Maintaining Behavioral Consistency during Schema Evolution

We examine the problem of how to ensure behavioral consistency of an object-oriented system after its schema has been updated. The problem is viewed from the perspective of both the strongly typed and the untyped language model. Solutions are compared in both models using C++ and CLOS as examples. 1 Introduction Schema evolution and transformations have recently received increasing attention in the literature in both the area of object-oriented languages and especially in the area of object-oriented database systems: [Opd92, Ber92, Ber91, Cas91, CPLZ91, DZ91, Bar91, LH90, AH88, BKKK87, PS87, SZ86]. Most of this work has been done from the object-oriented database point of view where the focus is naturally on the structural, rather than behavioral, aspects of the evolving schema. Systems such as ORION [BKKK87], GemStone [PS87], and OTGen [LH90] guarantee the correctness of the performed schema changes and reflect the impact on the persistent instances in the database (structural consis..