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When the subscribing client connects to a server, it can publish its connection state on a topic that contains an ID identifying it as a critical applicationSparkplug Primary Host Application.

In the event that the subscribing client becomes disconnected from the server, the server will publish a Last Will and Testament (LWT) message on the same topic setting the state to offline. Any connecting client that subscribes on the state topic will then be notified of critical application Primary Host Application state and can key off this to do different things when that subscriber status changes. 

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Prior to the v3.0.0 release of the specification, the STATE message included a UTF-8 string payload that was either ONLINE or OFFLINE. Sparkplug™ B payloads are not used for encoding in this payload and this allowed criticalHost/backend application(s) to work across Sparkplug™ payload types.

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