DISTRIBUTED DATABASE SYSTEMS - Database Design and Development: An Essential Guide for IT Professionals - page 549

Local
data

Local
data

DB

DB

DBMS

London

Chicago

DBMS

Local
data

DB

DBMS

DBMS

Paris Local

data

Local
data

DB

DB

Tokyo

Local
data

Boston

DBMS

DBMS

DDBMS
Local
data

DB

DB

Hong Kong

DBMS

New York

Figure 18-3 Basic distributed database configuration.

• Each site needs database software to manage its local data.
• Each site must have autonomy to own, control, and manage its data and
operations.
• Each site must also collaborate with any other site that needs data stored at
this site.
• Similarly, each site must also be able to access data from any other site.
• There must be some overall database software to enable cooperation among
sites for data access.

Considering these factors, we find that distributed database systems fall into two
major types: homogeneous and heterogeneous systems. Figure 18-4 illustrates how
these two types are configured in addition to showing a centralized database system
for comparison.

Homogeneous Systems The first feature of a homogeneous system is the degree
of homogeneity and the second feature is the degree of autonomy. In a homoge-
neous system, all sites use identical software.
The server at each site uses the same DBMS as that in every other site. Clients
at every site use identical software for data access. Each local DBMS is allowed
to function as a stand-alone database management system, thus providing local
autonomy.

Heterogeneous Systems In this case, different sites run under the control of
different DBMSs. However, the sites are connected in such a manner to enable data
access across different sites. A heterogeneous system, also known as a multidata-
base system, provides a high degree of local autonomy. Because a heterogeneous
system consists of a federation of autonomous database systems, it is also called a
federated multidatabase system.