Week 5 Lecture

Distributed Database Management Systems

Samuel Conn, Asst Professor

Suggestions for using the Lecture Slides

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In this lecture, 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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Evolution of DDBMS

Decentralized database management systems (DDBMS) Interconnected computer systems Data/processing functions reside on multiple sites

1970’s: Centralized DBMS 1980’s: Social and Technical Changes

Ad hoc capability required Decentralized management structure common

1990’s: New forces Internet and the World Wide Web used for data access and

distribution Data analysis through data mining and data warehousing

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DDBMS Advantages

Data located near site with greatest demand

Faster data access Faster data processing Growth facilitation Improved communications Reduced operating costs User-friendly interface Less danger of 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 data

environment Increased training costs

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Distributed Processing

Shares database’s logical processing among physically, networked independent sites

Figure 10.1

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Distributed Database

Stores logically related database over physically independent sites

Figure 10.2

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Distributed Database vs. Distributed Processing

Distributed processing Does not require distributed database May be based on a single database on single

computer Copies or parts of database processing

functions must be distributed to all data storage sites

Distributed database Requires distributed processing

Both Require a network to connect components

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Functions of DDBMS

Application/end user interface Validation to analyze data requests Transformation to determine request components Query optimization to find the best access

strategy Mapping to determine the data location I/O interface to read or write data Formatting to prepare the data for presentation Security to provide data privacy Backup and recovery DB Administration Concurrency Control Transaction Management

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Centralized Database

Figure 10.3

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Fully Distributed Database Management System

Figure 10.4

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DDBMS Components

Computer workstations Network hardware and software

components Communications media Transaction processor (TP)

Also called application manager (AP) or transaction manager (TM)

Data processor (DP) Also called data manager (DM)

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Distributed Database Components

Figure 10.5

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DDBMS Protocols

Interface with network to transport data and commands between DPs and TPs

Synchronize data received from DPs and route to appropriate TPs

Ensure common database functions Security Concurrency control Backup and recovery

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Levels of Data and Process Distribution

Database systems can be classified based on process distribution and data distribution

Table 10.1

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

All processing on single CPU or host computer

All data are stored on host computer disk

DBMS located on the host computer DBMS accessed by dumb terminals Typical of mainframe and minicomputer

DBMSs Typical of 1st generation of single-user

microcomputer database

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Single-Site Processing, Single-Site Data (con’t.)

Figure 10.6

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

• Requires network file server • Applications accessed through LAN • Variation known as client/server architecture

Figure 10.7

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

Fully distributed DDBMS with support for multiple DPs and TPs at multiple sites Homogeneous I

• Integrate one type of centralized DBMS over the network

Heterogeneous • Integrate different types of centralized

DBMSs over a network

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Heterogeneous Distributed Database Scenario

Figure 10.8

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Distributed DB Transparency

Allows end users to feel like only database user

Hides complexities of distributed database

Transparency features Distribution Transaction Failure Performance Heterogeneity

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Distribution Transparency

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

Three Levels Fragmentation transparency Location transparency Local mapping transparency

Table 10.2

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Transaction Transparency

Ensures transactions maintain integrity and consistency

Completed only if all involved database sites complete their part of the transaction

Management mechanisms Remote request Remote transaction Distributed transaction Distributed request

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Remote Request

Figure 10.10

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Remote Transaction

Figure 10.11

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Distributed Transaction

Figure 10.12

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Distributed Requests

Figure 10.13

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Distributed Requests (con’t.)

Figure 10.14

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Distributed Concurrency Control

Multi-site, multiple-process operations more likely to create data inconsistencies and deadlocked transactions

Problems Transaction committed by local DP One DP could not commit transaction’s result Yields inconsistent database

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Two-Phase Commit Protocol

DO-UNDO-REDO protocol Write-ahead protocol Two kinds of nodes

• Coordinator • Subordinates

Phases Preparation

• Coordinator sends message to all subordinates • Confirms all are ready to commit or abort

Final Commit • Ensures all subordinates have committed or aborted

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Performance Transparency and Query Optimization

Objective: Minimize total cost associated with execution of request

Main costs Access time Communication CPU time

Basis for query optimization algorithms Optimum execution order Sites accessed to minimize communication costs

Dynamic or static optimization Statistically based vs. rule-based query

optimization algorithms

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Distributed Database Design

Partition database into fragments Horizontal Vertical Mixed

Fragments to replicate Storage of data copies at multiple sites Fully, partially, un-replicated databases

Data allocation Where to locate data Centralized, partitioned, replicated

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Client/Server Advantages Over DDBMS

Client/server less expensive Client/server solutions allow use of

microcomputer’s GUI More people with PC skills than

mainframe skills PC is well established in workplace Numerous data analysis and query tools

exist Considerable cost advantages to off-

loading application development

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Client/Server Disadvantages

Creates more complex environment with different platforms Increased number of users and sites creates security problems Training issues become more complex and expensive

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Date’s 12 Commandments for Distributed Databases

1. Local Site Independence 2. Central Site Independence 3. Failure Independence 4. Location Transparency 5. Fragmentation Transparency

6. Replication Transparency

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Date’s 12 Commandments for Distributed Databases

7. Distributed Query Processing 8. Distributed Transaction Processing 9. Hardware Independence 10. Operating System Independence 11. Network Independence

12. Database Independence