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A short review on Distributed database Systems

Before going on to Distributed Database Systems, let me briefly explain you about Centralised Database Systems. Here all system components i.e the database and the Database Management System (DBMS) reside at a single computer or site. Users may be able to access the Centralised Database System remotely via terminals connected to the site; however all the data access and processing takes place at the central site. The following figure shows a Centralised DB System.

Centralized database System

Centralized database System

Distributed database In a Distributed Database System the database is stored/spread physically across computers or sites in different locations that are connected together by some form of data communication network. They may be spread over WAN or LAN. The computers may be of different types such as IBM Mainframes, VAXs, SUN work station, PCs etc managed by different operating systems and each fragment of the data base may be managed by a different DBMS such as Oracle, Ingress, and Microsoft SOL server.

Distributed database management system (DDBMS) In a DDS, database applications running at any of the system's sites should be able to operate on any of the database fragments transparently i.e., as if the data come from a single database managed by one DBMS. The software that manages a distributed database in such a way is called DDBMS.

The notion of distributed database is different from that of decentralised database. The latter does not imply sharing of data by a communication network. The former implies a collection of sites connected together with some kind of network and where each site has a database in its own right, but the sites work together as if data was stored at only one site.

Distributed database System Decentralized database System

Distributed database System     Decentralized database System

Distributed database design: The methodology used for the logical design of a centralized database applies to the design of the distributed one as well. However, for a distributed database three additional factors have to be considered.

Types of DDBMS A DDBMS can be classified as :

DBMS Transparency:
The term DBMS transparency refers to the ability of DDBMS to hide the implementation details from the user. Thus the fact that a distributed database is split into several fragments that can be stored on different computers and perhaps replicated is hidden from the user. The objective of the transparency is to make the distributed system appear like a centralized system.
We can identify four main types of transparency in a DDBMS:





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