协议交互 - MQTT 教程

1. 连接建立流程

sequenceDiagram participant C as Client participant B as Broker C->>B: CONNECT
(clientId, username, password,
will, cleanSession) B-->>C: CONNACK
(returnCode: 0=成功) Note over C,B: 连接建立完成

# CONNECT 报文示例
Packet Type: 0001 (CONNECT)
Flags: 0000
Remaining Length: variable

Payload:
  - Protocol Name: "MQTT"
  - Protocol Level: 4 (MQTT 3.1.1) or 5 (MQTT 5.0)
  - Connect Flags:
    * Clean Session: 1
    * Will Flag: 0
    * Will QoS: 00
    * Will Retain: 0
    * Password Flag: 1
    * Username Flag: 1
  - Keep Alive: 60 秒
  - Client ID: "my-client-001"
  - Username: "user123"
  - Password: "pass456"

2. QoS 0 发布流程

sequenceDiagram participant Pub as Publisher participant B as Broker participant Sub as Subscriber Pub->>B: PUBLISH (QoS 0)
topic: "sensors/temp"
payload: "25.5°C" B->>Sub: PUBLISH (QoS 0)
topic: "sensors/temp" Note over Pub,Sub: 无确认，消息可能丢失

📌 QoS 0 特点

最多一次：消息最多传递一次，不保证送达
无确认机制：发送后不等待任何响应
最低开销：只有一个 PUBLISH 报文
适用场景：传感器数据频繁上报，允许少量丢失

3. 发布/订阅流程（QoS 1）

sequenceDiagram participant Pub as Publisher participant B as Broker participant Sub as Subscriber Sub->>B: SUBSCRIBE
(topic: "sensors/temp", QoS: 1) B-->>Sub: SUBACK
(Message ID: 1, Return QoS: 1) Note over Pub,Sub: 订阅完成，开始发布消息 Pub->>B: PUBLISH
(topic: "sensors/temp"
payload: "25.5°C") B-->>Pub: PUBACK
(Message ID: 100) B->>Sub: PUBLISH
(delivered to subscriber) Sub-->>B: PUBACK
(acknowledge)

4. QoS 2 完整握手流程

sequenceDiagram participant Pub as Publisher participant B as Broker participant Sub as Subscriber Pub->>B: PUBLISH (QoS 2)
Message ID: 123 B-->>Pub: PUBREC
Message ID: 123 Pub->>B: PUBREL
Message ID: 123 B-->>Pub: PUBCOMP
Message ID: 123 Note over B,Sub: Broker 将消息分发给订阅者 B->>Sub: PUBLISH (QoS 2) Sub-->>B: PUBREC B->>Sub: PUBREL Sub-->>B: PUBCOMP

📌 QoS 2 四次握手说明

PUBLISH: 发布者发送消息
PUBREC: Broker 确认收到（Publish Received）
PUBREL: 发布者释放消息（Publish Release）
PUBCOMP: Broker 确认完成（Publish Complete）

通过 4 次握手保证消息"恰好送达一次"，既不丢，也不重复

5. 遗嘱消息（Last Will）

sequenceDiagram participant C as Client participant B as Broker participant S as Subscribers C->>B: CONNECT
Will Topic: "device/status"
Will Payload: "offline"
Will Retain: true B-->>C: CONNACK
连接成功 Note over C,B: 客户端正常在线 C--xB: ⚠️ 异常断开 Note over B: 检测到连接丢失 B->>S: PUBLISH (Will)
topic: "device/status"
payload: "offline"
(Retained)

6. 心跳保活机制

sequenceDiagram participant C as Client participant B as Broker C->>B: CONNECT
Keep Alive: 60s B-->>C: CONNACK loop 每 < 60 秒 C->>B: PINGREQ B-->>C: PINGRESP end Note over C,B: 如果 1.5 倍 Keep Alive 时间
内无消息，Broker 断开连接

⚠️ 保活超时

如果 Broker 在 1.5 倍 Keep Alive 时间内未收到任何消息，将断开连接并触发遗嘱消息（如果已设置）。

7. 断开连接

sequenceDiagram participant C as Client participant B as Broker C->>B: DISCONNECT
优雅断开 B-->>C: 连接关闭 Note over B: 不触发遗嘱消息

8. 完整会话流程

sequenceDiagram participant Pub as Publisher participant B as Broker participant Sub as Subscriber Pub->>B: CONNECT B-->>Pub: CONNACK Sub->>B: CONNECT B-->>Sub: CONNACK Sub->>B: SUBSCRIBE
(topic: "home/#") B-->>Sub: SUBACK loop 消息发布 Pub->>B: PUBLISH
(topic: "home/temp") B-->>Pub: PUBACK B->>Sub: PUBLISH Sub-->>B: PUBACK end Pub->>B: DISCONNECT Sub->>B: DISCONNECT

MQTT 协议交互流程