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.

Figure B.47illustrates that Publishersand Subscribersonly interact with the Message Oriented Middlewarewhich provides the means to forward the data to one or more receivers.

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Figure B.47– Publish Subscribe Model Overview

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 identity of the Subscribers and the forwarding of published data to the Subscribersis the responsibility of 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.