Maintaining Database Consistency in the Presence of Schema Evolution

With rapid advances in computer network technology and the increasing interest in global information sharing, grows the need for facilities that can eeectively maintain the database consistency and program compatibility as the schema evolves. In this paper, we present a general framework based on versions of schema for supporting seamless schema evolution in large-scale object-oriented software systems. We argue that the eeectiveness of using the schema version model to materialize schema evolution relies not only on the management of version derivation of schema, but also on the ability to maintain consistency of the database instances with the new schema versions, and the exibility of sharing instance access scopes among versions of schema. Semantics of schema versioning is studied with the objective to facilitate instance adaptation and program compatibility in the presence of schema updates. A number of options for sharing of instance access scopes among versions of a schema is developed. Applications may derive versions of schema upon requests for schema updates, and deene the instance access scope for each new version of schema by either creating their own instance access scope or inheriting the instance access scope of its ancestor schema versions in terms of a selection of options. The signiicance of our approach is the abilities for maintaining database consistency, in the presence of schema modiication, without irreversibly changing the objects that exist before the schema modiication, and the facilities that allow multi-users and applications to create and manipulate diierent collections of objects under diierent versions of schema. As a consequence, many organizational investments of the existing customer set, such as application design and development, can remain operational in the presence of schema changes.