Computer Software Design -1.2.pptx

7/30/2019 Computer Software Design -1.2.pptx

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Designing Computer

Software Applications


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Outline

The Nature of Software

The Role of Design

Design Process

Software Development Models

The Waterfall Model Advantages & Disadvantages

The Spiral Model Advantages & Disadvantages

The Rapid Prototyping Model Advantages &

Disadvantages

Software Development Lifecycle

Objectives of a Design Document

Example


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The Nature of Software

Software lies at the nerve centre of most engineered artifacts and business

processes, i.e. from consumer electronics to financial modelling, and from

automotives to medical applications

A single defect in millions of lines of code can result in a system failure (safety

critical systems are and exception)

Typically 50% of project costs are allocated to software design, of which 50%

are spent on testing

Software is among the most complex of engineered artifacts


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The Role of Design

Requirements Design Models Code


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Design Process

Architectural design: deciding on the subsystems and their

relationships

Subsystem design: provide an abstract specification for each

subsystem

Component design: decomposition of subsystems into components

Interface design: define the interface for each subsystem

Data structure design: data structuring decisions

Algorithm design: algorithmic decisions


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Software Development Models

The 3 key Software Development Models are:

Waterfall Model Involves Sequential process of analysis,design, implementation, testing, and deployment

Spiral Model Breaks down process steps into multiplephases; early phases involve construction of prototypes

Rapid Prototyping Model Customer OrientedDevelopment; Emphasizes Requirements, Analysis andValidation


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The Waterfall Model


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Advantages

Easy to understand and implement

Widely used and known (in theory!)

Reinforces good habits: define-before- design,

design-before-code

Identifies deliverables and milestones

Document driven, URD, SRD, etc. Published

documentation standards, e.g. PSS-05.


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Disadvantages

Unrealistic to expect accurate requirements so earlyin project

Software is delivered late in project, delays discoveryof serious errors

Difficult and expensive to make changes todocuments


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The Spiral Model


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Advantages

Accurately reflects the iterative nature of softwaredevelopment on projects with unclear requirements

Comprehensive model decreases risk

Good project visibility.


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Disadvantages

Needs technical expertise in risk analysis to reallywork

Model is poorly understood by non-technicalmanagement, hence not so widely used

Complicated model, needs competent professionalmanagement


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The Rapid Prototyping Model

Requirements Capture

Quick Design

Build Prototype

Customer Evaluation of

Prototype

Engineer FinalProduct

Iterate


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Advantages

Reduces risk of incorrect user requirements

Good where requirements are changing/uncommitted

Regular visible progress aids management

Supports early product marketing


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Disadvantages

Badly implemented prototype often becomes the finalproduct

Costs customers money

Difficult to know how long project will last


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Software Development Lifecycle

Standard Waterfall Model


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Feasibility & Analysis

Decide what the project is suppose to do

Do not think about how the program willaccomplish tasks

Output: requirements document

Describes what program will do oncecompleted

User manual: tells how user will operate

program

Performance criteria


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Design

Plan how to implement the system

Discover structures that underlie problemto be solved

Decide what classes and methods you need

Output:

Description of classes and methods

Diagrams showing the relationshipsamong the classes


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Implementation

Write and compile the code

Code implements classes and methodsdiscovered in the design phase

Output: completed program


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Testing

Run tests to verify the program workscorrectly

Output: a report of the tests and their

results


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Deployment & Maintenance

Users install program

Users use program for its intended purpose

Developers support the program for defects

Developer resolve defects


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Objectives of a Design Document

A formal design document in any engineering project is used to : -

Provide an overall guidance of the architecture of the software project

Coordinate a large team under a single vision

Outline all parts of the software and how they will work

Give a fairly complete description while maintaining a high-level view ofthe application

Convey the technical and operational requirements of am IT system

coupled with specifications, concepts, analysis, diagrams, and schematics

Reflect the users perspective of the system design


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Software Development Cycle Funny

Example


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ThankYou

Computer Software Design -1.2.pptx

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