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>/stateunless 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！
詳細說明：http://mosquitto.org/man/mosquitto-conf-5.html
$SYS/broker 詳細說明：http://mosquitto.org/man/mosquitto-8.html