Subscribers are the consumers of NetworkMessagesfrom theMessage Oriented Middleware. They may be OPC UA Clients, OPC UA Serversor applications that are neither Clientnor Serverbut only understand the structure of OPC UA PubSubmessages. Figure 7illustrates a Subscriberwith filtering, decoding and dispatching of NetworkMessages.


Figure 7– Subscriber details

To determine for which DataSetMessages and on which Message Oriented Middlewareto subscribe, the Subscriberneed to be configured and/or use discovery mechanisms.

Subscribers shall be prepared to receive messages that they do not understand or are irrelevant. Each NetworkMessageprovides unencrypted data in the NetworkMessageheader to support identifying and filtering of relevant Publishers, DataSetMessages, DataSetClassesor other relevant message content (see 5.3).

If a NetworkMessageis signed or signed and encrypted, the Subscriberwill need the proper security keys (see 5.3.5) to verify the signature and decrypt the relevant DataSetMessages.

Once a DataSetMessagehas been selected as relevant, it will be forwarded to the corresponding DataSetReaderfor decoding into a DataSet. See further information about this DataSetreading process. The resulting DataSetis then further processed or dispatched in the Subscriber.

If the Subscriberis 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 Modelfor PubSubconfiguration is specified in Clause 9.

Figure 8illustrates the process inside a Subscriberwhen receiving, decoding and interpreting messages and the parameter model required for accomplishing it. As for the Publisher, the components should be considered abstract.


Figure 8– Subscriber message reception sequence

The Subscriberneed to select the required Message Oriented Middlewareand establish a connection to it using the provided Address. Such a connection may simply be a multi-cast address when using OPC UA UDP or a connection to a message Brokerwhen using MQTT or AMQP. Once subscribed, the Subscriberwill start listening. The sequence starts when a NetworkMessageis received. The Subscribermay have configured filters (like a PublisherId, DataSetWriterIdor a DataSetClassId) so that it can drop all messages that do not match the filter.

Once a NetworkMessagehas been accepted, it is decrypted and decoded. The security parameters are the same as for the Publisher.

Each DataSetMessageof interest is passed on to a DataSetReader. Here, the DataSetMetaDatais used to decode the DataSetMessagecontent to a DataSet. The DataSetMetaDatain particular provides the complete field syntax including the name, data type, and other relevant Propertieslike engineering units. Version information that exists in both the DataSetMessageand theDataSetMetaDataallows the Subscriber to detect version changes. If a major change occurs, theSubscriberneeds to get an updated DataSetMetaData.

Any further processing is application-specific. For example, an additional dispatching step may map the received values to Nodesin the SubscribersOPC UA AddressSpace. The configuration for such a dispatching is called SubscribedDataSet. The two concrete SubscribedDataSetoptions with standard OPC UA configuration are TargetVariablesand SubscribedDataSetMirror. The configuration of TargetVariablesallows the dispatching of DataSetMessagefields to existing Variablesin the SubscribersOPC UA AddressSpace. The configuration of SubscribedDataSetMirroris used if the received DataSetfields should be represented as Variablesin the SubscribersOPC UA AddressSpacebut the Variablesdo not exist and must be created as part of the Subscriberconfiguration.