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.
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 126.96.36.199) controls which attributes of a value shall be encoded.
- The DataSetMessageContentMask (see 188.8.131.52.2) controls which header fields shall be encoded.
- The KeyFrameCount (see 184.108.40.206) 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 220.127.116.11), the DataSetClassId (see 18.104.22.168), the ConfigurationVersion of the DataSetMetaData (see 22.214.171.124.5), 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 126.96.36.199) defined on the PubSubConnection. The structure of this message is protocol specific. If the SecurityMode (see 188.8.131.52) requires message security, the SecurityGroupId (see 184.108.40.206) 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.