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