The Distributed Information Search COmponent (Disco) is a prototype heterogeneous distributed database that accesses underlying data sources. The Disco prototype currently focuses on three central research problems in the context of these systems. First, since the capabilities of each data source is di erent, transforming queries into subqueries on data source is di cult. We call this problem the weak data source problem. Second, since each data source performs operations in a generally unique way, the cost for performing an operation may vary radically from one wrapper to another. We call this problem the radical cost problem. Finally, existing systems behave rudely when attempting to access an unavailable data source. We call this problem the ungraceful failure problem. Disco copes with these problems. For the weak data source problem, the database implementor de nes precisely the capabilities of each data source. For the radical cost problem, the database implementor (optionally) d...