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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 have 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 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.
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 6.2.3.2) controls which attributes of a value shall be encoded.
- The DataSetMessageContentMask (see 6.3.1.2.2) controls which header fields shall be encoded.
- The KeyFrameCount controls whether a key frame or a delta frame DataSetMessage is to be created.
The resulting DataSetMessage is passed on to the next step together with the DataSetWriterId (see 6.2.3.1), the DataSetClassId (see 6.2.2.2), the ConfigurationVersion of the DataSetMetaData (see 6.2.2.1.5), 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 6.2.6.1) defined on the WriterGroup. The structure of this message is protocol specific. If the SecurityMode (see 6.2.4.2) requires message security, the SecurityGroupId (see 6.2.4.3) is used to fetch the SecurityPolicy and the security keys from the SKS (see 5.4.3). 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.