Two-level-hierarchies of objects: a unit of reference for distributed object-oriented databases

We introduce and formalize the concept of two-level-hierarchies (t-l-hs) of objects. The t-l-hs are special partial directed acyclic graphs having as vertices objects. An equivalence relation is also defined on their edges. They are used for modelling complex objects by supporting, in a uniform way, the well known abstractions like association, aggregation, grouping as well as specialization and generalization. Considering the t-l-h as the design unit, we develop a new design process (Uni-Design) for the distributed object-oriented databases (DOODBs) by combining the main design approaches (Top-Down and Bottom-Up) in a consistent and complementary way leading to capture and represent explicitly most of the semantics of a system. We also choose the t-l-h as the elementary unit of distribution in order to adopt a distribution design strategy (a DOODB is now a set of possibly interrelated t-l-hs) and reconsider the notions of fragmentation and replication by integrating the distribution process at the last phase of the Uni-Design process.