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SOFTWARE LIFE-CYCLE MODELS

CS 3320--Chapter 3

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MODELS• Build-And-Fix Model• Waterfall Model• Rapid Prototyping Model• Incremental Model• Spiral Model• Object-Oriented Model

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BUILD-AND-FIX MODEL• No Specifications• No design• Reworked many times• Totally unsatisfactory

• Need life-cycle model– “Game Plan”– Phases– Milestones

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WATERFALL MODEL• First developed by Royce in early70’s• Documentation Driven• Verification/Test at each phase• Feedback loops

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ANALYSIS OF THE WATERFALL MODEL• Advantages– Enforced discipline– documentation-driven model– testing by SQA– Maintenance easier

• Disadvantages– Requirements & Spec Analysis– graphical aids are used

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RAPID PROTOTYPING MODEL

• Rapid Prototype used for requirements and specifications analysis.

• Rapid Prototype contains a subset of the functionality of the target product

• Structure of prototype not important, but speed of development process is--RAPID

• Prototype can be used in requirement phase of WF model

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RAPID PROTOTYPING MODEL cont.• Advantages– Helps get requirements right– Good insight to estimate budget and time

requirements– Help in design phase

==> less feedback loops• Disadvantages– Prototype adds to the cost (overall it may

reduce cost)– Temptation to turn prototype into product

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RAPID PROTOTYPING MODEL cont.• Key points– Do not turn it into product– Prototype may replace requirement phase

but never the design phase– Waterfall model-- try to get right the first

time– Rapid Prototype-- Allows better feedback

from client at earliest stage of the process.

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INCREMENTAL MODEL

• The product is constructed step by step• The product is divided into a series of

incremental builds• Each build consist of a set of module that

provide a functionality• Product provided to client build by build• Each new build is integrated with previous

builds and delivered to client • Production may stop at end of any build

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ANALYSIS OF THE INCREMENTAL MODEL• Waterfall: operational quality of complete

product at end• Incremental model: deliver operational quality

of portion of product in “short” time• Reduces the effect of introducing a new system

on users: less traumatic• Smaller capital outlay• The process can be stopped at end of any build• Need open architecture-- good for maintenance

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ANALYSIS OF THE INCREMENTAL MODELProblems:• If broken into too few builds then it

degenerates into the build-and-fix model• If broken into too many builds then integration

of the builds can turn out to be too costly.• Note: Requirements, Specs and Design are

done for whole product. Only Implementation and Integration are incremental.

• A more risky version: incremental Specs and Design

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SYNCHRONIZE-AND-STABILIZE MODEL• Microsoft’s life-cycle model– Requirements analysis--interview potential

customers– Draw up specifications– Divide project into 3 to 4 builds– Each build carried out by small teams working in

parallel– At end of each day--Synchronize (integrate

modules into build, test & debug)– At end of each build-Stabilize (freeze build)

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SPIRAL MODEL• Simplified form– Waterfall model plus risk analysis

• Precede each phase by– Alternatives– Risk analysis: performance, budget, milestones

• Follow each phase by– Evaluation– Planning of next phase

• Prototypes may be used to analyze risks

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SPIRAL MODEL• Refer to Figure • radial dimension shows cumulative cost to date• Angular dimension shows progress through the

spiral• Each cycle corresponds to a phase

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ANALYSIS OF SPIRAL MODEL

Advantages:• It is a risk-driven model• Risk analysis highlights the need to stay within

budget, on schedule and quality assurance.• If risks can not be resolved, terminate project• No distinction between development and

maintenance.

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ANALYSIS OF SPIRAL MODELRestrictions:• Intended for internal (in-house) software– Contract software: risk analysis can not be

done at each phase.• Applicable to only large-scale software– cost of risk analysis

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OBJECT-ORIENTED MODELS• Intended to overcome shortcomings of previous

models• Emphasis on incorporating – iterations, refinement, and feedback– overlap and parallelism between phases– incremental development

• Danger– Can degenerate into CABTAB (Code A Bit

Test A Bit)

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CONCLUSION• Different life-cycle models• Build-and-Fix should be avoided• WF: well known with strengths and weaknesses• Rapid-Prototype: strengthens WF model• Incremental model• Synchronize-and-Stabilize: used with success

by Microsoft• Spiral: risk analysis driven• Best suggestion: “Mix-and-match” life-cycle