Schema Transformation Processors For Federated Objectbases

In contrast to three schema levels in centralized objectbases, a reference architecture for federated objectbase systems proposes five levels of schemata. This paper investigates the fundamental mechanisms to be provided by an object model to realize the processors transforming between these levels. The first process is schema extension, which gives the possibility to define views. The second process is schema filtering, that allows to define subschemata. The third one, schema composition, brings together several (so far unrelated) objectbases. It is shown, how composition and extension are used for stepwise bottom-up integration of existing objectbases into a federation; and how extension and filtering support authorization on different levels in a federation. A powerful View definition mechanism and the possibility to define subschemas (i.e., parts of a schema) are the key mechanisms used in these processes. Keywords: Object-oriented database systems, federated objectbases, five-le..