Clause 5 describes the general OPC UA PubSub concepts.
The DataSet constitutes the payload of messages provided by the Publisher and consumed by the Subscriber. The DataSet is described in 5.2. The mapping to messages is described in 5.3. The participating entities like Publisher and Subscriber are described in 5.4.
The abstract communication parameters are described in Clause 6.
The mapping of this model to concrete message and transport protocol mappings is defined in Clause 7.
The OPC UA Information Model for PubSub configuration in Clause 9 specifies the standard Objects in an OPC UA AddressSpace used to create, modify and expose an OPC UA PubSub configuration.
Figure 2 provides an overview of the Publisher and Subscriber entities. It illustrates the flow of messages from a Publisher to one or more Subscribers. The PubSub communication model supports many other scenarios; for example, a Publisher may send a DataSet to multiple Message Oriented Middleware and a Subscriber may receive messages from multiple Publishers.
Figure 2 – Publisher and Subscriber entities
Publishers and Subscribers are loosely coupled. They often will not even know each other. Their primary relation is the shared understanding of specific types of data (DataSets), the publish characteristics of messages that include these data, and the Message Oriented Middleware.
The “messages” in Figure 2 represent NetworkMessages. Each NetworkMessage includes header information (e.g. identification and security data) and one or more DataSetMessages (the payload). The DataSetMessages may be signed and encrypted in accordance with the configured message security. A Security Key Server is responsible for the distribution of the security keys needed for message security.
Each DataSetMessage is created from a DataSet. A component of a Publisher called DataSetWriter generates a continuous sequence of DataSetMessages. Syntax and semantics of DataSets are described by DataSetMetaData. The selection of information for a DataSet in the Publisher and the data acquisition parameters are called PublishedDataSet. DataSet, DataSetMetaData and PublishedDataSet are detailed in 5.2.
Publishers and Subscribers are typically configured through a configuration tool. The configuration can be done through a generic OPC UA PubSub configuration tool using the PubSub configuration Information Model defined in Clause 9 or through product-specific configuration tools. To support the PubSub configuration Information Model, Publishers and Subscribers must be also OPC UA Server.
NOTE The PubSub directory is an optional entity that allows Publishers to advertise their PublishedDataSets and their communication parameters. This directory functionality is planned for a future version of this document.