文档介绍：Data & Knowledge Engineering 4 (1989) 43-67 43
North-Holland
Schema evolution in object-oriented
database systems
. NGUYEN and D. RIEU
INRIA and IMAG, Universitd de Grenoble, BP 53, 38041 Grenoble Cedex, France
Abstract. Object-oriented database systems ususally exhibit specific advantages over traditional database
management systems and programming languages. Among them stand the ease of writing, maintaining and
debugging application programs, code modularity, inheritance, persistency and sharability. Of particuiar
interest to software engineering and computer-aided design applications is also the ability to dynamically
change the object definitions and the opportunity to define incrementally composite objects. This paper gives
an overview of current research efforts directed towards evolving data definitions in object-oriented database
systems. Ti~e emphasis is on their ability to support two complementary aspects: supporting evolving
schemas, and propagating the changes on the object instances.
Several projects are analyzed: Cadb, Encore, GemStone, Orion, and Sherpa. Current results indicate that
if most of them provide schema evolution facilities, they seldom support automatic propagation mechanisms.
A proposal is described that enables Sherpa to fully support the propagation of changes and the dynamic
classification of the instances whose class definitions are modified. This approach is an extension of
techniques used in artificial intelligence for knowledge representation. It extends previous classification
mechanisms with a dymanic capability which adequately supports evolving class definitions and instances.
Keywords. Object-oriented models, Databases, Dynamic schemas, Inheritance Propagation, Classification.