MQTT Subscriber Sparkplug behaviour
This page lists how the MQTT Subscriber treats the Sparkplug B messages and metrics that it receives from a Message Queuing Telemetry Transport (MQTT) broker. Sparkplug B is an MQTT payload format for industrial data. For the settings that control the subscription, and for the paths where the tags appear, refer to MQTT Subscriber.
The subscriber applies data from both node-level and device-level publishers.
It never writes command or state messages to a tag, even under the default spBv1.0/# filter.
Messages the subscriber applies
| Message | What the subscriber does |
|---|---|
NBIRTH |
Starts the node session, which must begin at sequence 0, and makes the node's metrics known. Metrics from an earlier session that are missing from it become Stale. |
NDATA |
Writes the metric values to their tags. |
DBIRTH |
Makes the device's metrics known. Metrics of that device that are missing from it become Stale. |
DDATA |
Writes the device's metric values to their tags. |
NDEATH |
Marks every metric of that node Stale and ends the node session. |
DDEATH |
Marks every metric of that device Stale. |
The subscriber ignores NCMD, DCMD and STATE messages.
A broker disconnect marks all subscriber tags Stale.
Host state
While connected, the subscriber announces itself to the broker as online under its Host ID. It publishes an offline state when the subscriber stops. The state message is retained on the broker, so a Sparkplug-aware consumer can see the last known state.
Sequence gaps and metric death
When a message arrives out of sequence, the subscriber asks the edge node to republish its data. It does not apply the out-of-sequence values as current values.
When a metric dies, or a later announcement from the edge node omits a metric that was known before, the tag is kept and its quality is set to Stale. History and dashboard bindings against that tag path keep working and show the value as no longer live. Refer to Tags for how data quality is shown.
Data types and malformed payloads
The subscriber supports scalar metric types only: boolean, string, the integer widths, float, double, and DateTime. It skips dataset, template, bytes, and property-set values. It also skips a metric whose name is not yet known, because the edge node has not announced it. A payload that cannot be decoded is logged and dropped, and the subscriber stays connected.