In PubSub the participating OPC UA Applicationscan assume the roles Publisher and Subscriber. Publishersare the sources of data, while Subscribers consume that data. Communication in PubSubis message-based. 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 that supports sending and receiving messages between distributed systems. The implementation of this distribution depends on the Message Oriented Middleware.

Figure 1illustrates 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 1– Publish Subscribe model overview

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