SPEAKER : KAI-JIA CHANG

ADVISER : QUINCY WU

DATA : 2010-03-17

Spiral Model

Outline

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IntroductionThe spiral modelEvaluationConclusionsReferences

Introduction

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A Risk-Driven Approach Different idea of software development.How does this project affect the developers

and the clients?How does each step in the project affect its

overall development? Not used in previous development models.– Usually code-driven or document-driven.

Introduction

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Background on software process models1950 ： Code-and-fix model 1956 ： Stagewise model (Bengington ) 1970 ： Waterfall model (Royce)1971 ： Incremental model(Mills) 1977 ： Prototyping model(Bally and others) 1988 ： Spiral model(Boehm)

Introduction

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Code-and-fix modelFirst, elementary modelWrite code now; fix it laterNo planning involvedProblems:– Code is poorly structured.– The software developed was usually a poor

match for the users’ needs.

Introduction

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Waterfall model Introduced:– Feedback loops across multiple stages: Validation and verification steps.– Prototyping via a “build it twice” step alongside ofrequirements and design.Difficulties exposed even as revisions were made to

the model.– Required elaborated documents. (Document-driven)– Led to pursuing stages of development in the wrongorder.

The spiral model

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A Typical Cycle of the spiral

Risk AnalysisPrototypeDesign/ValidationPlanningAlternativesAnd repeatMeasure of Cumulative Cost and Progress

The spiral model

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Initiating and terminating the spiralFour fundamental questions arise in considering

this presentation of the spiral model: (1) How does the spiral ever get started? (2) How do you get off the spiral when it is

appropriate to terminate a project early? (3) Why does the spiral end so abruptly? (4) What happens to software enhancement (or

maintenance)?

The spiral model

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Initiating and terminating the spiral Initiating the process: – The spiral gets started by a hypothesis that a

particular operational mission (or set of missions) could be improved by a software effort.

Terminating the process: – The spiral process then involves a test of this

hypothesis:At any time, if the hypothesis fails the test, the

spiral is terminated.

The spiral model

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The spiral model

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The spiral model

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Cycle RequirementsEach cycle is completed by a review by

the people concerned with the project.

Plans for the next cycle should be introduced.

With each succeeding level in the spiral the level of detail increases.

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The spiral model

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Prototype Throwaway prototypingThrowaway or Rapid Prototyping refers to the creation of a

model that will eventually be discarded rather than becoming part of the final delivered software.

Evolutionary prototypingThe main goal when using Evolutionary Prototyping is to build a

very robust prototype in a structured manner and constantly refine it.

Incremental prototypingThe final product is built as separate prototypes. At the end the

separate prototypes are merged in an overall design. Extreme prototypingIt is a development process is used especially for developing

web applications.

The spiral model

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Advantages of prototypingReduced time and costs Improved and increased user involvement

Disadvantages of prototyping Insufficient analysisUser confusion of prototype and finished systemDeveloper misunderstanding of user objectivesDeveloper attachment to prototypeExcessive development time of the prototypeExpense of implementing prototyping

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Risk item Risk management techniquesPersonnel shortfalls Staffing with top talent, job matching; teambuilding;

morale building; cross-training; pre-scheduling key people

Unrealistic schedules and Budgets

Detailed, multisource cost and schedule estimation;design to cost; incremental development; software reuse; requirements scrubbing

Developing the wrong software Functions

Organization analysis; mission analysis; ops-conceptformulation; user surveys; prototyping; early users’manuals

Developing the wrong user Interface

Task analysis; prototyping; scenarios; user characterization (functionality, style, workload)

Gold plating Requirements scrubbing; prototyping; cost-benefit analysis; design to cost

Continuing stream of requirement Changes

High change threshold; information hiding; incrementaldevelopment (defer changes to later increments)

Shortfalls in externally furnished Components

Benchmarking; inspections; reference checking; compatibility analysis

Shortfalls in externallyperformed tasks

Reference checking; pre-award audits; award-fee contracts; competitive design or prototyping;Teambuilding

Real-time performanceShortfalls

Simulation; benchmarking; modeling; prototyping;instrumentation; tuning

Straining computer-science Capabilities

Technical analysis; cost-benefit analysis; prototyping;reference checking

The spiral model

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The Risk Management Plan

Identify the project’s top 10 risk items.Present a plan for resolving each risk item.Update list of top risk items, plan, and results

monthly.Highlight risk-item status in monthly project

reviews.Compare with previous month’s rankings, status. Initiate appropriate corrective actions.

Spiral model usage: TRW Software Productivity System, Round 0. --Feasibility

study.

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Spiral model usage: TRW Software Productivity System, Round 1.--Concept of

operations.

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Spiral model usage: TRW Software Productivity System, Round 2.--Top-level requirements

specification.

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Evaluation

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Advantages Estimates become more realistic as work progresses,

because important issues are discovered earlier. It is more able to cope with the changes that software

development generally entails. Software engineers can get their hands in and start

working on a project earlier.

Disadvantages Highly customized limiting re-usability Applied differently for each application Risk of not meeting budget or schedule

Conclusions

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The risk-driven nature provides adaptability for a full range of software projects.

The model has been successful in a large application, the TRW-SPS.

The model is not yet fully elaborated.Even partial implementations of the model,

such as the risk management plan, are compatible with the other process models.

References

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Rapid Prototyping Tools Revisited Alan M. Davis, "Operational Prototyping: A new

Development Approach. IEEE Software, September 1992. Page 70-78.

Wikipedia: Software Prototyping B. Boehm, " A Spiral Model of Software Development

and Enhancement, ACM SIGSOFT Software Engineering Notes, Volume 11 , Issue 4 (August 1986) Pages: 14 - 24.

Barry Boehm et al., "Using the WinWin Spiral Model: A Case Study, IEEE Computer, July 1998 (vol. 31 no. 7) pp. 33-44.