mqtt - 有什么方法可以为 MQTT Broker 中的消息提供 TTL

Question

在 MQTT 规范中，有控制服务器和客户端上的保留和状态的连接标志。

清理 session specification建议:

3.1.2.4 Clean Session Position: bit 1 of the Connect Flags byte.

This bit specifies the handling of the Session state. The Client and Server can store Session state to enable reliable messaging to continue across a sequence of Network Connections. This bit is used to control the lifetime of the Session state.

If CleanSession is set to 0, the Server MUST resume communications with the Client based on state from the current Session (as identified by the Client identifier). If there is no Session associated with the Client identifier the Server MUST create a new Session. The Client and Server MUST store the Session after the Client and Server are disconnected [MQTT-3.1.2-4].

After the disconnection of a Session that had CleanSession set to 0, the Server MUST store further QoS 1 and QoS 2 messages that match any subscriptions that the client had at the time of disconnection as part of the Session state

因此，每当客户端在断开连接后再次连接时，代理将中继所有未确认的 QoS-1 和 QoS-2 消息。所以，经纪人有一个状态。为了维护每个客户端的状态，服务器将使用一些数据库进行保留。有没有办法为这些消息状态(服务器将存储)提供生存时间。如何告诉代理丢弃(即不中继)任何早于的消息；比如说 5 天。

基本上，我们想使用 clean session = 0 和 QoS-1；但是如何用旧消息清除代理状态？不想收到早于 5 天的消息，即使未收到消息也是如此。

Answer 1

MQTT 5.0 允许为发布设置消息过期间隔。

https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901112

3.3.2.3.3 Message Expiry Interval

2 (0x02) Byte, Identifier of the Message Expiry Interval.

Followed by the Four Byte Integer representing the Message Expiry Interval.

If present, the Four Byte value is the lifetime of the Application Message in seconds. If the Message Expiry Interval has passed and the Server has not managed to start onward delivery to a matching subscriber, then it MUST delete the copy of the message for that subscriber [MQTT-3.3.2-5].

If absent, the Application Message does not expire.

The PUBLISH packet sent to a Client by the Server MUST contain a Message Expiry Interval set to the received value minus the time that the Application Message has been waiting in the Server [MQTT-3.3.2-6]. Refer to section 4.1 for details and limitations of stored state.