What is the mechanism for receiving the message that the homeassistant terminal device is deleted
Please explain
the homeassistant terminal device is that i use mqtt add to homeassistant
when i delete it can my mqtt client get the delete message
when i delete it can my mqtt client get the delete message
Yes, just listen to the topic
which topic
Everything in MQTT is done with a topic
yes， but how to let homeassistant send message to my mqtt client when i deleted devcie on homeassistant
Ok, now I understand the question. You want a MQTT client to detect you deleted a device (or entity ?) in HA.
Question: any device/entity, or just the ones that were created using MQTT discovery ?
Yeah, that’s it
The ‘or’ in that sentence means you have to choose one or the other. 
device create using MQTT disvovery
device created using MQTT discovery,like this
{
"~": "homeassistant/light/moorgen",
"name": "moorgen",
"uniq_id": "moorgen",
"cmd_t": "~/set",
"stat_t": "~/state",
"availability":
[
{
"topic":"~/online",
"value_template": "{{ value_json.state }}"
}
],
"schema": "json",
"brightness": true,
"brightness_scale":100,
"retain":true,
"optimistic":false,
"device":
{
"name":"moorgen",
"model":"moorgen",
"manufacturer":"moorgen",
"identifiers":"moorgen"
}
}
HA deletes it by sending an empty message to the topic that created it.
i try it but not receive delete message
topic: homeassistant/light/moorgen/config
when deleted receive tcp message but not mqtt message
not delete by mqtt client， deleted by HA can send message to my mqtt client
You can easily test it. Install mqtt explorer (https://mqtt-explorer.com/) and delete a device create by mqtt discovery. When you delete it in HA, you will see that HA sends an empty message to the topic that created it, which deletes it from the broker.