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A DataSet can be thought of as a list of name and value pairs representing an Event or a list of Variable Values.
A DataSet can be created from an Event or from a sample of Variable Values. The configuration of this application-data collector is called PublishedDataSet. DataSet fields can be defined to represent any information, for example, they could be internal Variables in the Publisher, Events from the Publisher or collected by the Publisher, network data, or data from sub-devices.
DataSetMetaData described in 5.2.3 defines the structure and content of a DataSet.
For publishing, a DataSet will be encoded into a DataSetMessage. One or more DataSetMessages are combined to form the payload of a NetworkMessage.
Figure 3 illustrates the use of DataSets for publishing.
Figure 3 – DataSet in the process of publishing
A PublishedDataSet is similar to either an Event MonitoredItem or a list of data MonitoredItems in the Client Server Subscription model. Similar to an Event MonitoredItem, a PublishedDataSet can select a list of Event fields. Similar to data MonitoredItems, the PublishedDataSet can contain a list of Variables.
A DataSet does not define the mechanism to encode, secure and transport it. A DataSetWriter handles the creation of a DataSetMessage for a DataSet. The DataSetWriter contains settings for the encoding and transport of a DataSetMessage. Most of these settings depend on the selected Message Oriented Middleware.
The configuration of DataSets and the way the data is obtained for publishing can be configured using the PubSub configuration model defined in clause 8.2 or with vendor specific configuration tools.
DataSets can be individual for a Publisher or they can be derived from a DataSetClass. Such a DataSetClass acts as template declaring the content of a DataSet. The DataSetClass is identified by a globally unique id – the DataSetClassId (see 6.2.2.2).
The DataSetMetaData is identical for all PublishedDataSets that are configured based on this DataSetClass. The DataSetClassId shall be in the corresponding field of the DataSetMetaData.
When all DataSetMessages of a NetworkMessage are created from DataSets that are instances of the same DataSetClass, the DataSetClassId of this class can be provided in the NetworkMessage header.
DataSetMetaData describes the content and semantic of a DataSet. The structure description includes overall DataSet attributes (e.g. name and version) and a set of fields with their name and data type. The order of the fields in the DataSetMetaData shall match the order of values in the published DataSetMessages.
The DataSetMetaDataType is defined in 6.2.2.1.2.
Example description (simplified, in pseudo-language):
Name: “Temperature-Sensor Measurement”
Fields: [1] Name=DeviceName, Type=String
[2] Name=Temperature, Type=Float, Unit=Celsius, Range={1,100}
Subscribers use the DataSetMetaData for decoding the values of a DataSetMessage to a DataSet. Subscribers may use name and data type for further processing or display of the published data.
Each DataSetMessage also includes the version of the DataSetMetaData that it complies with. This allows Subscribers to verify if they have the corresponding DataSetMetaData. The related ConfigurationVersionDataType is defined in 6.2.2.1.5.
DataSetMetaData may be specific to a single PublishedDataSet or identical for all PublishedDataSets that are configured based on a DataSetClass (see 5.2.2).
There are multiple options for Subscribers to get the initial DataSetMetaData:
- The Subscriber is an OPC UA Client and is able to get the necessary configuration information from the PubSub configuration model (see 9.1.4.2.1) provided by the Publisher, from a configuration server or from a directory server.
- The Subscriber supports the OPC UA configuration Methods defined in the PubSub configuration model.
- The Subscriber receives the DataSetMetaData as NetworkMessage from the Publisher. This may require an option for the Subscriber to request this NetworkMessage from the Publisher.
- The Subscriber is configured with product specific configuration means.
There are multiple options to exchange the DataSetMetaData between Publisher and Subscriber if the configuration changes.
- The DataSetMetaData is sent as a NetworkMessage from the Publisher to the Subscriber before DataSetMessages with changed content are sent. The used Message Oriented Middleware should ensure reliable delivery of the message. The mapping for the Message Oriented Middleware defines a way for the Subscriber to request the DataSetMetaData. The Subscriber goes to an error state if it has not received the new DataSetMetaData that matches the ConfigurationVersion of the received DataSetMessage.
- The Subscriber is automatically updated via the OPC UA configuration Methods defined in the PubSub configuration model when the DataSet in the Publisher is updated.
- The Subscriber is an OPC UA Client and is able to obtain the update from the Publisher or a configuration server via the information exposed by the PubSub configuration model.
- The Subscriber is updated with product specific configuration means when the DataSet in the Publisher is changed.