DISTRIBUTED DATABASE SYSTEMS - Database Design and Development: An Essential Guide for IT Professionals - page 545

London

Chicago

Paris

Tokyo

Boston

Hong Kong

New York

Figure 18-2 Distributed database system for a bank.

transactions. A distributed database is not stored in its entirety at a single physical
location but spread across several locations on a computer network.
Let us also give a formal definition for a distributed database management system
(DDBMS).

A DDBMS

is a software system that manages the distributed database and makes the distribution
of data transparent to users.

Basic Concepts

Let us go over the basic concepts of a distributed database system by considering
an example. Figure 18-2 presents a distributed database system for a bank that has
domestic and international branches.
Observe the figure, note the following, and understand the underlying concepts:

• The distributed database system is a single, logical database.
• The database is split into a number of fragments.
• Each fragment is stored separately at a location, also known as a site or a node.
• All the sites or nodes, storing parts of the database, are linked by a communi-
cation network.
• The data at each site constitute a physical database managed by its own DBMS.
• A distributed database system is a network of loosely coupled sites not sharing
physical components.
• Each site has substantial autonomy over the data and rarely depends on any
type of centralized control or service.