ROVER: flexible yet consistent evolution of relationships

摘要

Relationships have been repeatedly identified as an important object-oriented modeling construct. Most emerging modeling standards such as the object database management group (ODMG) object model and UML have some support for relationships. However object-oriented database (OODB) systems have largely ignored the existence of relationships during schema evolution. We are the first to propose comprehensive support for relationship evolution. A complete schema evolution facility for any OODB system must provide primitives to manipulate all object model constructs, and maintenance strategies for the structural and referential integrity of the database under such evolution. We propose a set of basic evolution primitives for relationships as well as a compound set of changes that can be applied to the same. However, given the myriad of possible change semantics a user may desire in the future, any pre-defined set is not sufficient. Rather we present a flexible schema evolution framework that allows the user to define new relationship transformations as well as to extend existing ones. Addressing the second problem, namely of updating schema evolution primitives to conform to the new set of invariants, can be a very expensive re-engineering effort. In this paper we present an approach that de-couples the constraints from the schema evolution code, thereby enabling their update without any re-coding effort. We also present an approach that can be used to verify the correctness of these complex evolution operations using the de-coupled constraints.