OPC UA PubSub over MQTT

Unlike the client-server model requiring direct sessions, PubSub over MQTT introduces a message broker that completely decouples publishers from subscribers. A publisher sends a Network Message containing a DataSet to a configured MQTT topic. Any number of subscribers can then receive that data by subscribing to the topic, without the publisher needing any knowledge of their existence. This spatial and temporal decoupling eliminates point-to-point connection complexity and allows dynamic addition of consumers without reconfiguring the data source.

The standard encoding for OPC UA PubSub over MQTT is JSON, making payloads human-readable and directly consumable by cloud services and web applications. Each MQTT message carries a single DataSetMessage, which contains a header with metadata like the DataSetWriterId and timestamp, followed by a payload of key-value pairs representing the published data fields. This contrasts with the binary UA Binary encoding used in client-server or TSN profiles, prioritizing interoperability with IT systems over raw bandwidth efficiency.

OPC UA PubSub defines a standardized mapping of its logical concepts to the MQTT topic namespace. The topic structure typically follows a pattern that encodes the PublisherId and optionally the DataSetWriterId, allowing subscribers to filter messages using MQTT wildcards. For example, a topic like opcua/Factory1/Cell3/PressStatus enables hierarchical topic structures that mirror the physical or logical layout of a facility. This allows MQTT brokers to efficiently route messages and enables subscribers to selectively receive only relevant data streams.

Security is implemented by layering OPC UA's application-level security on top of MQTT's transport-level security. The connection between the OPC UA Publisher or Subscriber and the MQTT broker is secured using TLS, which provides encryption and broker authentication. OPC UA then adds message-level signing and encryption using its established Security Policies, ensuring end-to-end data integrity and confidentiality independent of the broker. This dual-layer approach means that even if the broker is compromised, the payload remains protected by OPC UA's proven security model.

OPC UA PubSub maps its Network Message concept directly to an MQTT PUBLISH packet. The specification leverages MQTT's Quality of Service (QoS) levels to control delivery guarantees. QoS 0 (at most once) is suitable for high-frequency, loss-tolerant telemetry. QoS 1 (at least once) ensures delivery for critical events like alarms but may introduce duplicates. The choice of QoS allows system architects to balance the trade-off between reliability and bandwidth for different classes of industrial data within the same broker infrastructure.

To enable true plug-and-produce interoperability, OPC UA PubSub over MQTT includes a mechanism for publishing DataSetMetaData to a dedicated topic. This metadata describes the structure, names, and data types of the fields within a DataSet, allowing a subscriber to dynamically interpret incoming payloads without prior out-of-band configuration. A subscriber can listen on a well-known metadata topic, receive the JSON-encoded description of a DataSet, and automatically configure its parsing logic. This is critical for maintaining semantic understanding in loosely coupled, large-scale systems.