Subscribers are the consumers of NetworkMessages from the Message Oriented Middleware. They may be OPC UA Clients, OPC UA Servers or applications that are neither Client nor Server but only understand the structure of OPC UA PubSub messages. Figure 7 illustrates a Subscriber with filtering, decoding and dispatching of NetworkMessages.
To determine for which DataSetMessages and on which Message Oriented Middleware to subscribe, the Subscriber need to be configured and/or use discovery mechanisms.
Subscribers shall be prepared to receive messages that they do not understand or are irrelevant. Each NetworkMessage provides unencrypted data in the NetworkMessage header to support identifying and filtering of relevant Publishers, DataSetMessages, DataSetClasses or other relevant message content (see 5.3).
If a NetworkMessage is signed or signed and encrypted, the Subscriber will need the proper security keys (see 5.3.5) to verify the signature and decrypt the relevant DataSetMessages.
Once a DataSetMessage has been selected as relevant, it will be forwarded to the corresponding DataSetReader for decoding into a DataSet. See 5.4.2.2 for further information about this DataSet reading process. The resulting DataSet is then further processed or dispatched in the Subscriber.
If the Subscriber is an OPC UA Server, it can expose the reader configuration in its AddressSpace. This information may be created through product-specific configuration tools or through the OPC UA defined configuration model. The OPC UA Information Model for PubSub configuration is specified in Clause 9.