Improvements of MQTT Retain Message Storage Mechanism

A287 to the disadvantage that MQTT agreement only saving one latest retain message which may result in missed and duplicate messages, it redesigned SUBSCRIBE and PUBLISH control packet format of MQTT, and realized new retain messages storage model in server and sending and receiving mechanism of retain messages. The experimental results showed that when publisher frequently publishes messages and subscriber disconnect, the subscriber will receive all the missed messages after being reconnected under new mechanism, while only the last missed message was received under old mechanism. When subscribers disconnect frequently and the server holds the retain message, subscribers received 0% repetitive messages under new mechanism while 100% repetitive messages was received under old mechanism. It proves that the new retain message storage mechanism can effectively prevent missing messages and repeatedly receiving messages.