBACK | 11 | 請求取消訂閱的ACK |
| PINGREQ | 12 | Ping請求 |
| PINGRESP | 13 | Ping回應 |
| DISCONNECT | 14 | 請求斷線 |
| Reserved | 15 | 保留之後使用 |
Comparison of MQTT Implementations
https://en.wikipedia.org/wiki/Comparison_of_MQTT_Implementations
MQTT 5.0 new feature
http://docs.oasis-open.org/mqtt/mqtt/v5.0/cs02/mqtt-v5.0-cs02.html#_Toc514345565