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5.9 Query Service Set ToC Previous Next

5.9.2 Querying Views ToC Previous Next

A View is a subset of the AddressSpace available in the Server. See OPC 10000-5 for a description of the organisation of Views in the AddressSpace.

For any existing View, a Query may be used to return a subset of data from the View. When an application issues a Query against a View, only data defined by the View is returned. Data not included in the View but included in the original AddressSpace is not returned.

The Query Services supports access to current and historical data. The Service supports a Client querying a past version of the AddressSpace. Clients may specify a ViewVersion or a Timestamp in a Query to access past versions of the AddressSpace. OPC UA Query is complementary to Historical Access in that the former is used to Query an AddressSpace that existed at a time and the latter is used to Query for the value of Attributes over time. In this way, a Query can be used to retrieve a portion of a past AddressSpace so that Attribute value history may be accessed using Historical Access even if the Node is no longer in the current AddressSpace.

Servers that support Query are expected to be able to access the AddressSpace that is associated with the local Server and any Views that are available on the local Server. If a View or the AddressSpace also references a remote Server, query may be able to access the AddressSpace of the remote Server, but it is not required. If a Server does access a remote Server the access shall be accomplished using the user identity of the Client as described in 5.5.1.

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