Database schema evolution through the specification and maintenance of changes on entities and relationships

Abstract

An information system usually consists of complex application programs structured on top of a database whose schema can change in order to facilitate the needs of new application programs. It is neither easy nor practical to modify the application programs when the database schema is changed. In order to facilitate the needs of new application programs, and to continue supporting existing applications, the database system needs to be able to cope with changing database schemas and maintaining consistency among objects created under different schemas. This thesis presents an approach to schema evolution through changes to the Entity-Relationship (ER) schema of a database. In this ER-based approach, changes to a database schema are specified in terms of changes to the ER diagram describing the database. In order to extend ER diagrams for specifying the relationships between an old schema and a new one, corresponding to the one before and the one after change, respectively, a specification language called LEVER and its graphical representation called EVER (EVolutionary ER) diagram were developed. Methods used for mapping the change specifications into the relational and object-oriented data models, for organizing the databases after schema change and for constructing database views for schema versions are described. Through the reconstruction of views after database reorganization, changes to an ER diagram can be made transparent to the application programs while all objects in the database remain accessible to the application programs.