We show how the use of a metaobject layer is useful to allow an easier evolution of a an object-oriented database integrating existing relational systems. A correct use of metaobjects allow to hide the persistence contingencies when linking an application with an external application. Moreover a metaobject layer allows the evolution of the database object model. We show in these lines how a underlying relational database can be used for final storage of data in a system allowing metamodeling abilities. We have designed a correspondence engine, driven by rules, linking standard object and relational structures. We want the designer to be able to alter these rules according to the evolution of the object model, in order to keep coherent relational schemes. So the correspondence engine has been designed as a specialisable protocol discriminating on the metaobjects of the object layer.
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