Unified Class Evolution by Object-Oriented Views

Object-oriented databases are said to support evolution and incremental development. On the schema level, a firm restriction in this evolution is that it can only be done by evolving class hierarchies downwards by subclassing. We show a unified approach to class evolution in object-oriented databases, where class hierarchies are allowed to grow in all directions, covering for evolution situations like generalisation, specialisation, and class versioning. We show how to make the evolution transparent, allowing existing and new clients to coexist and be clients of the same (existing and new) objects. A design of this approach based on object-oriented database views is shown. 1 Introduction In most database applications there is a need for letting the schema evolve. There are several reasons for this. There may be design flaws that are not discovered before (some of the) applications are implemented and the database is populated. The domain being modelled is evolving, new applications ha..