Distributed DBMS [Good]

8/7/2019 Distributed DBMS [Good]

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Distributed Database

Management SystemsChapter 10


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In this chapter, you will

learn: What a distributed database management

system (DDBMS) is and what its componentsare

How database implementation is affected bydifferent 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


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The Evolution of

Distributed DatabaseManagement Systems Distributed database management

system (DDBMS)

Governs storage and processing of

logically related data over

interconnected computer systems

in which both data and processingfunctions are distributed among

several sites


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The Evolution of Distributed

Database Management Systems(continued) Centralized database required that

corporate data be stored in a

single central site

Dynamic business environment

and centralized databases

shortcomings spawned a demandfor applications based on data

access from different sources at

multiple locations


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Centralized Database

Management System


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DDBMS Advantages Data are located near greatest demand site Faster data access

Faster data processing

Growth facilitation

Improved communications

Reduced operating costs

User-friendly interface

Less danger of a single-point failure

Processor independence


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DDBMS Disadvantages Complexity of management and control

Security

Lack of standards Increased storage requirements

Greater difficulty in managing the data

environment

Increased training cost


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Distributed Processing

Environment


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Environment


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Characteristics of Distributed

Management Systems Application interface

Validation

Transformation

Query optimization

Mapping

I/O interface

Formatting

Security

Backup and recovery DB administration

Concurrency control

Transaction management


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Characteristics of

Distributed ManagementSystems (continued) Must perform all the functions of a

centralized DBMS

Must handle all necessary

functions imposed by the

distribution of data and processing

Must perform these additional

functions transparentlyto the end

user


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A Fully Distributed

Database ManagementSystem


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DDBMSComponents Must include (at least) the following components:

Computer workstations

Network hardware and software

Communicati

ons

media

Transaction processor (or, application processor,or transaction manager)

Software component found in each computer thatrequests data

Data processor or data manager Software component residing on each computer

that stores and retrieves data located at the site

May be a centralized DBMS


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System Components


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Database Systems:

Levels of Data andProcess Distribution


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Single-Site Processing,

Single-Site Data (SPSD) All processing is done on single CPU or host

computer (mainframe, midrange, or PC)

All data are stored on host computers local disk

Processing cannot be done on end users side ofthe system

Typical of most mainframe and midrangecomputer DBMSs

DBMS is located on the host computer, which isaccessed by dumb terminals connected to it

Also typical of the first generation of single-usermicrocomputer databases


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Single-Site Processing,Single-Site Data(Centralized)


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Multiple-SiteProcessing, Single-SiteData (MPSD) Multiple processes run on different

computers sharing a single data

repository

MPSD scenario requires a network

file server running conventional

applications that are accessedthrough a LAN

Many multi-user accounting

applications, running under a

personal computer network, fit


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Multiple-SiteProcessing, Single-SiteData


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Multiple-Site Processing,

Multiple-Site Data (MPMD) Fully distributed database management system

with support for multiple data processors and

transaction processors at multiple sites Classified as either homogeneous or

heterogeneous

Homogeneous DDBMSs Integrate only one type of centralized DBMS

over a network


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Multiple-Site Processing,Multiple-Site Data (MPMD)(continued)

Heterogeneous DDBMSs

Integrate different types of centralized DBMSs

over a network

Fully heterogeneous DDBMS

Support different DBMSs that may even support

different data models (relational, hierarchical, ornetwork) running under different computer

systems, such as mainframes and

microcomputers


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HeterogeneousDistributedDatabase Scenario


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Transparency Features Allow end user to feel like databases only

user

Features include:Distribution transparency

Transaction transparency

Failure transparencyPerformance transparency

Heterogeneity transparency


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Distribution

Transparency Allows management of a physically dispersed

database as though it were a centralized

database

Three levels of distribution transparency are

recognized:

Fragmentation transparency

Location transparency

Local mapping transparency


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A Summary of

Transparency Features


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Fragment Locations


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Transaction

Transparency Ensures database transactions will

maintain distributed databases

integrity and consistency


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Distributed Requestsand DistributedTransactions

Distributed transaction

Can update or request data from several

different remote sites on a network Remote request

Lets a single SQL statement access data to beprocessed by a single remote database

processor

Remote transaction

Accesses data at a single remote site


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Distributed Requestsand DistributedTransactions (continued) Distributed transaction

Allows a transaction to referenceseveral different (local or remote)

DP sites

Distributed request

Lets a single SQL statement

reference data located at several

different local or remote DP sites


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A Remote Request


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A Remote Transaction


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A Distributed

Transaction


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A Distributed Request


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Another Distributed

Request


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DistributedConcurrency

Control Multisite, multiple-process

operations are much more likely to

create data inconsistencies anddeadlocked transactions than are

single-site systems


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The Effect of a

Premature COMMIT


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Two-Phase Commit

Protocol Distributed databases make it possible for a

transaction to access data at several sites

Final COMMIT must not be issued until allsites have committed their parts of the

transaction

Two-phase commit protocol requires each

individual DPs transaction log entry be

written before the database fragment is

actually updated


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PerformanceTransparencyand QueryOptimization Objective of query optimization

routine is to minimize total cost

associated with the execution of arequest

Costs associated with a request

are a function of the:

Access time (I/O) cost

Communication cost

CPU time cost


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Performance Transparency

and QueryOptimization (continued) Must provide distribution transparency as well as

replica transparency

Replica transparency:DDBMSs ability to hide the existence of multiple

copies of data from the user

Query optimization techniques:

Manual or automatic

Static or dynamic

Statistically based or rule-based algorithms


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Design Data fragmentation:

How to partition the database into fragments

Data replication:

Which fragments to replicate

Data allocation:

Where to locate those fragments and replicas


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Data Fragmentation

Breaks single object into two or more

segments or fragments

Each fragment can be stored at any site overa computer network

Information about data fragmentation is

stored in the distributed data catalog (DDC),from which it is accessed by the TP to

process user requests


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Data Fragmentation

Strategies Horizontal fragmentation:

Division of a relation into subsets (fragments)

of tuples (rows)

Vertical fragmentation:

Division of a relation into attribute (column)

subsets

Mixed fragmentation:

Combination of horizontal and vertical

strategies


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A Sample CUSTOMER

Table

H i t l


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HorizontalFragmentation of the

CUSTOMERTable byState


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Table Fragments in

Three Locations


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Vertically Fragmented

Table Contents


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Mixed Fragmentation oftheCUSTOMERTable


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Data Replication

Storage of data copies at multiple sites served

by a computer network

Fragment copies can be stored at several sitesto serve specific information requirements

Can enhance data availability and response time

Can help to reduce communication and total

query costs


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Table Contents After theMixed FragmentationProcess


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Data Replication


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Replication Scenarios Fully replicated database:

Stores multiple copies ofeach databasefragment at multiple sites

Can be impractical due to amount of overhead

Partially replicated database:Stores multiple copies ofsome database

fragments at multiple sites

Most DDBMSs are able to handle the partiallyreplicated database well

Unreplicated database:Stores each database fragment at a single

site

No duplicate database fragments


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Data Allocation

Deciding where to locate data Allocation strategies:

Centralized data allocation Entire database is stored at one site

Partitioned data allocation Database is divided into several disjointed parts

(fragments) and stored at several sites

Replicated data allocation Copies of one or more database fragments are

stored at several sites

Data distribution over a computer network isachieved through data partition, datareplication, or a combination of both


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Client/Server vs. DDBMS

Way in which computers interact to form a

system

Features a userof resources, or a client, and

a providerof resources, or a server

Can be used to implement a DBMS in which

the client is the TP and the server is the DP


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Client/Server Advantages Less expensive than alternate minicomputer ormainframe solutions

Allow end user to use microcomputers GUI,thereby improving functionality and simplicity

More people with PC skills than with mainframe

skills in the job market

PC is well established in the workplace

Numerous data analysis and query tools exist tofacilitate interaction with DBMSs available in the

PC market Considerable cost advantage to offloading

applications development from the mainframe topowerful PCs


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Client/Server Disadvantages

Creates a more complex environment, in whichdifferent platforms (LANs, operating systems,

and so on) are often difficult to manage

An increase in the number of users and

processing sites often paves the way for securityproblems

Possible to spread data access to a much wider

circle of users increases demand for people with

broad knowledge of computers and softwareincreases burden of training and cost of

maintaining the environment


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C. J. Dates TwelveCommandments forDistributed Databases

1. Local site independence

2. Central site independence

3. Failure independence

4. Location transparency5. Fragmentation transparency

6. Replication transparency

7. Distributed query processing

8. Distributed transaction processing

9. Hardware independence

10. Operating system independence

11. Network independence

12. Database independence


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Summary

Distributed database stores logically related

data in two or more physically independent

sites connected via a computer network

Database is divided into fragments

Distributed databases require distributed

processing

Main components of a DDBMS are the

transaction processor and the data processor


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Summary (continued)

Current database systems can be classified byextent to which they support processing anddata distribution

DDBMS characteristics are best described as a

set of transparencies A transaction is formed by one or more

database requests

A database can be replicated over several

different sites on a computer network Client/server architecture refers to the way in

which two computers interact over a computernetwork to form a system

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