In PubSub the participating OPC UA Applications can assume the roles Publisher and Subscriber. Publishers are the sources of data, while Subscribers consume that data. Communication in PubSub is message-based. 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.

Message Oriented Middleware is 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 1 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 1 – Publish Subscribe model overview

To cover a large number of use cases, OPC UA PubSub supports two largely different Message Oriented Middleware variants: