External Database Coexistence

JADE external database coexistence provides:

• Mapping technology to enrich relational schemas with object model semantics

• Tables (base, derived, or predefined views) mapped to subclasses of ExternalObject

• Columns mapped to attributes

• Foreign key definitions are used to determine inverse relationships

• Tuples (or rows) map to instances of external classes

• Each external database is represented by a subclass of ExternalDatabase and a singleton instance

• Object identifier to primary key mapping enables JADE to uniquely identify a tuple and map this to a proxy object in JADE

• Inverse relationships can be defined and implemented using references to class types

• Multiple-valued properties can be implemented with Dictionary, Set, or Array types

JADE can coexist with existing legacy or heritage relational databases in a seamless fashion. JADE provides the External Schema Wizard, to enable you to map your relational database to a JADE object model. (For details about using the External Schema Wizard, see Chapter 3, "Transforming an External Relational Database".)

You can map one or more external relational databases into a JADE object model, by using a Global Conceptual Schema (GCS). The GCS provides a high-level integrated view of external relational databases as part of an existing JADE schema, hiding structural differences between the JADE object model and the relational model, giving the appearance of accessing a single unified database. The relationships between tables in the relational database are mapped to bi-directional relationships using inverse references.

Access relational objects in an external database in a procedural manner, by using the JADE language rather than constructing Structured Query Language (SQL) statements. The External Schema Wizard enables you to map:

• Tuples (which may be the result of a project/join or a view) to objects

• SQL types to JADE primitive types

• Joins to bi-directional relationships based on foreign and primary keys

The mapping is specified declaratively rather than programmatically, to reduce coding requirements and to increase flexibility.

The schema mapping technology is supported by Open Database Connectivity (ODBC), which provides the gateway to access relational databases. ODBC is used during the schema transformation phase to access catalog information, and at run time to populate virtual object instances using SQL queries. As ODBC uses SQL as its database access language, a single application can access different database management systems with the same source code. The JADE query engine calls functions in the ODBC interface, which are implemented in drivers; that is, database-specific modules.