Closed External Schemas in Object-Oriented Databases

Schema closure is a property to guarantee that classes in a schema have not references to classes that are not included into it (external references). The existing methodologies solve this point including referenced classes into the schema. In this paper, a new kind of schema closure (reduction closure) is put forward. Reduction closure is based on the transformation of classes with external references, removing these references to fulfil the schema closure property. The main benefit of reduction closure is that it can be used as a method to simplify the external schema definition process, because explicit definition of derived classes for hiding external references is avoided. In addition, the explicit definition of derived classes to update the references to modified classes is also avoided. This closure algorithm is part of an external schema generation system for ODMG databases that we are developing, which is also briefly described in this paper.