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MQTT Engine must have a client connection to a broker to use the publish mechanism. This is configured here

If the MQTT Engine client connection is configured to use Primary Host ID, the client will include an MQTT Will message in its connect message with a topic in the format "spBv1.0/STATE/primary_host_id" and a payload {"online" : false, "timestamp" : 1668114759262}.
When the MQTT Engine client goes off-line, the broker will publish the MQTT Will message

on behalf of

MQTT Engine for MQTT clients subscribing on this topic.