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CS 425/625 Software EngineeringCS 425/625 Software Engineering

Software ProcessesSoftware Processes

Based on Chapter 3 of the textbook [Somm00]: Ian Sommerville, Software Engineering, 6th Ed., Addison-Wesley, 2000

and on Ch3 PowerPoint presentation fromwww.comp.lancs.ac.uk/computing/resources/IanS/SE6/Slides/index.html

September 3, 2003

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OutlineOutline

Software Process ModelsSoftware Process Models Waterfall modelWaterfall model Evolutionary developmentEvolutionary development Formal developmentFormal development Reuse-oriented developmentReuse-oriented development Incremental modelIncremental model Spiral modelSpiral model

Software Process ActivitiesSoftware Process Activities SpecificationSpecification Design and implementationDesign and implementation ValidationValidation EvolutionEvolution

Automated Process SupportAutomated Process Support

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Software Process ModelsSoftware Process Models

Software processSoftware process = organized set of activities = organized set of activities aimed at building a software systemaimed at building a software system

Software process modelSoftware process model = an abstract = an abstract representation of a software processrepresentation of a software process

Fundamental software process activities:Fundamental software process activities: Software specificationSoftware specification Software design and implementationSoftware design and implementation Software validationSoftware validation Software evolutionSoftware evolution

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Software Process Models: Waterfall..Software Process Models: Waterfall..

The Waterfall modelThe Waterfall model [Somm00, Fig 3.1][Somm00, Fig 3.1]

Requirementsdefinition

System andsoftware design

Implementationand unit testing

Integration andsystem testing

Operation andmaintenance

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Software Process Models: .Waterfall.Software Process Models: .Waterfall.

Main characteristics: Also called classic software life cycle or

sequential model Process activities (phases/stages) are clearly

separated After a number of iterations, phases of the life

cycle (such as specification and design) are “frozen”

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Software Process Models: ..WaterfallSoftware Process Models: ..Waterfall

Advantages: Organized approach, provides robust separation of

phases Reflects common engineering practice

Disadvantages:Disadvantages: Doesn’t cope well with changes required by the client Doesn’t cope well with changes required by the client Development teams might wait for each otherDevelopment teams might wait for each other A working version of the product is available only lateA working version of the product is available only late

Applicability:Applicability: When requirements are well known and few changes When requirements are well known and few changes

are likely to be neededare likely to be needed Can be used also for parts of larger software systemsCan be used also for parts of larger software systems

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Software Process Models: Software Process Models: Evolutionary Development…Evolutionary Development…

Evolutionary Development model Evolutionary Development model [Somm00, Fig 3.2][Somm00, Fig 3.2]

ValidationFinal

version

DevelopmentIntermediate

versions

SpecificationInitial

version

Outlinedescription

Concurrentactivities

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Software Process Software Process Models: .Evolutionary Development..Models: .Evolutionary Development..

Main characteristics: The phases of the software construction are

interleaved Feedback from the user is used throughout the entire

process The software product is refined through many versions

Types of evolutionary development: Exploratory development Throw-away prototyping

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Software Process Software Process Models: ..Evolutionary Development.Models: ..Evolutionary Development.

Advantages: Deals constantly with changes Provides quickly an initial version of the system Involves all development teams

Disadvantages: Quick fixes may be involved “Invisible” process, not well-supported by

documentation The system’s structure can be corrupted by

continuous change

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Software Process Models: …Software Process Models: …Evolutionary DevelopmentEvolutionary Development

Disadvantages [cont’d]: Special tools and techniques may be necessary The client may have the impression the first

version is very close to the final product and thus be less patient

Applicability: When requirements are not well understood When the client and the developer agree on a

“rapid prototype” that will be thrown away Good for small and medium-sized software

systems

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Software Process Models: Software Process Models: Formal Development….Formal Development….

The Formal System Development modelThe Formal System Development model [Somm00, Fig 3.3][Somm00, Fig 3.3]

Requirementsdefinition

Formalspecification

Formaltransformation

Integration andsystem testing

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Software Process Models: Software Process Models: .Formal Development….Formal Development…

The Formal Transformation process The Formal Transformation process [Somm00, Fig 3.4][Somm00, Fig 3.4]

R2Formal

specificationR3

Executableprogram

P2 P3 P4

T1 T2 T3 T4

Proofs of transformation correctness

Formal transformations

R1

P1

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Software Process Models: Software Process Models: ..Formal Development....Formal Development..

Main characteristics: Approach based on formal, mathematics-based

techniques for specification, refinement, and verification

A formal specification is produced, than transformed using mathematical methods into the software product’s design and code; formal verification is also used

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Software Process Models: Software Process Models: ...Formal Development....Formal Development.

Advantages: Transformations between stages are sufficiently close

to ensure better verification and less expensive proofs Errors, omissions, ambiguities, and contradictions in

specifications are more completely detected The final product is more likely to satisfy the The final product is more likely to satisfy the

requirements requirements The product’s reliability is increased The product’s reliability is increased

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Software Process Models: Software Process Models: ….Formal Development….Formal Development

Disadvantages:Disadvantages: Requires knowledge of and experience with Requires knowledge of and experience with

formal methodsformal methods Are resource consumingAre resource consuming

Applicability:Applicability: Safety- and security-critical systemsSafety- and security-critical systems Parts of the systems that must be thoroughly Parts of the systems that must be thoroughly

verified verified

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Software Process Models: Software Process Models: Reuse-Oriented Development…Reuse-Oriented Development…

The Reuse-oriented Development modelThe Reuse-oriented Development model [Somm00, Fig 3.5][Somm00, Fig 3.5]

Requirementsspecification

Componentanalysis

Developmentand integration

System designwith reuse

Requirementsmodification

Systemvalidation

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Software Process Models: .Reuse-Oriented..Software Process Models: .Reuse-Oriented..

Main characteristics:Main characteristics: Makes intensive use of existing reusable Makes intensive use of existing reusable

componentscomponents The focus is on integrating the components The focus is on integrating the components

rather than on creating them from the scratchrather than on creating them from the scratch

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Software Process Models: ..Reuse-Oriented.Software Process Models: ..Reuse-Oriented.

Advantages:Advantages: Reduces considerably the software to be Reduces considerably the software to be

developed “in-house”developed “in-house” Allows faster deliveryAllows faster delivery In principle, more reliable systems, due to In principle, more reliable systems, due to

using previously tested componentsusing previously tested components

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Software Process Models: …Reuse-OrientedSoftware Process Models: …Reuse-Oriented

Disadvantages:Disadvantages: Compromises in requirements are neededCompromises in requirements are needed Less control over the system’s evolutionLess control over the system’s evolution

Applicability:Applicability: When there is a pool of existing components that When there is a pool of existing components that

could satisfy the requirements of the new could satisfy the requirements of the new product product

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Software Process Models: Software Process Models: Incremental Development…Incremental Development…

The Incremental Model The Incremental Model [Somm00, Fig 3.6][Somm00, Fig 3.6]

Valida teincrement

Develop systemincrement

Design systemarchitecture

Integrateincrement

Valida tesystem

Define outline requirements

Assign requirements to increments

System incomplete

Finalsystem

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Software Process Models: .Incremental..Software Process Models: .Incremental..

Main characteristics:Main characteristics: Hybrid model that combines elements of the Hybrid model that combines elements of the

waterfall and evolutionary paradigmswaterfall and evolutionary paradigms The specification, design, and implementation The specification, design, and implementation

phases are broken in smaller incrementsphases are broken in smaller increments

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Software Process Models: ..Incremental.Software Process Models: ..Incremental.

Advantages:Advantages: Provides better support for process iterationProvides better support for process iteration Reduces rework in the software construction processReduces rework in the software construction process Some decisions on requirements may be delayedSome decisions on requirements may be delayed Allows early delivery of parts of the systemAllows early delivery of parts of the system Supports easier integration of sub-systemsSupports easier integration of sub-systems Lower risk of project failure Lower risk of project failure Delivery priorities can be more easily set Delivery priorities can be more easily set

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Software Process Models: ...IncrementalSoftware Process Models: ...Incremental

Disadvantages:Disadvantages: Increments need be relatively smallIncrements need be relatively small Mapping requirements to increments may not be Mapping requirements to increments may not be

easyeasy Common software facilities may be difficult to Common software facilities may be difficult to

identify identify Applicability:Applicability:

When it is possible to deliver the system “part-by-When it is possible to deliver the system “part-by-part” part”

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Software Process Models: Spiral Model..Software Process Models: Spiral Model..

Boehm’s Spiral ModelBoehm’s Spiral Model [Somm00, Fig 3.7][Somm00, Fig 3.7]

Riskanalysis

Riskanalysis

Riskanalysis

Riskanalysis Proto-

type 1

Prototype 2Prototype 3

Opera-tionalprotoype

Concept ofOperation

Simulations, models, benchmarks

S/Wrequirements

Requirementvalidation

DesignV&V

Productdesign Detailed

design

CodeUnit test

IntegrationtestAcceptance

testService Develop, verifynext-level product

Evaluate alternativesidentify, resolve risks

Determine objectivesalternatives and

constraints

Plan next phase

Integrationand test plan

Developmentplan

Requirements planLife-cycle plan

REVIEW

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Software Process Models: .Spiral Model.Software Process Models: .Spiral Model.

Main characteristics:Main characteristics: Also a hybrid model that support process iterationAlso a hybrid model that support process iteration The process is represented as a spiral, each loop in The process is represented as a spiral, each loop in

the spiral representing a process phasethe spiral representing a process phase Four sectors per loop: objective setting, risk Four sectors per loop: objective setting, risk

assessment and reduction, development and assessment and reduction, development and validation, planningvalidation, planning

Risk is explicitly taken into considerationRisk is explicitly taken into consideration

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Software Process Models: ..Spiral ModelSoftware Process Models: ..Spiral Model

Advantages:Advantages: Risk reduction mechanisms are in placeRisk reduction mechanisms are in place Supports iteration and reflects real-world practicesSupports iteration and reflects real-world practices Systematic approachSystematic approach

Disadvantages: Requires expertise in risk evaluation and reduction Complex, relatively difficult to follow strictly Applicable only to large systems

Applicability: Internal development of large systems

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Process Activities: SpecificationProcess Activities: Specification Requirements engineering Requirements engineering [Somm00, Fig 3.8][Somm00, Fig 3.8]

Feasibilitystudy

Requirementselicitation and

analysisRequirementsspecification

Requirementsvalidation

Feasibilityreport

Systemmodels

User and systemrequirements

Requirementsdocument

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Process Activities: Design & ImplementationProcess Activities: Design & Implementation

A general model for design [Somm00, Fig 3.9]A general model for design [Somm00, Fig 3.9]

Architecturaldesign

Abstractspecification

Interfacedesign

Componentdesign

Datastructuredesign

Algorithmdesign

Systemarchitecture

Softwarespecification

Interfacespecification

Componentspecification

Datastructure

specification

Algorithmspecification

Requirementsspecification

Design activities

Design products

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Process Activities: TestingProcess Activities: Testing

The debugging process [Somm00, Fig 3.10]The debugging process [Somm00, Fig 3.10]

The testing process [Somm00, Fig. 3.11]The testing process [Somm00, Fig. 3.11]

Locateerror

Designerror repair

Repairerror

Re-testprogram

Sub-systemtesting

Moduletesting

Unittesting

Systemtesting

Acceptancetesting

Componenttesting

Integration testing Usertesting

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Process Activities: EvolutionProcess Activities: Evolution

System evolutionSystem evolution [Somm00, Fig 3.13][Somm00, Fig 3.13]

Assess existingsystems

Define systemrequirements

Propose systemchanges

Modifysystems

Newsystem

Existingsystems

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Automated Process SupportAutomated Process Support Classification of CASE technology Classification of CASE technology [Somm00, Fig 3.16][Somm00, Fig 3.16]

Single-methodworkbenches

General-purposeworkbenches

Multi-methodworkbenches

Language-specificworkbenches

Programming TestingAnalysis and

design

Integratedenvironments

Process-centredenvironments

Filecomparators

CompilersEditors

EnvironmentsWorkbenchesTools

CASEtechnology