Schema evolution in object databases by catalogs

We are concerned with schema evolution in object oriented databases (OODB) that is processed by a modification of the classes of the schema. We present a new categorization of the different modifications in three categories: primitive, composite and complex modifications. On this basis, we propose a method by decomposition for addressing conceptual schema evolution: "real-life" complex schema modifications are solved by decomposition into simpler well known and controlled modifications, called composites, which in turn may be reduced to so-called primitives. A key step in making this approach practical, is the provision of two catalogs: one for the primitives based on a sound object model and one for the composites built on top of the primitive catalog. Such catalogs raise the level of abstraction, further reuse and are prerequisites for effective tool support. We define the three types of schema modifications, describe these catalogs, put them into the context of our decomposition approach and provide a process for schema evolution.