Learn from Taiji Maker - mqtt Chapter 2 (I) QoS service quality level

Video link ：https://www.bilibili.com/video/BV1FK4y1L7TQ/?spm_id_from=333.788.recommend_more_video.-1&vd_source=b91967c499b23106586d7aa35af46413
The data link ：http://www.taichi-maker.com/homepage/esp8266-nodemcu-iot/iot-tuttorial/mqtt-tutorial/mqtt-qos/

Why develop multiple quality of service levels , Instead of directly adopting a ?

answer ： This is determined by the attributes of the Internet of things , The bandwidth of the Internet of things is very valuable in some cases , And sometimes it is very unstable . Considering these characteristics ,MQTT The agreement sets multiple quality of service levels . Because the service bandwidth occupied by the lower QoS level is also the lowest , The transmission stability is also the lowest . contrary , For messages with high quality of service , The occupied service bandwidth is also relatively high , The transmission stability is also relatively high .

summary ：
If you want to achieve QoS = 1/2 Communication for , The following three steps are required .

First step , When the client publishes a message ,QoS Set to 1/2 ,
The second step , When the client connects to the server , You have to cleanSession Set to false,
The third step , When the client subscribes to a topic , Subscribe to the SUBSCRIBE In the packet QoS Must be set to 1/2.

These three steps are critical , Be short of one cannot .

summary ： The lowest level shall prevail .