Distributed Database Management Systems

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Chapter 12
Distributed Database Management
Systems Database Systems Design,
Implementation, and Management, Seventh Edition,
Rob and 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
  databases 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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DDBMS Advantages and Disadvantages (continued)

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

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DDBMS Advantages and Disadvantages (continued)
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DDBMS Advantages and Disadvantages (continued)
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DDBMS Advantages and Disadvantages (continued)
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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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Characteristics of Distributed Management Systems
(continued)

• 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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Characteristics of Distributed Management Systems
(continued)
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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 computers local disk
• Processing cannot be done on end users 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 databases 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 databases 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 Requests and Distributed Transactions
(continued)
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Distributed Requests and Distributed Transactions
(continued)
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Distributed Requests and Distributed Transactions
(continued)
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Distributed Requests and Distributed Transactions
(continued)
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Distributed Requests and Distributed Transactions
(continued)
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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 DPs 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
• DDBMSs 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 Fragmentation (continued)
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Data Fragmentation (continued)
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Data Fragmentation (continued)
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Data Fragmentation (continued)
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Data Fragmentation (continued)
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Data Fragmentation (continued)
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Data Fragmentation (continued)
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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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Data Replication (continued)

• 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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Data Replication (continued)

• 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 microcomputers 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 vs. DDBMS (continued)

• 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 vs. DDBMS (continued)

• 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 vs. DDBMS (continued)

• 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. Dates 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. Dates 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