2.10. Service

A virtual database (VDB) provides a unified view of data residing in multiple physical repositories. A VDB is composed of various data models and configuration information that describes which data sources are to be integrated and how. In particular, source models are used to represent the structure and characteristics of the physical data sources, and view models represent the structure and characteristics of the integrated data exposed to applications.

The access layer is the interface through which applications submit queries (relational, XML, XQuery and procedural) to the VDB via JDBC, ODBC or Web services.

When applications submit queries to a VDB via the access layer, the query engine produces an optimized query plan to provide efficient access to the required physical data sources as determined by the SQL criteria and the mappings between source and view models in the VDB. This query plan dictates processing order to ensure physical data sources are accessed in the most efficient manner.

Translators and resource adapters are used to provide transparent connectivity between the query engine and the physical data sources. A translator is used to convert queries into source-specific commands, and a resource adapter provides communication with the source.