客户端如何了解另一个客户端是否已连接 [英] Mqtt How a client can get to know that another client is connected or not

问题描述

大家好，我的问题是我需要保持设备(客户端)的状态，因为它们是否在线.

Hello guys my problem is that I need to keep device(clients) status as they are online or not.

我正在通过客户端ID进行连接，通过订阅该主题，我可以知道该设备已断开连接，但是如果它们重新连接，我如何检查它们是否再次连接.

I am connecting through a client id and from subscribing there will topic I can get to know that device is disconnected but if they connect again how can I check they connect again.

推荐答案

简短的答案是您不同意(在协议级别).

The short answer is you don't (at a protocol level).

发布者和订阅者完全不了解彼此，消息发送到主题而不是特定的订阅客户端.

Publishers and subscribers are totally unaware of each other, messages are sent to topics not to specific subscribing clients.

稍长的版本:

您可以对保留的消息和LWT(最后的遗嘱)进行某些操作

You could do something with retained messages and LWT (Last Will & Testament)

例如

每个客户端都将保留的消息发布到以下主题

Each client publishes a retained message to the following topic

online/<clientID>

当连接时有效负载为true.如果客户端由于网络故障而断开连接，它还将设置LWT，该LWT将发布false的有效负载.如果客户端完全断开连接，则它需要在下线之前发布false有效负载，这是它的最后一个操作.

With a payload of true when it connects. It also sets up LWT that will publish a payload of false if the client is disconnected by a network fault. If the client cleanly disconnects then it will need to publish the false payload as it's last action before going offline.

要判断客户端是否在线，请订阅online/+的通配符主题，然后检查主题的最后一段以获取clientid和状态的有效负载.

To tell if client is online you subscribe to the wildcard topic of online/+ and then check the last segment of the topic for clientid and the payload for the state.

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