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Stumpf and TeagueObject-Oriented Systems Analysis and Design with

UML

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Chapter 7I nform ation System Design

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Learning Objectives

• Describe how analysts and designers view system requirements from different perspectives.

• Explain why analysts and designers view the boundary between analysis and design differently than managers do.

• State some goals of information system design.

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Learning Objectives (continued)

• Distinguish among batch, online, interactive, and real-time systems.

• Name and state the purpose of each of the layers of a three-tier system architecture.

• Give examples of each of the three generic types of hardware components of a computer information system.

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Learning Objectives (continued)

• Explain the function of infrastructure and administration components in a real information processing system.

• Discuss what additional design decisions and system components are required in distributed systems.

• Describe the principal subsystems into which a computer information system is partitioned.

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Overview

Analysis defines users’ requirements for a new information processing system.

The principal goal of design is to specify a realizable information processing system which satisfies the performance standards of the requirements specification.

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Overview (continued)

Designing the overall system structure involves partitioning the system into three major subsystems – the application programs, the user interface, and the data base.

Thereafter, these subsystems may be treated as relatively independent design problems.

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Overview (continued)

This partitioning is implemented as a layered architecture, which is considered to be best practice.

The hardware of an information- processing system consists of three generic types of components: channels (which transport information), containers (which store information), and processors (which transform information).

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Overview (continued)

A real information-processing system must contain components which compensate for the imperfections of the hardware.

Before the end of design, the design must be made consistent with the decisions about the hardware and system software environment and with the specifics of the system acceptance tests.

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System Design in theRational Unified Process

FI GURE 7.1

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The Transition from Analysis to Design

Analyst’s Viewpoint: Analysis defines a problem. It focuses on an essential description of what the system must do.

Designer’s Viewpoint: Analysis offers clues to a solution and to tests to determine whether a proposed design is satisfactory.

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Technical and ManagementIssues in the Transition

• The technical criterion for exiting analysis is: “Have all the users’ requirements been adequately defined?”

• The managerial criterion for exiting analysis is: “Is the project worth continuing?”

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Goals of ComputerInformation System Design

1. Solving the problem posed in the requirements specification

2. Satisfying the performance requirements determined during systems analysis

3. Deriving an automated system whose structure fits the structure of the problem

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Goals of Computer Information System Design

(continued)

4. Considering alternative system designs to select the one most suitable for the organization

5. Matching the application software design to the hardware and system software environment in which it will operate

6. Creating a system whose structure makes it easy to understand, construct, and modify

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Characteristics of a System Design Specification

• Explicit: spells out what is critical• Complete: in scope and detail• Unambiguous: only one possible

interpretation• Consistent: no internal conflicts• Accurate: no mistakes requiring

subsequent correction• Minimally redundant: facilitates

changes to the design

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Batch, Interactive, and Real-Time Systems

In a batch system the system inputs are stored at or near the system boundary. They are processed in batches. The storage introduces a time delay.

In an interactive system inputs enter the system one at a time. Time delays are handled internally.

A real-time system responds rapidly enough for its output to affect or control events in its environment.

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The Three- Tier Layered System

ArchitectureFI GURE 7.5

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Generic Hardware Components

• Processors: transform information• Containers: store data• Channels: transport information

Appropriately allocating channels, containers, and processors to each of the layers of the three-tier architecture is the fundamental design problem at the system level.

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Channels, Containers, and Processors

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FI GURE 7.6

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Technology of a Real Information Processing

SystemIn contrast to an essential system,

a real system must incorporate additional components to compensate for imperfections in real-world technology.

• Infrastructure provides additional communication.

• Administration provides additional quality control and coordination.

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Distributed Systems

Distributed systems have processors or data bases in more than one location.

They are usually organized as client/server systems with three types of components:

• Client machine requests a service from another machine in the system.

• Server machine carries out the request and returns the results to the client.

• Communications network transmits messages between client and server.

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Communications Networks

• Network types: Local, metropolitan, wide area networks

• Network protocols: Specify formats for messages between processes

• Network components: Routers, switches, hubs, modems, codecs, multiplexers

• Telecommunications software: Manages the communications in distributed systems

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Other Components of an Implementation

• Application software: Carries out application-specific transformations

• System software: Manages computer resources and supports applications

• Data base: Stores data in an organized structure in non-volatile memory

• User interface: Enables communication between humans and computers

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Activities of System Design

.FI GURE 7.9

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System Acceptance Tests

The system acceptance tests measure whether a completed system is acceptable to its users.

They are based on the performance requirements specified in analysis.

Ideally, the tests should be defined by people who are not responsible for developing the system.

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Activities of Designing the System

Structure1. Specify the external interface.

2. Allocate essential use cases to processors.

3. Allocate stored data to storage devices.

4. Establish the infrastructure for communication.

5. Add the intra- and interprocessor administration.

6. Select the best system structure.

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Alternative A – Public University System

. FI GURE 7.13

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Alternative B – Public University System

. FI GURE 7.14

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Alternative C – Public University System

. FI GURE 7.15

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Summary

A layered system architecture of at least three tiers represents best practice for information system design.

The fundamental system design problem is to appropriately allocate processors, containers, and channels to each layer of this architecture.

Once the system architecture is defined, the design may be partitioned into program, database, and user interface design problems.