A database in which the data is contained within a number of separate subsystems, usually in different physical locations. If the constituent subsystems are essentially similar, the system is said to be homogeneous, otherwise it is said to be heterogeneous. Distributed database systems may vary very considerably. At one extreme is the type where the complete system was conceived, designed, and implemented as a single entity; such systems exist within large commercial organizations and are usually homogeneous. At the other extreme is the case where a number of existing systems, originally planned as isolated systems, continue in their normal operation but in addition are loosely linked to provide a larger distributed system; in this instance the system is often heterogeneous.
Distributed database systems are currently an active topic for database research and development, largely because of the availability of national and international communication facilities.