Chapter 2 SW Process Models

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Chapter 2 SW Process Models

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Objectives Understand various process models Understand the pros and cons of each model Evaluate the applicability of each model for a

certain type of software development

Overview Why SW process models? What are SW process models? The waterfall model Evolutionary models The Spiral CBSE Which is best?

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Why SW process models? Some problems with ad-hoc development

Difficult to distinguish between tasks and therefore some important tasks may be ignored

Problem if multiple people are involved, as it is not easy to review their work.

If problems are found late, the more costly it is to fix.

Division of work Helps in managing the process

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What is it? A SW process is a structured set of activities

required to develop a software system Specification; Design and Implementation; Validation; Evolution.

A software process model is an abstract representation of a process. It presents a description of a process from some particular perspective.

May take months or years to complete.

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SW Process Models Waterfall model

Separate and distinct phases of specification and development. Evolutionary models

Specification, development and validation are interleaved. Exploratory development The incremental model The prototyping model The spiral model

Component-based development model The system is assembled from existing components

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The Waterfall Model Oldest model Called “linear sequential model” Most widely used model for SW engineering Documentation is produced at each stage.

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The Waterfall model phases Requirements analysis and definition System and software design Implementation and unit testing Integration and system testing Operation and maintenance

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The Waterfall model

Requirements

definition

System andsoftware design

Implementationand unit testing

Integration andsystem testing

Operation and

maintenance

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The Waterfall model Disadvantages Inflexible partitioning of the project into

distinct stages makes it difficult to respond to changing customer requirements.

Only appropriate when the requirements are well-understood and changes will be fairly limited during the design process.

The waterfall model is mostly used for large systems engineering projects.

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Evolutionary Models The Exploratory Model

Objective is to work with customers and to evolve a final system from an initial outline specification. Should start with well-understood requirements and add new features as proposed by the customer.

The Incremental Model Rather than deliver the system as a single delivery, the

development and delivery is broken down into increments with each increment delivering part of the required functionality.

The Prototyping Model Objective is to understand the system requirements. Should start with

poorly understood requirements to clarify what is really needed. The Spiral Model

Process is represented as a spiral rather than as a sequence of activities with backtracking

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The Exploratory ModelConcurrentactivities

ValidationFinal

version

DevelopmentIntermediate

versions

SpecificationInitial

version

Outlinedescription

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The Exploratory Model Problems

Lack of process visibility; Systems are often poorly structured;

Applicability For small or medium-size interactive systems; For parts of large systems (e.g. the user

interface); For short-lifetime systems.

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The Incremental Model User requirements are prioritised and the

highest priority requirements are included in early increments.

Once the development of an increment is started, the requirements are frozen though requirements for later increments can continue to evolve.

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The Incremental Model

Validateincrement

Develop systemincrement

Design systemarchitecture

Integrateincrement

Validatesystem

Define outline requirements

Assign requirements to increments

System incomplete

Finalsystem

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The Incremental Model- Advantages Customer value can be delivered with each

increment so system functionality is available earlier.

Early increments act as a prototype to help elicit requirements for later increments.

Lower risk of overall project failure. The highest priority system services tend to

receive the most testing.

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The Incremental Model- Disadvantages Increments should be relatively small (20,000

lines of code) Can be difficult to map the customer’s

requirements onto increments of the right size Hard to identify common functions

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The Prototyping Model When a customer defines a set of general

objectives for a software but does not identify detailed input, processing, or output requirement.

It consists of the iterating phases: Requirements gathering Design and build SW prototype Evaluate prototype with customer Refine requirements

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The Prototyping Model

Build/ revise prototype

Customer test drives prototype

Listen to customer

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The Prototyping Model Advantages

Users get a feel for the actual system Developers get to build something immediately Specifications can be developed incrementally

Disadvantages The developer may make implementation compromises in

order to get a prototype working quickly. The process in not visible (few documents that reflect

every version of the system) Systems poorly structured

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The Spiral Model Defined by Barry Boehm in his 1988 article A Spiral

Model of Software Development and Enhancement. Process is represented as a spiral rather than as a

sequence of activities with backtracking. Each loop in the spiral represents a phase in the

process. Suitable for large, expensive and complicated

projects

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The Spiral Model

Riskanalysis

Riskanalysis

Riskanalysis

Riskanalysis Proto-

type 1

Prototype 2

Prototype 3Opera-tionalprotoype

Concept ofOperation

Simulations, models, benchmarks

S/Wrequirements

Requirementvalidation

DesignV&V

Productdesign Detailed

design

Code

Unit test

IntegrationtestAcceptance

testService Develop, verifynext-level product

Evaluate alternatives,identify, resolve risks

Determine objectives,alternatives and

constraints

Plan next phase

Integrationand test plan

Developmentplan

Requirements planLife-cycle plan

REVIEW

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The Spiral Model Objective setting

Specific objectives for the phase are identified. Risk assessment and reduction

Risks are assessed and activities put in place to reduce the key risks.

Development and validation A development model for the system is chosen which can be

any of the generic models. Planning

The project is reviewed and the next phase of the spiral is planned.

The Spiral Model Risk driven process model

Different risk patterns can lead to choosing different process models

What is a risk? Situations or possible events that may cause a project to fail to

meet its goal. Example risks:

Experienced staff leave the project Hardware which is essential for the system will not be delivered on

schedule

(more about risks in Chapter 3)24

The Spiral Model- Advantages Risks are explicitly assessed and resolved

throughout the process. Software engineers can start working on the

project earlier rather than wading through a lengthy early design process.

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The Spiral Model- Disadvantages Requires highly skilled people in risk analysis

and planning Requires more time, and is more expensive Estimates of budget and time are harder to

judge at the beginning of the project since the requirements evolve through the process

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Component Based Software Engineering (CBSE) Based on systematic reuse where systems are

integrated from existing components or COTS (Commercial-off-the-shelf) systems.

Process stages Component analysis; Requirements modification; System design with reuse; Development and integration.

This approach is becoming increasingly used as component standards have emerged.

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Component Based Software Engineering (CBSE)

Requirementsspecification

Componentanalysis

Developmentand integration

System designwith reuse

Requirementsmodification

Systemvalidation

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Component Based Software Engineering (CBSE) Advantages:

Reduce amount of software to be developed Reduce costs and risks Faster delivery

Disadvantages: Requirements compromises, system does not

meet real needs of users Control over system evolution is lost

Which model is best? The choice of a model depends on the project

circumstances and requirements. A combination of models is used sometimes get the

benefits of more than one model. Criteria for evaluating models:

Risk management Quality / cost control Visibility of progress Early system functionality Customer involvement and feedback

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Model/Criteria Matrix Rate each model 1-5 in each of the categories shown:

WaterfallIncrementalPrototypingSpiralCBSE

Risk Management

QualityCost Control

System Functionality

Visibility of Progress

Customer Involvement

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Risk Management22152QualityCost Control

System Functionality



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Risk Management22152QualityCost Control43253System Functionality



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Risk Management22152QualityCost Control43253System Functionality15443Visibility of Progress


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Risk Management22152QualityCost Control43253System Functionality15443Visibility of Progress54243Customer Involvement

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Risk Management22152QualityCost Control43253System Functionality15443Visibility of Progress54243Customer Involvement24532

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Chapter 2 SW Process Models

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