Classification of multi-database languages

This paper defines a formal classification of multi-database languages into five levels of database integration with increasing degree of global control and decreasing degree of local autonomy. First, the fundamental interoperability mechanisms are identified for each of these levels. Their consequences on local autonomy as well as implementation draw-backs are discussed. Second, various multi-database languages are classified into these categories. In addition to our own language COOL, other proposals are analyzed, including SQL Net, Multibase, Superviews, VODAK, Pegasus, and O SQL.