Client-Server Processing and

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Chapter 17

• Client-Server Processing and
• Distributed Databases

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Outline

• Overview of Distributed Processing and
  Distributed Data
• Client-Server Database Architectures
• Web Database Connectivity
• Architectures for Distributed Database Management
  Systems
• Transparency for Distributed Database Processing
• Distributed Database Processing

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Evolution of Distributed Processing and
Distributed Data

• Need to share resources across a network
• Timesharing (1970s)
• Remote procedure calls (1980s)
• Client-server computing (1990s)

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Timesharing Network
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Resource Sharing with a Network of Personal
Computers
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Client-Server Processing with Distributed
processing only
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Distributed processing and data
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Motivation for Distributed Processing

• Flexibility the ease of maintaining and adapting
  a system
• Scalability the ability to support scalable
  growth of hardware and software capacity
• Interoperability open standards that allow two
  or more systems to exchange and use software and
  data

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Motivation for Distributed Data

• Data control locate data to match an
  organizations structure
• Communication costs locate data close to data
  usage to lower communication cost and improve
  performance
• Reliability increase data availability by
  replicating data at more than one site

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Summary of Distributed Processing and Data
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Client-Server Database Architectures

• Client-Server Architecture is an arrangement of
  components (clients and servers) among computers
  connected by a network.
• A client-server architecture supports efficient
  processing of messages (requests for service)
  between clients and servers.

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Design Issues

• Division of processing the allocation of tasks
  to clients and servers.
• Process management interoperability among
  clients and servers and efficiently processing
  messages between clients and servers.
• Middleware software for process management

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Tasks to Distribute

• Presentation code to maintain the graphical user
  interface
• Validation code to ensure the consistency of the
  database and user inputs
• Business logic code to perform business
  functions
• Workflow code to ensure completion of business
  processes
• Data access code to extract data to answer
  queries and modify a database

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Middleware

• A software component that performs process
  management.
• Allow clients and servers to exist on different
  platforms.
• Allows servers to efficiently process messages
  from a large number of clients.
• Often located on a dedicated computer.

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Client-Server Computing with Middleware
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Types of Middleware

• Transaction-processing monitors relieve the
  operating system of managing database processes
• Message-oriented middleware maintain a queue of
  messages
• Object-request brokers provide a high level of
  interoperability and message intelligence
• Data access middleware provide a uniform
  interface to relational and non relational data
  using SQL

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Two-Tier Architecture
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Two-Tier Client-Server Architecture

• A PC client and a database server interact
  directly to request and transfer data.
• The PC client contains the user interface code.
• The server contains the data access logic.
• The PC client and the server share the validation
  and business logic.

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Three-Tier Architecture (Middleware Server)
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Three-Tier Architecture (Application Server)
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Three-Tier Architecture

• To improve performance, the three-tier
  architecture adds another server layer either by
  a middleware server or an application server.
• The additional server software can reside on a
  separate computer.
• Alternatively, the additional server software can
  be distributed between the database server and PC
  clients.

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Multiple-Tier Architecture

• A client-server architecture with more than three
  layers a PC client, a backend database server,
  an intervening middleware server, and application
  servers.
• Provides more flexibility on division of
  processing
• The application servers perform business logic
  and manage specialized kinds of data such as
  images.

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Multiple-Tier Architecture
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Multiple-Tier Architecture with Software Bus
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Web Database Connectivity

• Internet commerce depends heavily on database
  access for websites.
• Web database connectivity allows a database to be
  manipulated through a Web page.
• A user may use a Web form to change a database or
  view a report generated from a database.

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Internet Basics

• Network of networks
• Uses standard protocols TCP/IP
• TCP splits messages into packets
• IP routes messages
• Each computer on the Internet has a unique
  numeric address known as an IP address.

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Internet and Intranet Relationship
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World Wide Web

• Most popular application on the Internet
• Supports browsing pages located on any computer
  on the Internet
• Hypertext Transport Protocol (HTTP) establishes a
  session between a browser and a Web server.
• Each page has a unique address known as a URL.

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Web Page Request Cycle
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XML/XSL

• Solutions to HTML limitations
• eXtensible Markup Language (XML)
• Separates content and structure of a document
• Use document type declaration or schema to
  specify document structure
• eXtensible Style Language (XSL) supports
  transformation into display languages
• Both are extensible languages

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The Common Gateway Interface (CGI)

• CGI is an interface that allows a Web server to
  invoke an external program on the same computer.
• The external program uses the parameters passed
  by the Web server to produce output that is sent
  back to the browser.
• Usually, the output contains HTML/XML so that the
  browser can display it properly.

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Straight CGI
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Hybrid CGI
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Server-side connectivity

• Server-side connectivity bypasses the external
  program needed with the CGI approaches.
• Specialized Web server or middleware server is
  needed
• SQL statements and database logic are kept in a
  web page or external file.
• The database code can execute stored procedures
  on the database server.

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Server-Side Connectivity Approach
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Server-Side Connectivity with a Middleware Server
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Client-Side Connectivity

• Client computing capacity can be more fully
  utilized without storing code on the client.
• Provides a more customized interface than
  permitted by HTML
• Supports data buffering by the client to improve
  performance

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Web Page Request Cycle with Client-Side
Connectivity
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Summary of Web Connectivity
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Architectures for Distributed Database Management
Systems

• DBMSs need fundamental extensions.
• Underlying the extensions are a different
  component architecture and a different schema
  architecture.
• Component Architecture manages distributed
  database requests.
• Schema Architecture provides additional layers of
  data description.

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Global Requests
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Component Architecture
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Schema Architecture I
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Schema Architecture II
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Transparency for Distributed Database Processing

• Transparency is related to data independence.
• With transparency, users can write queries with
  no knowledge of the distribution, and
  distribution changes will not cause changes to
  existing queries and transactions.
• Without transparency, users must reference some
  distribution details in queries and distribution
  changes can lead to changes in existing queries.

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Motivating Example
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Fragments Based on the CustRegion Field
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Fragments Based on the WareHouseNo Field
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Fragmentation Transparency

• Fragmentation transparency provides the highest
  level of data independence.
• Users formulate queries and transactions without
  knowledge of fragments (locations, or local
  formats).
• If fragments change, queries and transactions are
  not affected.

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Location Transparency

• Location transparency provides a lesser level of
  data independence than fragmentation
  transparency.
• Users need to reference fragments in formulating
  queries and transactions.
• However, knowledge of locations and local formats
  is not necessary.

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Local Mapping Transparency

• Local mapping transparency provides a lesser
  level of data independence than location
  transparency.
• Users need to reference fragments at sites in
  formulating queries and transactions.
• However, knowledge of local formats is not
  necessary.

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Distributed Database Processing

• Distributed data adds considerable complexity to
  query processing and transaction processing.
• Distributed database processing involves movement
  of data, remote processing, and site
  coordination.
• Performance implications sometimes cannot be
  hidden.

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Distributed query processing

• Involves both local (intra site) and global
  (inter site) optimization.
• Multiple optimization objectives
• The weighting of communication costs versus local
  processing costs depends on network
  characteristics.
• There are many more possible access plans for a
  distributed query.

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Distributed Transaction Processing

• Distributed DBMS provides concurrency and
  recovery transparency.
• Independently operating sites must be
  coordinated.
• New kinds of failures exist because of the
  communication network.
• New protocols are necessary.

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Distributed Concurrency Control

• The simplest scheme involves centralized
  coordination.
• Centralized coordination involves the fewest
  messages and the simplest deadlock detection.
• The number of messages can be twice as much in
  distributed coordination.
• Primary Copy Protocol is used to reduce overhead
  with locking multiple copies.

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Centralized Coordination
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Distributed Recovery Management

• Distributed DBMSs must contend with failures of
  communication links and sites.
• Detecting failures involves coordination among
  sites.
• The recovery manager must ensure that different
  parts of a partitioned network act in unison.
• The protocol for distributed recovery is the two
  phase commit protocol (2PC).

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Voting and Decision Phases
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Summary

• Utilizing distributed processing and data can
  significantly improve DBMS services but at the
  cost of new design challenges.
• Several client-server architectures provide
  alternatives among cost, complexity, and benefit
  levels.
• Architectures for distributed DBMSs differ in the
  integration of the local databases and level of
  data independence.