20.1. Varieties of distributed systems

Homogeneous and heterogeneous distributed databases are possible. In the homogeneous case, each local database is managed by the same DBMS. In a heterogeneous system, local databases can even refer to different data models. Network integration of heterogeneous databases is a pressing, but very complex problem. Many solutions are known at the theoretical level, but so far they have not been able to cope with the main problem - the lack of efficiency of integrated systems.

Note that the intermediate task is practically more successfully solved - the integration of heterogeneous SQL-oriented systems. It is clear that standardization of the SQL language and the general adherence of database vendors to the principles of open systems largely contribute to this.

We restrict ourselves to the consideration of the problems of homogeneous distributed DBMS on the example of System R *.