Learning Tai Chi Maker - mqtt Chapter II (VI) mqtt wills

Video link : https://www.bilibili.com/video/BV1B54y1t7j5/?spm_id_from=autoNext&vd_source=b91967c499b23106586d7aa35af46413

The data link ：http://www.taichi-maker.com/homepage/esp8266-nodemcu-iot/iot-tuttorial/mqtt-tutorial/mqtt-last-will/

When the client disconnects normally , Will send to the server DISCONNECT message , After the server receives the message , You know , The client is disconnected normally , Instead of accidentally disconnecting .

However , When the server does not receive DISCONNECT In the case of messages , Discover clients “ heartbeat ” Stopped. , At this time, the server knows that the client is disconnected accidentally .

Give another example through the client software ：

When connected normally ,

When you click Disconnect after , This is software （ client ） Will go to MQTT Server sends a DISCONNECT Data packets , In this case , The server knows that the client is disconnected normally . However , If I close the software directly or the computer shuts down accidentally , here , Software （ client ） I won't go to MQTT Server send DISCONNECT Data packets , That is to say, as long as the client does not MQTT Server send DISCONNECT Data packets , This is an accidental disconnection .

（ Only clients that have subscribed to the will can receive the will information when the published client accidentally disconnects .）

1、 How the client sends the will message to the server

Pass below MQTT.fx This software is used to operate how to use MQTT will ,

First , Click on the gear

After setting up , Click on OK , It's set up .

below , Let's open another one MQTT.fx Software , Subscribe to the topics published by the above client as a client , But there is a very important step , That's it

1、 Delete the contents of the will set before , Because it is subscribed as a client , So there is no need for these contents .

2、 Click regenerate Client ID, It's very important .

after , Click on OK, It's set up .

Now? , We accidentally disconnected the client that issued the will （ Close the software directly ）, See what happens .

If the will retention information is checked , What kind of result will happen ？

First open a software and set it ,

Open another software （ It is the same as the setting for opening another software mentioned above ）

after , We accidentally disconnected the client that issued the will （ Close the software directly ）, See what happens .

Will also receive information about wills .

Now? , Open another software （ The settings are the same , Mainly regenerate ClientID）, Then click to subscribe to the will topic .

You will find that you have received the will information immediately .

And if the connection was normally disconnected before ？

When the connection is normally disconnected, it will not send , When other clients subscribe to the will information , I will receive the reserved wills immediately , Instead of waiting until you accidentally disconnect .

Last , Let's talk about it MQTT Will operation suggestions

（ How to demonstrate , I'll introduce it later ）