7.5 Schema Query

   Some clients may want to query the schema information in greater detail than is possible with the feature tests defined in the previous section. This specification aims to accommodate these clients without requiring every data provider to implement a full

   schema query facility1.

   If a data provider supports schema queries, they are performed via its ResourceQueryService interface in the same way as general population queries. The meta-model supporting schema query is derived from the RDF schema candidate recommendation [2].

   A data provider that supports schema query must, at a minimum, translate the following URI-references to ResourceIDs and implement the designated classes and properties in its ResourceQueryService.

   1. Experience with developing data providers indicates that the schema query capabilities add significant cost and complexity and may even dominate the overall implementation cost. The design rationale adopted here is that implementations should only pay for what they use.

   7-4 UMS Data Access Facility, v2.0.1 July 2005

   A proxy data provider is required when clients require schema query features but the ultimate data providers do not support the foregoing metadata. The proxy must supply the missing metadata, which may be available from a third data provider.