DISTRIBUTED DATABASE SYSTEMS - Database Design and Development: An Essential Guide for IT Professionals - page 546

• A distributed database system may be thought of as a collaboration of partic-
ipating remote sites storing parts of the logical database.
• Each site, however, may participate in transaction execution when the transac-
tion needs to access data from that site.
• Applications at a site not requiring data from any other sites are known as local
applications. Applications that do need data from other sites are called global
applications. If all applications at every site are local applications, then there is
no necessity for a DDBMS.
Motivation and Goals
At the beginning of this chapter, we discussed broadly the evolution and purposes
of distributed database systems. Let us now formalize the motivation for these
systems. Why distributed databases? Here are the major reasons.
Efficient access of local data. In many global organizations or in those with a
number of geographically dispersed locations, most of the database operations are
on the local data. Maintaining the pertinent data at the local site allows efficient
and immediate access to local data.
Improved customer service. Global organizations must be flexible enough to meet
the needs of local customers. Each location, although part of the overall
organization, must primarily look after the customers at that location. Local owner-
ship and control of the local data are essential.
Enhanced reliability. If the computer system at one site fails or if the communica-
tion link between two locations goes down, presumably the other sites can continue
working. Furthermore, if replication techniques are used to store copies of the same
data at more than one site, then other operating sites may still supply the required
data.
Availability of global data. Each site in a global organization, although autonomous,
is still part of the overall organization. Therefore, users at a site would require data
from other sites as well through their global applications. Global data must also be
available at every site.
Advantages and Disadvantages
You have been introduced to the concept of distributed database systems. You have
understood the motivation that has guided the development of these systems. Let
us review a list of advantages provided by distributed database systems. From the
list of advantages, you will note that, in practice, distributed database systems are
fulfilling the aspirations that prompted their development. These systems also have
a few disadvantages.
We will get into in-depth discussions of distributed data systems in the next
sections. We will cover the components, the types, and the configurations.
DDBMSs have to be more sophisticated than DBMSs. You will learn about