This Service Set is used to issue a Query to a Server. OPC UA Query is generic in that it provides an underlying storage mechanism independent Query capability that can be used to access a wide variety of OPC UA data stores and information management systems. OPC UA Query permits a Client to access data maintained by a Server without any knowledge of the logical schema used for internal storage of the data. Knowledge of the AddressSpace is sufficient.
An OPC UA Application is expected to use the OPC UA Query Services as part of an initialization process or an occasional information synchronization step. For example, OPC UA Query would be used for bulk data access of a persistent store to initialise an analysis application with the current state of a system configuration. A Query may also be used to initialise or populate data for a report.
A Query defines what instances of one or more TypeDefinitionNodes in the AddressSpace should supply a set of Attributes. Results returned by a Server are in the form of an array of QueryDataSets. The selected Attribute values in each QueryDataSet come from the definition of the selected TypeDefinitionNodes or related TypeDefinitionNodes and appear in results in the same order as the Attributes that were passed into the Query. Query also supports Node filtering on the basis of Attribute values, as well as relationships between TypeDefinitionNodes.
See Annex B for example queries.