Transparent schema evolution (TSE) using object-oriented view technology: Transparency theory, methodology and system.

Abstract

To safely update industrially critical software interoperating on a shared Object-Oriented Database (OODB), it is important to keep the existing applications running while the shared OODB is modified to satisfy newly emerging data service requirements. The new requirements may be due to adding new applications, extending the functionality of existing applications, correcting initial design errors, etc. In this dissertation, we present the transparent schema evolution methodology (TSE) that addresses the problem by allowing one user to change the database schema without adversely impacting other users. The methodology achieves transparent evolution by constructing a database view over the shared OODB that reflects the intention of the schema change operation instead of modifying the existing schema in-place. To overcome the inherent limitation of view mechanisms not being able to augment the capacity of the database, the methodology is further refined for capacity-augmenting changes into the following three steps: (1) the underlying base schema is physically restructured to augment the database capacity; (2) a target view, which achieves the semantics of the schema change, is generated from the modified base schema; (3) the original base schema is reconstructed as a view over the modified base schema to preserve the interface required by existing users. To show the soundness of the TSE methodology, we first present a formal definition of the transparent schema changes. Then we show that if a schema change operation is achievable by the TSE methodology, then the operation is indeed performed transparently. To show the practical completeness, we demonstrate how the comprehensive set of both basic and hard schema change operations, such as transforming values into objects, can be handled by our TSE methodology. We have validated the feasibility of the TSE methodology by implementing schema change operations using object-oriented (OO) view technology. A standard OO view model has been defined and implemented on top of a commercial OODB GemStone. The view model defines the update semantics of views such that the views preserve the update semantics of the base schema. Such views are required in TSE to shield users from the fact that they are operating on views rather than base schemas.