Schema Versioning and Class Hierarchy Modifications in Object-Oriented Databases

Schema Modification Management is the ability to perform changes to a database schema when the database may already be populated, and is particularly concerned about how schema changes propagate to instances to retain compatibility objects and their specification. In this paper we argue that the schema versioning approach, which allows for multiple versions of the schema to coexist, is preferable to the schema evolution approach, for which all database objects and application programs must conform with the most recent schema version. Furthermore, we argue that the granularity of change should be the complete schema level, in order to properly handle interclass modifications such as properties moved between classes and reorganization of class and aggregation hierarchies: These problems have considerable interest in OO programming language communities, however have received little attention in the OODB area.