Dynamic evolution of integrated schemas for federated objectbase systems
D'Silva, Albert A. Allwyn
Schema integration has been a topic of active research for several years and many methodologies have been presented. Most integration methodologies assume that the local databases participating in the federation do not change, and hence these methodologies ignore the post-integration phase. Typically, local schemas are unified to form one or more integrated schemas that do not change over time. However, changes made to the local schemas should be propagated to the integrated schemas so that the integrated schemas remain coherent. Schema evolution and integration are closely related and their association is evident in the post-integration phase of schema integration. In this thesis, the local schemas considered for integration are dynamic in nature and integration is performed in a federated environment. The proposed methodology uses an axiomatic model of dynamic schema evoluti n to represent the local schemas. The axiomatic model serves as a common foundation for evolving schemas when several systems with different object models participate in a federation. Other work has shown that the popular Orion object model and the purely behavioral object model of Tigukat can be represented in the axiomatic model. Two additional object models, the GemStone and the O$\sb2$ models, are axiomatized to illustrate the diversity of the object models represented in the axiomatic form. A new method of generating the initial integrated schema is presented, wherein related types form a subtype hierarchy that describes a similar real world concept. The axiomatic model automatically supports the process of forming subtype hierarchies. The essential schema evolution operations that effect the integrated schema are presented in terms of the axiomatic model. Changes made to the local schemas are propagated to the integrated schema without the need to carry out the re-integration process repeatedly and in its entirety.