DataSet

A DataSet is a collection of Variable Node values, aggregated from an OPC UA Server's Address Space. Instead of publishing individual value changes, the Publisher groups related data points—such as temperature, pressure, and speed from a single machine module—into one cohesive payload. This reduces message overhead and ensures data consistency by delivering a snapshot of correlated process variables simultaneously.

Every DataSet is described by DataSetMetaData, which provides the schema and semantic context required for a Subscriber to decode the payload. The metadata includes:

• Name and Description: Human-readable identifiers.
• Field Definitions: The name, data type, and engineering units for each value.
• Configuration Version: A version stamp that signals structural changes, ensuring Subscribers can detect and handle schema mismatches gracefully.

The logical DataSet is decoupled from its wire format. The same DataSet can be serialized into a Network Message using different encodings based on the transport profile:

• UA Binary: A compact, high-performance format for deterministic field-level communication (e.g., OPC UA FX over TSN).
• JSON: A human-readable format ideal for cloud integration and web-friendly transports (e.g., PubSub over MQTT). This allows a single data definition to serve both real-time control and enterprise analytics.

A DataSetWriter is the Publisher's internal component responsible for assembling and sending a specific DataSet. It manages the timing of data collection from the local Address Space and constructs the Network Message. Multiple DataSetWriters can exist within a single Publisher, each handling a distinct DataSet, allowing a single device to publish different data streams at independent rates.

To optimize bandwidth, DataSet messages can be sent as Key Frames or Delta Frames:

• Key Frame: Contains the complete, current value of every field in the DataSet. Essential for initial synchronization and recovery.
• Delta Frame: Contains only the fields whose values have changed since the last message. This drastically reduces payload size for high-frequency, sparsely changing data. Subscribers reconstruct the full DataSet by applying Delta Frames to the last received Key Frame.

For real-time control, a DataSet can be configured for cyclic publishing at a strict, deterministic interval. The DataSetWriter samples the configured Variables at a precise Sampling Offset and publishes the Network Message at a guaranteed Publishing Offset within the communication cycle. This time-aware behavior, fundamental to OPC UA FX over TSN, ensures bounded latency and jitter for controller-to-controller communication.