Database Systems - Chapter 12.pdf - po ang - Januszek66

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DISTRIBUTED DATABASE MANAGEMENT SYSTEMS

In this chapter, you will learn:

What a distributed database management system (DDBMS) is and what its components are

How database implementation is affected by different levels of data and process distribution

How transactions are managed in a distributed database environment

How database design is affected by the distributed database environment

In this chapter, you learn that a single database can be divided into several fragments. The

fragments can be stored on different computers within a network. Processing, too, can be

dispersed among several different network sites, or nodes.The multisite database forms the

core of the distributed database system.

P review

The growth of distributed database systems has been fostered by the dispersion of business

operations across the country and the world, along with the rapid pace of technological

change that has made local and wide area networks practical and more reliable. The

network-based distributed database system is very flexible: it can serve the needs of a small

business operating two stores in the same town while at the same time meeting the needs

of a global business.

Although a distributed database system requires a more sophisticated DBMS, the end user

should not be burdened by increased operational complexity.That is, the greater complexity

of a distributed database system should be transparent to the end user.

The distributed database management system (DDBMS) treats a distributed database as a

single logical database; therefore, the basic design concepts you learned in earlier chapters

apply. However, although the end user need not be aware of the distributed database’s

special characteristics, the distribution of data among different sites in a computer network

clearly adds to a system’s complexity. For example, the design of a distributed database must

consider the location of the data and the partitioning of the data into database fragments.

You examine such issues in this chapter.

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12.1 THE EVOLUTION OF DISTRIBUTED DATABASE MANAGEMENT SYSTEMS

A distributed database management system ( DDBMS ) governs the storage and processing of logically related

data over interconnected computer systems in which both data and processing functions are distributed among several

sites. To understand how and why the DDBMS is different from the DBMS, it is useful to briefly examine the changes

in the business environment that set the stage for the development of the DDBMS.

During the 1970s, corporations implemented centralized database management systems to meet their structured

information needs. Structured information is usually presented as regularly issued formal reports in a standard format.

Such information, generated by procedural programming languages, is created by specialists in response to precisely

channeled requests. Thus, structured information needs are well served by centralized systems.

The use of a centralized database required that corporate data be stored in a single central site, usually a mainframe

computer. Data access was provided through dumb terminals. The centralized approach, illustrated in Figure 12.1,

worked well to fill the structured information needs of corporations, but it fell short when quickly moving events

required faster response times and equally quick access to information. The slow progression from information request

to approval, to specialist, to user simply did not serve decision makers well in a dynamic environment. What was

needed was quick, unstructured access to databases, using ad hoc queries to generate on-the-spot information.

FIGURE

12.1

Centralized database management system

Request

End user

DBMS

Data

Read

L oc al d at aba se

Database management systems based on the relational model could provide the environment in which unstructured

information needs would be met by employing ad hoc queries. End users would be given the ability to access data when

needed. Unfortunately, the early relational model implementations did not yet deliver acceptable throughput when

compared to the well-established hierarchical or network database models.

The last two decades gave birth to a series of crucial social and technological changes that affected database

development and design. Among those changes were:

Business operations became decentralized.

Competition increased at the global level.

Customer demands and market needs favored a decentralized management style.

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DISTRIBUTED DATABASE MANAGEMENT SYSTEMS

Rapid technological change created low-cost computers with mainframe-like power, impressive multifunction

handheld portable wireless devices with cellular phone and data services, and increasingly complex and fast

networks to connect them. As a consequence, corporations have increasingly adopted advanced network

technologies as the platform for their computerized solutions.

The large number of applications based on DBMSs and the need to protect investments in centralized DBMS

software made the notion of data sharing attractive. Data realms are converging in the digital world more and

more. As a result, single applications manage multiple different types of data (voice, video, music, images, etc.),

and such data are accessed from multiple geographically dispersed locations.

Those factors created a dynamic business environment in which companies had to respond quickly to competitive and

technological pressures. As large business units restructured to form leaner and meaner, quickly reacting, dispersed

operations, two database requirements became obvious:

Rapid ad hoc data access became crucial in the quick-response decision-making environment.

The decentralization of management structures based on the decentralization of business units made

decentralized multiple-access and multiple-location databases a necessity.

During recent years, the factors just described became even more firmly entrenched. However, the way those factors

were addressed was strongly influenced by:

The growing acceptance of the Internet as the platform for data access and distribution . The World Wide

Web (WWW or just the Web) is, in effect, the repository for distributed data.

The wireless revolution . The widespread use of wireless digital devices, such as personal digital assistants

(PDAs) like Palm and BlackBerry and multipurpose “smart phones” like the iPhone, has created high demand

for data access. Such devices access data from geographically dispersed locations and require varied data

exchanges in multiple formats (data, voice, video, music, pictures, etc.) Although distributed data access does

not necessarily imply distributed databases; performance and failure tolerance requirements often make use of

data replication techniques similar to the ones found in distributed databases.

The accelerated growth of companies providing “application as a service” type of services . This new type

of service provides remote application services to companies wanting to outsource their application develop-

ment, maintenance, and operations. The company data is generally stored on central servers and is not

necessarily distributed. Just as with wireless data access, this type of service may not require fully distributed

data functionality; however, other factors such as performance and failure tolerance often require the use of

data replication techniques similar to the ones found in distributed databases.

The increased focus on data analysis that led to data mining and data warehousing . Although a data

warehouse is not usually a distributed database, it does rely on techniques such as data replication and

distributed queries that facilitate data extraction and integration. (Data warehouse design, implementation, and

use are discussed in Chapter 13, Business Intelligence and Data Warehouses.)

ONLINE CONTENT

To learn more about the Internet

s impact on data access and distribution, see Appendix I in the Student

Online Companion, Databases in Electronic Commerce.

’

At this point, the long-term impact of the Internet and the wireless revolution on distributed database design and

management is still unclear. Perhaps the success of the Internet and wireless technologies will foster the use of

distributed databases as bandwidth becomes a more troublesome bottleneck. Perhaps the resolution of bandwidth

problems will simply confirm the centralized database standard. In any case, distributed databases exist today and many

distributed database operating concepts and components are likely to find a place in future database developments.

The decentralized database is especially desirable because centralized database management is subject to problems

such as:

Performance degradation due to a growing number of remote locations over greater distances.

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CHAPTER 12

High costs associated with maintaining and operating large central (mainframe) database systems.

Reliability problems created by dependence on a central site (single point of failure syndrome) and the need

for data replication.

Scalability problems associated with the physical limits imposed by a single location (power, temperature

conditioning, and power consumption.)

Organizational rigidity imposed by the database might not support the flexibility and agility required by

modern global organizations.

The dynamic business environment and the centralized database’s shortcomings spawned a demand for applications

based on accessing data from different sources at multiple locations. Such a multiple-source/multiple-location database

environment is best managed by a distributed database management system (DDBMS).

12.2 DDBMS ADVANTAGES AND DISADVANTAGES

Distributed database management systems deliver several advantages over traditional systems. At the same time, they

are subject to some problems. Table 12.1 summarizes the advantages and disadvantages associated with a DDBMS.

TABLE

12.1

Distributed DBMS Advantages and Disadvantages

ADVANTAGES

DISADVANTAGES

• Data are located near the greatest demand site

The data in a distributed database system are dis-

persed to match business requirements.

• Faster data access

• Complexity of management and control

. Applications

must recognize data location, and they must be able to

stitch together data from various sites. Database admin-

istrators must have the ability to coordinate database

activities to prevent database degradation due to data

anomalies.

• Technological difficulty

. End users often work with only

a locally stored subset of the company

’

s data.

• Faster data processing

. A distributed database sys-

tem spreads out the systems workload by process-

ing data at several sites.

• Growth facilitation

. Data integrity, transaction man-

agement, concurrency control, security, backup, recov-

ery, query optimization, access path selection, and so

on, must all be addressed and resolved.

• Security

. New sites can be added to

the network without affecting the operations of

other sites.

• Improved communications

. The probability of security lapses increases

when data are located at multiple sites. The responsi-

bility of data management will be shared by different

people at several sites.

• Lack of standards

. Because local sites are

smaller and located closer to customers, local sites

foster better communication among departments

and between customers and company staff.

• Reduced operating costs

. There are no standard communica-

at the database level

. It is more cost-effective to

add workstations to a network than to update a

mainframe system. Development work is done

more cheaply and more quickly on low-cost PCs

than on mainframes.

• User-friendly interface

tion protocols

. (Although TCP/IP

is the de facto standard at the network level, there is

no standard at the application level.) For example,

different database vendors employ different

—

and

techniques to manage the distri-

bution of data and processing in a DDBMS

environment.

• Increased storage and infrastructure requirements

often incompatible

—

. PCs and workstations are

usually equipped with an easy-to-use graphical

user interface (GUI). The GUI simplifies training

and use for end users.

• Less danger of a single-point failure

. Mul-

tiple copies of data are required at different sites, thus

requiring additional disk storage space.

• Increased training cost . Training costs are generally

higher in a distributed model than they would be in a

centralized model, sometimes even to the extent of

offsetting operational and hardware savings.

• Costs

. When one of

the computers fails, the workload is picked up by

other workstations. Data are also distributed at

multiple sites.

• Processor independence . The end user is able to

access any available copy of the data, and an end

user ’ s request is processed by any processor at the

data location.

. Distributed databases require duplicated infra-

structure to operate (physical location, environment,

personnel, software, licensing, etc.)

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Distributed databases are used successfully but have a long way to go before they will yield the full flexibility and power

of which they are theoretically capable. The inherently complex distributed data environment increases the urgency for

standard protocols governing transaction management, concurrency control, security, backup, recovery, query

optimization, access path selection, and so on. Such issues must be addressed and resolved before DDBMS technology

is widely embraced.

The remainder of this chapter explores the basic components and concepts of the distributed database. Because the

distributed database is usually based on the relational database model, relational terminology is used to explain the basic

concepts and components of a distributed database.

12.3 DISTRIBUTED PROCESSING AND DISTRIBUTED DATABASES

In distributed processing , a database’s logical processing is shared among two or more physically independent sites

that are connected through a network. For example, the data input/output (I/O), data selection, and data validation

might be performed on one computer, and a report based on that data might be created on another computer.

A basic distributed processing environment is illustrated in Figure 12.2, which shows that a distributed processing

system shares the database processing chores among three sites connected through a communications network.

Although the database resides at only one site (Miami), each site can access the data and update the database. The

database is located on Computer A, a network computer known as the database server .

FIGURE

12.2

Distributed processing environment

Computer A

Site 1

Miami user Joe

DBMS

E m pl oyee data ba s e

Site 2

New York user Donna

Computer B

Site 3

Atlanta user Victor

Computer C

Updat e

payrol l

data

Generate

payroll

report

Communications network

Database records are processed in different locations

A distributed database , on the other hand, stores a logically related database over two or more physically

independent sites. The sites are connected via a computer network. In contrast, the distributed processing system uses

only a single-site database but shares the processing chores among several sites. In a distributed database system, a

database is composed of several parts known as database fragments . The database fragments are located at

different sites and can be replicated among various sites. Each database fragment is, in turn, managed by its local

database process. An example of a distributed database environment is shown in Figure 12.3.

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