DISTRIBUTED DATABASE SYSTEMS - Database Design and Development: An Essential Guide for IT Professionals - page 548

More complex design. The design of a distributed database system, at both the
logical and physical levels, is more complicated than that of a centralized database
system.
Difficult integrity control. Distributed query and transaction processing, despite the
benefits they offer, are more prone to problems relating to concurrency conflicts,
deadlocks, and recovery from failures.
Involved security systems. Security protection must be duplicated at every site.
Added maintenance cost. Maintenance cost must increase to include maintenance
of local computer systems, local DBMSs in addition to the overall DDBMS, and the
communications network.
Lack of standards. Standards for data communications and data access, critical in
distributed database environments, are only emerging now at a slow pace.
Limited acceptance. Despite the numerous advantages, general-purpose distrib-
uted database systems are not widely used. Therefore, there is not much industry
experience.
DISTRIBUTED DATABASES
When you decide to distribute the corporate data and spread it across the various
locations, you are faced with a number of design and implementation issues. You
need to plan what type of distributed system you must have. How about the data-
base software at each location? Then how are you going create segments of data
and determine which segment gets stored in which location? The next considera-
tion relates to the features necessary in the overall database software to manage
the distribution. What about moving data from one site to another whenever
needed? What are the characteristics of the communications network?
We will explore these issues in this section. You will learn about the variations
available for distributing data and managing data at each site. You will examine the
components of DDBMS. You will note implementation options and review design
issues.
Types and Configurations
Let us begin by looking at a basic configuration. Figure 18-3 shows a plain distrib-
uted database environment.
Look at the figure and note the following points that indicate what components
and features are needed in the configuration to make it a distributed database
environment.
• At each location, also known as site or node, there is a computer system.
• Local data are stored at each site.