The IRO-DB (Interoperable Relational and Object DataBases) ESPRIT project provides a set of tools to achieve interoperability of pre-existing relational databases and new object-oriented databases. In contrast with current available technology, the project is based on the object-oriented paradigm, both for describing the heterogeneous databases and for accessing the federated databases. The system architecture is divided into three layers: (1) The local layer alleviates model differences and provides a standard ODMG interface for each participating system; (2) the communication layer assures the exchange of requests and objects between client and server sites; (3) the interoperable layer provides integrated views of the various participating databases and integrates them within the environment of a home system. The paper presents the design choices of IRO-DB, the IRO-DB system architecture, and the system services. The paper focuses successively on the local database mapping, the query and object interchange protocol, the interoperable object manager, the schema integrator workbench, and the user interface.

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