Decentralization of Databases and the Communication between Them,

Database management systems DBMS have been used to develop centralized information systems, where the database and the application programs are stored at a central computer. DBMS have also resulted in decentralized information systems in which data and programs are stored at several sites with none or very little communication between the sites. With the advent of distributed database management systems DDBMS new possibilities for developing information systems have emerged. An ideal DDBMS supports an information system database stored at several sites in a computer network in such a way that users can access data at any site in the network as if they were stored at the local site. This paper looks at some of the necessary properties of an ideal DDBMS including location, replication and fragmentation transparencies. The advantages of DDBMS compared with CDBMS -- such as local control of data, better system availability etc. -- are discussed together with some of the disadvantages of DDBMS -- such as updating of replicated data and a generally more complex environment than in a central DBMS-sytem. The advantages of a distributed information system compared to a decentralized information system are also discussed. Another area creating new opportunities for interconnection between information systems is the development of application level protocols within the ISO Open Systems Interconnection Model.