Publish a Sparkplug B host application STATE message so edge nodes can detect when the SCADA/host application goes offline
domain: sparkplug.eclipse.org · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Have the host application publish a retained message to spBv1.0/STATE/<primary_host_id> on connect, with QoS 1
Set the payload to JSON of the form {"online": true, "timestamp": <epoch-millis>}
Use the MQTT Will message mechanism so the broker automatically publishes {"online": false, "timestamp": ...} to the same topic if the host disconnects ungracefully
Have edge nodes subscribe to the host's STATE topic to detect host availability before sending NBIRTH/DBIRTH messages
On graceful shutdown, have the host explicitly publish the offline payload before disconnecting
Known gotchas
this JSON payload format is specific to Sparkplug B v3.0 -- v2.2 deployments use a plain-text ONLINE/OFFLINE payload on a topic without the spBv1.0/ prefix, so mixed-version deployments can misinterpret each other's STATE messages
the STATE message must be retained so edge nodes connecting after the host published it still see the current state immediately
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