A Formal Framework for the Implementation and Evolution of DOOD Schemas

We present a formal framework for the implementation of a concep tual data model in terms of an internal data model within a two level architecture taking into account schema consistency and instance com patibility Within that framework the consequences of schema changes are investigated leading to a homogeneous formalization of existing ap proaches to schema evolution On that basis we introduce the notion of a schema evolution strategy which consists of a syntactic and semantic characterization of the range of applicability and de nes the mappings re quired for implementing schema evolution within our two level framework These mappings clearly distinguish those consequences of a schema mod i cation which are model inherent from those which are implementation dependent and they separate consequences on schemas from consequences on instances Schema evolution strategies are used as the unifying con cept required for combining and exibly choosing among di erent schema evolution techniques in an advanced DBMS Our formalism is applied to Chimera a powerful active DOOD language