Mosquitto 如何訂閱 bridge 連線狀態?

一般來說如果要得知 mqtt client 是不是離線了,可以透過 will message 來設定。但是 mosquitto 的貼心功能 bridge 要怎麼實現這個功能呢?

本來看使用說明 /$SYS 這部分沒找到,原來藏在 mosquitto.conf 的說明文檔中…

notifications [ true | false ]

If set to true, publish notification messages to the local and remote brokers giving information about the state of the bridge connection. Retained messages are published to the topic $SYS/broker/connection/<clientid>/state unless otherwise set with notification_topics. If the message is 1 then the connection is active, or 0 if the connection has failed. Defaults to true.

notification_topic topic

Choose the topic on which notifications will be published for this bridge. If not set the messages will be sent on the topic $SYS/broker/connection/<clientid>/state.

很明顯可以看出只要訂閱 $SYS/broker/connection/<clientid>/state 即可,上線會傳 0 反之就是 1

如此一來就不用自己做 keepalive 功能啦!

$SYS/broker 詳細說明: