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Distributed DatabaseManagement Systems

Lecture - 1

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1-Distributed Database Systems (2nd Edition) by T.M., Ozsu, P. Valdusiez

2- Distributed Database Systems. By D. Bell, J. Grimson,

Addison-Wesley, 1992

References

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3- Distributed Systems: Concepts and Design, 4th Edition, by G. Coulouris, J. Dollimore, T. Kindberg, Addison-Wesley

Prerequisites: Database Management Systems, Computer Networks

References

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BRIEFLY COURSE INTRODUCTION

• Introduction to database and Distributed Systems in general

• Architectures and Design Issues of DDBS

• Technological Aspects and designs

• Theoretical Aspects of the topic

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LITTLE BIT OF HISTORY

• Traditional File Processing System: the very first form of business data processing

• Each program contains data description that it manipulates

• Redundancy of data• Problems in maintenance

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Program and Data Interdependence

RegistrationApplications

RegistrationDataFiles

Registration

ExaminationApplications

ExaminationDataFiles

Examination

LibraryApplications

LibraryDataFiles

Library

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Library Exam Registration

Reg_Number Reg_Number Reg_Number

Name Name Name

Father Name Address Father Name

Books Issued Class Phone

Fine Semester Address

Grade Class

File Processing Systems

Duplication of DataVulnerable to Inconsistency

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Traditional File Processing

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HISTORY CONTINUES

Database Approach: (Also called centralized database) Database is a shared collection of logically related data

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Database

Data Description

Data Manipulation

….

PROGRAM 1

PROGRAM 2

PROGRAM 3

Database Approach

Takes care of all major drawbacks of File System Environment plus more

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DISTRIBUTED COMPUTING SYSTEM

A number of autonomous processing elements that are connected through a computer network and that cooperate in performing their assigned tasks

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DISTRIBUTED COMPUTING SYSTEMS• Distributed System Software enables

computers to coordinate and share

• The word distributed?• Processing logic• Functions• Data• Control; All are relevant and important here

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CLASSIFICATIONS OF DCS• Degree of Coupling• How closely systems are connected• May be the measured as ratio of

messages interchanged to the local processing• Could be Weak (over the network) or

Strong (if components are shared)

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CLASSIFICATIONS OF DCS• Interconnection structure• Could be point to point or a common

interconnection channel

• Interdependence of Components

• Synchronization

• Factors are not totally independent

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WHY DCS?• Suits some of the Organizational Structures;

more reliable and responsive• Nature of some applications• Technological Push

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DCS’S ALERTS• Information pieces and Lack of

Standards• Difficulties in Large Application

Design• Too Many Options Available

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DISTRIBUTED DB AND DBMS

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Distributed Database: A collection of logically interrelated databases that are spread physically across multiple locations connected by a data communications link.

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MAIN CHARACTERISTICS

Data at multiple sites

DM at each site

Local requirements

Global perspective

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WHERE TO APPLY• Major two reasons that make an

application a candidate to be DDBS application

• Large Number of Users

• Operation spread large geographical area

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Example Applications

BankingAir TicketingBusiness at multiple locations

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Distributed DBMS: A software system that permits the management of DDB and makes the distributed transparent environment to the users

Decentralized Database:

A collection of independent databases on non-networked computers.

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RESEMBLING SETUPS

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• Distributed Files: A collection of files stored on different computers of a network; not a DDBS• DDBS is logically related, common

structure among files, and accessed via same interface

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RESEMBLING SETUPS• Multiprocessor System: multiple processors that

share some form of memory

ProcessorUnit

ProcessorUnit

ProcessorUnit

Memory

I/O SystemShared EverythingTight Coupling

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RESEMBLING SETUPS

CPU

Memory

Computer System

CPU

Memory

Computer System

CPU

Memory

Computer System

SharedSecondary

MemoryShared EverythingLoose Coupling

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RESEMBLING SETUPS

CPU

Memory

Computer System

CPU

Memory

Computer System

CPU

Memory

Computer System

Switch

Shared Nothing

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RESEMBLING SETUPS• DDBS is also different from a centralized system having C/S system involving network

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REASONS FOR DDBS

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• Local units want control over data.

• Consolidate data for integrated decisions

• Reduce telecommunication costs.

• Reduce the risk of telecommunication failures.

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Distributed DBMS

DBMS 1DBMS 1 DBMS n• • • •

Global User Global User

Node 1 Node n

Global Schema

Local User

Local User

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Objectives/Promises of DDBSs

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TransparencyUser View System View

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• DATA INDEPENDENCE• Data independence is a fundamental

form of transparency that we look for within a DBMS• It is also the only type that is important

within the context of a centralized DBMS

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DATA INDEPENDENCE• Two types, Logical Data Independence and Physical Data Independence

• Logical Data Independence

• refers to the immunity of user applications to changes in the logical structure (i.e., schema) of the database

• Physical Data Independence

• it deals with hiding the details of the storage structure from user applications

• A transparent system hides the implementation details from its users

• When a user application is written, it should not be concerned with the details of physical data organization. Therefore, the user application should not need to be modified when data organization changes occur due to performance considerations

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DATA INDEPENDENCE

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NETWORK TRANSPARENCY• User should not only be free from network management

activities rather it should be unaware of even existence of the network

• Then there would be no difference between database applications that would run on a centralized database and those that would run on a distributed database. This type of transparency is referred to as network transparency or distribution transparency

• Location Transparency and Naming Transparency • Naming transparency means that a unique name is provided

for each object in the database

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REPLICATION TRANSPARENCY• For performance, reliability, and availability reasons, it is usually desirable

to be able to distribute data in a replicated fashion across the machines on a network

• Such replication helps performance since diverse and conflicting user requirements can be more easily accommodated

• if one of the machines fails, a copy of the data are still available on another machine on the network

• Assuming that data are replicated, the transparency issue is whether the users should be aware of the existence of copies or whether the system should handle the management of copies and the user should act as if there is a single copy of the data

• Replication transparency refers only to the existence of replicas, not to their actual location

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FRAGMENTATION TRANSPARENCY• The final form of transparency that needs to be addressed within the context of a

distributed database system is that of fragmentation transparency

• This is commonly done for reasons of performance, availability, and reliability

• Furthermore, fragmentation can reduce the negative effects of replication. Each replica is not the full relation but only a subset of it; thus less space is required and fewer data items need to be managed

• There are two general type of fragmentations are available named Horizontal fragmentation and the second one is the vertical fragmentation

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RESPONSIBILITY OF TRANSPARENCY• Transparency is desirable but there is a compromise between level of

Transparency and difficulty/cost (Gray argues that full transparency makes the management of distributed data very difficult and claims that “applications coded with transparent access to geographically distributed databases have: poor manageability, poor modularity, and poor message performance” [Gray, 1989])

• The language/Compiler: to provide uniform method of manipulating data. Avoid connectivity details.

• Operating System: already provides in form of Device Drivers• The third layer at which transparency can be supported is within the

DBMS• Practically, we get combination of all three

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Layers of Transparency

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Improved PerformanceThis has two potential advantages:

1. Since each site handles only a portion of the

database, contention for CPU and I/O services is not as

severe as for centralized databases

2. Localization reduces remote access delays that are

usually involved in wide area networks (for example,

the minimum round-trip message propagation delay in

satellite-based systems is about 1 second)

EASIER SYSTEM EXPANSION• It is much easier to accommodate increasing database sizes

• Expansion can usually be handled by adding processing and storage power to the network. Obviously, it may not be possible to obtain a linear increase in “power,” since this also depends on the overhead of distribution. However, significant improvements are still possible

• Grosh’s law:• It was commonly believed that it would be possible to purchase a fourfold powerful

computer if one spent twice as much

• As the time passed with the advent of microcomputers and workstations, and their price/performance characteristics, this law is considered invalid now but this doesn’t means that MAIN-FRAMES are dead

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THANKS