With PubSub, OPC UA Applicationsdo not directly exchange requests and responses. Instead, Publisherssend messages to a Message Oriented Middleware, without knowledge of what, if any, Subscribersthere may be. Similarly, Subscribersexpress interest in specific types of data, and process messages that contain this data, without knowledge of what Publishersthere are.

Message Oriented Middlewareis software or hardware infrastructure supporting sending and receiving messages between distributed systems. It depends on the Message Oriented Middlewarehow this distribution is implemented.

To cover a large number of use cases, OPC UA PubSubsupports two largely different Message Oriented Middlewarevariants. These are:

PubSubis used to communicate messages between different system components without these components having to know each other’s identity.

A Publisheris pre-configured with what data to send. There is no connection establishment between Publisherand Subscriber.

The knowledge about who Subscribers are and the forwarding of published data to the Subscribersis off-loaded to the Message Oriented Middleware. The Publisherdoes not know or even care if there is one or many Subscribers. Effort and resource requirements for the Publisherare predictable and do not depend on the number of Subscribers.

Part 14describes the details of the OPC UA PubSubmodel.