PowerPoint Chapter 12

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Chapter 12

Distributed Database Management Systems

Database Systems: Design, Implementation, and Management,

Seventh Edition, Rob and Coronel

Database Systems: Design, Implementation, & Management, 7th Edition, Rob & Coronel

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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


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The Evolution of Distributed Database Management Systems

• Distributed database management system (DDBMS) – Governs storage and processing of logically

related data over interconnected computer systems in which both data and processing functions 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 database’s shortcomings spawned a demand for applications based on data access from different sources at multiple locations


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The Evolution of Distributed Database Management Systems (continued)


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DDBMS Advantages and Disadvantages

• Advantages include:– Data are located near “greatest demand” site– Faster data access– Faster data processing – Growth facilitation – Improved communications


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DDBMS Advantages and Disadvantages (continued)

• Advantages include (continued):– Reduced operating costs – User-friendly interface – Less danger of a single-point failure – Processor independence


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• Disadvantages include:– Complexity of management and control – Security – Lack of standards– Increased storage requirements – Increased training cost


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Characteristics of Distributed Management Systems

• Application interface

• Validation

• Transformation

• Query optimization

• Mapping

• I/O interface


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Characteristics of Distributed Management Systems (continued)

• Formatting

• Security

• Backup and recovery

• DB administration

• Concurrency control

• Transaction management


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• Must perform all the functions of centralized DBMS

• Must handle all necessary functions imposed by distribution of data and processing– Must perform these additional functions

transparently to the end user


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DDBMS Components

• Must include (at least) the following components:– Computer workstations – Network hardware and software – Communications media – Transaction processor (application processor,

transaction manager)• Software component found in each computer

that requests data


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DDBMS Components (continued)

• Must include (at least) the following components (continued):– 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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DDBMS Components (continued)


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Levels of Data and Process 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 computer’s local disk

• Processing cannot be done on end user’s side of system


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Single-Site Processing, Single-Site Data (SPSD) (continued)

• Typical of most mainframe and midrange computer DBMSs

• DBMS is located on host computer, which is accessed by dumb terminals connected to it

• Also typical of first generation of single-user microcomputer databases


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Single-Site Processing, Single-Site Data (SPSD) (continued)


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Multiple-Site Processing, Single-Site Data (MPSD)

• Multiple processes run on different computers sharing single data repository

• MPSD scenario requires network file server running conventional applications that are accessed through LAN

• Many multiuser accounting applications, running under personal computer network, fit such a description


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Multiple-Site Processing, Single-Site Data (MPSD) (continued)


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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, or network) running under different computer systems, such as mainframes and microcomputers


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


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Distributed Database Transparency Features

• Allow end user to feel like database’s only user

• Features include: – Distribution transparency– Transaction transparency– Failure transparency– Performance transparency– Heterogeneity transparency


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Distribution Transparency

• Allows management of physically dispersed database as though it were a centralized database

• Following three levels of distribution transparency are recognized:– Fragmentation transparency – Location transparency – Local mapping transparency


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Distribution Transparency (continued)


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Distribution Transparency (continued)


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Transaction Transparency

• Ensures database transactions will maintain distributed database’s integrity and consistency


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Distributed Requests and Distributed Transactions

• Distributed transaction – Can update or request data from several

different remote sites on network

• Remote request– Lets single SQL statement access data to be

processed by single remote database processor

• Remote transaction– Accesses data at single remote site


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Distributed Requests and Distributed Transactions (continued)

• Distributed transaction – Allows transaction to reference several

different (local or remote) DP sites

• Distributed request– Lets single SQL statement reference data

located at several different local or remote DP sites


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Distributed Concurrency Control

• Multisite, multiple-process operations are much more likely to create data inconsistencies and deadlocked transactions than are single-site systems


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Distributed Concurrency Control (continued)


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Two-Phase Commit Protocol

• Distributed databases make it possible for transaction to access data at several sites

• Final COMMIT must not be issued until all sites have committed their parts of transaction

• Two-phase commit protocol requires each individual DP’s transaction log entry be written before database fragment is actually updated


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Performance Transparency and Query Optimization

• Objective of query optimization routine is to minimize total cost associated with execution of request

• Costs associated with request are function of:– Access time (I/O) cost – Communication cost – CPU time cost

• Must provide distribution transparency as well as replica transparency


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Performance Transparency and Query Optimization (continued)

• Replica transparency – DDBMS’s ability to hide existence of multiple

copies of data from user

• Query optimization techniques include: – Manual or automatic– Static or dynamic – Statistically based or rule-based algorithms


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Distributed Database Design

• Data fragmentation – How to partition 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 over computer network

• Information about data fragmentation is stored in distributed data catalog (DDC), from which it is accessed by TP to process user requests


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Data Fragmentation (continued)

• 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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Data Replication

• Storage of data copies at multiple sites served by computer network

• Fragment copies can be stored at several sites to serve specific information requirements– Can enhance data availability and response

time– Can help to reduce communication and total

query costs


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Data Replication (continued)


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• Replication scenarios– Fully replicated database

• Stores multiple copies of each database fragment at multiple sites

• Can be impractical due to amount of overhead – Partially replicated database

• Stores multiple copies of some database fragments at multiple sites

• Most DDBMSs are able to handle the partially replicated database well


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• Replication scenarios (continued)– Unreplicated database

• Stores each database fragment at 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


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Data Allocation (continued)

• Allocation strategies (continued)– Replicated data allocation

• Copies of one or more database fragments are stored at several sites

• Data distribution over computer network is achieved through data partition, data replication, or combination of both


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Client/Server vs. DDBMS

• Way in which computers interact to form system

• Features user of resources, or client, and provider of resources, or server

• Can be used to implement a DBMS in which client is the TP and server is the DP


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Client/Server vs. DDBMS (continued)

• Client/server advantages– Less expensive than alternate minicomputer

or mainframe solutions– Allow end user to use microcomputer’s GUI,

thereby improving functionality and simplicity– More people in job market have PC skills than

mainframe skills– PC is well established in workplace


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• Client/server advantages (continued)– Numerous data analysis and query tools exist

to facilitate interaction with DBMSs available in PC market

– Considerable cost advantage to offloading applications development from mainframe to powerful PCs


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• Client/server disadvantages– Creates more complex environment

• Different platforms (LANs, operating systems, and so on) are often difficult to manage

– An increase in number of users and processing sites often paves the way for security problems


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• Client/server disadvantages (continued)– Possible to spread data access to much wider

circle of users• Increases demand for people with broad

knowledge of computers and software• Increases burden of training and cost of

maintaining the environment


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C. J. Date’s Twelve Commandments for Distributed Databases

• Local site independence

• Central site independence

• Failure independence

• Location transparency

• Fragmentation transparency

• Replication transparency


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C. J. Date’s Twelve Commandments for Distributed Databases (continued)

• Distributed query processing

• Distributed transaction processing

• Hardware independence

• Operating system independence

• Network independence

• Database independence


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Summary

• Distributed database stores logically related data in two or more physically independent sites connected via computer network

• Distributed processing is division of logical database processing among two or more network nodes

• Distributed databases require distributed processing

• Main components of DDBMS are transaction processor and data processor


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Summary (continued)

• Current database systems can be classified by extent to which they support processing and data distribution

• Homogeneous distributed database system integrates only one particular type of DBMS over computer network

• Heterogeneous distributed database system integrates several different types of DBMSs over computer network


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Summary (continued)

• DDBMS characteristics are best described as set of transparencies

• Transaction is formed by one or more database requests

• Distributed concurrency control is required in network of distributed databases

• Distributed DBMS evaluates every data request to find optimum access path in distributed database


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Summary (continued)

• The design of distributed database must consider fragmentation and replication of data

• Database can be replicated over several different sites on computer network

• Client/server architecture refers to way in which two computers interact over computer network to form a system

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PowerPoint Chapter 12