The Publisher is the PubSub entity that sends NetworkMessages to a Message Oriented Middleware. It represents a certain information source, for example, a control device, a manufacturing process, a weather station, or a stock exchange.
Commonly, a Publisher is also an OPC UA Server. For the abstract PubSub concepts, however, it is an arbitrary entity and should not be assumed to be an individual or even a specific network node (an IP or a MAC address) or a specific application. A Publisher may consist of one or more network nodes sending messages and management node(s) responding to discovery probe messages and providing an OPC UA Server for configuration and diagnostics.
Figure 5 illustrates a Publisher with data collection, encoding and message sending.
Figure 5 – Publisher details
A single Publisher may support multiple PublishedDataSets and multiple DataSetWriters to one or more Message Oriented Middleware. A DataSetWriter is a logical component of a Publisher. See 18.104.22.168 for further information about the DataSet writing process.
If the Publisher is an OPC UA Server, it can expose the Publisher configuration in its AddressSpace. This information may be created through product-specific configuration tools or through the OPC UA defined Methods. The OPC UA Information Model for PubSub configuration is specified in Clause 9.
Figure 6 illustrates the process inside a Publisher when creating and sending messages and the parameters required to accomplish it. The components, like DataSet collection or DataSetWriter should be considered abstract. They may not exist in every Publisher as independent entities. However, comparable processes need to exist to generate the OPC UA PubSub messages.
Figure 6 – Publisher message sending sequence
The sending process is guided by different parameters for different logical steps. The parameters define for example when and how often to trigger the sending sequence and the encoding and security of the messages. The PubSub communication parameters are defined in Clause 6.
The first step is the collection of data (DataSet) to be published. The configuration for such a collection is called PublishedDataSet. The PublishedDataSet also defines the DataSetMetaData. Collection is a generic expression for various different options, like monitoring of Variables in an OPC UA Server AddressSpace, processing OPC UA Events, or for example reading data from network packets. In the end, the collection process produces values for the individual fields of a DataSet. The two concrete PublishedDataSet options with standard OPC UA configuration are PublishedDataItems for Variable base collection and PublishedEvents for Event based collection.
In the next step, a DataSetWriter takes the DataSet and creates a DataSetMessage. DataSetMessages from DataSetWriters in one WriterGroup can be inserted into a single NetworkMessage. The creation of a DataSetMessage is guided by the following parameters:
- The DataSetFieldContentMask (see 22.214.171.124) controls which attributes of a value shall be encoded.
- The DataSetMessageContentMask (see 126.96.36.199.2) controls which header fields shall be encoded.
- The KeyFrameCount (see 188.8.131.52) greater than or equal to 1 controls whether a key frame or a delta frame DataSetMessage is to be created. A KeyFrameCount of 0 is used for non-cyclic PublishedDataSets, like PublishedEvents. The resulting DataSetMessage is passed on to the next step together with the DataSetWriterId (see 184.108.40.206), the DataSetClassId (see 220.127.116.11), the ConfigurationVersion of the DataSetMetaData (see 18.104.22.168.5), and a list of values that match the configured propagated fields.
Next is the creation of the NetworkMessage. It uses the data provided from the previous step together with the PublisherId (see 22.214.171.124) defined on the PubSubConnection. The structure of this message is protocol specific. If the SecurityMode (see 126.96.36.199) requires message security, the SecurityGroupId (see 188.8.131.52) is used to fetch the SecurityPolicy and the security keys from the SKS (see 5.4.4). This information is used to encrypt and/or sign the NetworkMessage as required by the SecurityMode.
The final step is delivery of the NetworkMessage to the Message Oriented Middleware through the configured Address.