OPC UA PubSub

Unlike the classic client-server model, PubSub completely decouples data producers (Publishers) from data consumers (Subscribers). Publishers send data to a message-oriented middleware without knowing the identity or quantity of subscribers. This enables dynamic, scalable architectures where new analytics services can be added without reconfiguring PLCs or sensors. The decoupling is spatial, temporal, and synchronization-based, allowing systems to operate independently across different network topologies and lifecycles.

PubSub defines a transport-agnostic data model that can be mapped to multiple protocols:

• MQTT: Lightweight, ideal for constrained devices and cloud telemetry
• AMQP: Advanced queuing for enterprise message brokers
• UDP: For raw, low-latency multicast on local networks
• OPC UA Binary: For direct, broker-less communication This flexibility allows a single semantic data model to flow from the sensor to the cloud without protocol translation gateways.

PubSub supports two primary encoding formats:

• UADP (UA Datagram Protocol): A compact, high-performance binary format optimized for real-time industrial traffic with minimal overhead. It preserves full OPC UA type system fidelity.
• JSON: A human-readable, web-friendly format that enables direct consumption by cloud services, Node-RED flows, and REST-based analytics pipelines without proprietary parsers. This dual encoding strategy bridges the gap between OT determinism and IT interoperability.

Every published message is described by a DataSetMetaData structure that defines the fields, their data types, and semantic meaning. This self-describing payload allows subscribers to dynamically interpret data without prior out-of-band configuration. Combined with the PubSub Configuration Model, it enables automatic discovery of available data streams, supporting plug-and-produce scenarios where new equipment announces its data capabilities to the entire network upon connection.

PubSub inherits OPC UA's robust end-to-end security model. Messages can be signed and encrypted at the application layer, independent of the transport protocol's security. This ensures data integrity and confidentiality even when traversing untrusted networks or public MQTT brokers. The security model supports X.509 certificates, user authentication, and fine-grained authorization, allowing a single broker to securely multiplex data from multiple tenants or production lines.

For time-critical applications, PubSub supports deterministic, cyclic publishing with configurable intervals down to microseconds. The WriterGroup concept organizes publishers into synchronized groups that share a common timing model. Combined with TSN (Time-Sensitive Networking) at the Ethernet layer, PubSub can guarantee bounded latency and jitter for hard real-time control loops, making it suitable for motion control and safety-critical data distribution alongside cloud telemetry on the same wire.