When mqtt connects, the KeepAlive time is set, which method implements the heartbeat mechanism about mop HOT 3 CLOSED

Different MQTT client has different default value of the KeepAlive time. when the KeepAlive timeout, the client will send PingReq to the broker. The MoP server(proxy or broker) only needs to respond pongRes.
For MoP, both proxy and broker use Netty IdleStateHandler to monitor the MQTT client if or not timeout(KeepAlive * 1.5). If timeout, MoP server will close the client.

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When mqtt client sends connect command, the MoP server will process it add IdleStateHandler to that channel.
So if you like to see the implementation, please ref: MQTTProxyProtocolMethodProcessor#processConnect and DefaultProtocolMethodProcessorImpl#processConnect

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MQTTProxyProtocolMethodProcessor#processConnect Did not see the setting KeepAlive , DefaultProtocolMethodProcessorImpl#processConnect See that the initializeKeepAliveTimeout(channel, msg, clientId); method is set, what method do I want to implement, send heartbeat data to the client according to the heartbeat

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