Object Migration in ISA Hierarchies

When an object migrates from one class to another in an ISA hierarchy, it acquires or loses membership in respective classes in the ISA hierarchy. Since an object can be an instance of multiple classes in the ISA hierarchy, there is a question of whether to have a single object identifier (OID) or multiple OIDs assigned to an object as it migrates along the ISA hierarchy. We refer to this as the OID ambiguity problem. Rules for meaningful object migration are first described in order to establish a framework for studying this problem. Then, to address the problem, a number of variables are considered, viz. (1) the storage scheme chosen for the ISA hierarchy, (2) the representation for the OID, (3) the way a message despatched to an object is processed, and (4) the ability to decide if two (or more) OIDs refer to the same real world object. Four approaches are studied, each of which resolves the OID ambiguity problem. One of the approaches is recommended as a superior approach because it retains the desirable OID properties of uniqueness and immutability and has relatively less overheads.