With PubSub, OPC UA Applications do not directly exchange requests and responses. Instead, Publishers send messages to a Message Oriented Middleware, without knowledge of what, if any, Subscribers there may be. Similarly, Subscribers express interest in specific types of data, and process messages that contain this data, without knowledge of what Publishers there are.
Figure B.47 illustrates that Publishers and Subscribers only interact with the Message Oriented Middleware which provides the means to forward the data to one or more receivers.
Figure B.47 – Publish Subscribe Model Overview
PubSub is used to communicate messages between different system components without these components having to know each other’s identity.
A Publisher is pre-configured with what data to send. There is no connection establishment between Publisher and Subscriber.
The identity of the Subscribers and the forwarding of published data to the Subscribers is the responsibility of the Message Oriented Middleware. The Publisher does not know or even care if there is one or many Subscribers. Effort and resource requirements for the Publisher are predictable and do not depend on the number of Subscribers.