SAD8 Spiral Model and Incremental Model

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Evolutionary modelSpiral modelIncremental model


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Spiral modelThe spiral model is a software developmentprocess combining elements of both designand prototyping -in-stages, in an effort tocombine advantages of top-down and bottom-up concepts.

Also known as the spiral lifecycle model, it is asystems development method (SDM) used in Information Technology (IT).This model of development combines thefeatures of the prototyping model and thewaterfall model . The spiral model is intendedfor large, expensive and complicated projects.Each cycle involves the same sequence of steps as the waterfall process model.


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HistoryThe spiral model was defined by

Barry Boehm. the iterations were typically 6months to 2 years long.

Each phase starts with a design goal and ends with the client (whomay be internal) reviewing theprogress thus far.

Analysis and engineering effortsare applied at each phase of theproject, with an eye toward the endgoal of the project


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In the spiral model,

the radial coordinaterepresents costthe angular coordinaterepresents progress

in completion of acycle of the model


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traversing through four quadrants.

Each cycle involves traversing through four quadrants.

1. The first quadrant is todetermine objectives,alternatives, and constraints for the cycle.

2. The second quadrant is a riskanalysis and evaluation of alternatives for the cycle.

3. The third quadrant is to developand verify the next level product.

4. The fourth quadrant involves

planning for the next phases.


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The Spiral Model is intended to encompassother life cycle models such as the WaterfallModel, the Incremental Development model,

and the Throwaway Prototyping Model.During Risk Analysis, the key characteristicsof the project are determined, referred to asprocess drivers. The process drivers areused to determine which process model ismost appropriate for the project. In this way,the Spiral Model can be considered aprocess model generator [Boehm89].The spiral model consists of four quadrants:1. Defining Objectives, Alternatives, and Constraints2. Analyzing Risks3. Developing Product4. Spiral Planning


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Monday, September 7,1:48:54 PMSpiral Quadrant

Determine objectives,alternatives and constraints

Objectives: functionality, performance,hardware/software interface, criticalsuccess factors, etc.

Alternatives: build, reuse, buy, sub-contract, etc.Constraints: cost, schedule, interface, etc.


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Monday, September 7,1:48:54 PMSpiral Quadrant

Evaluate alternatives, identifyand resolve risks

Study alternatives relative to objectives andconstraintsIdentify risks (lack of experience, new

technology, tight schedules, poor process,etc.Resolve risks (evaluate if money could belost by continuing system development


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Spiral QuadrantDevelop next-level product

Typical activites:Create a designReview design

Develop codeInspect codeTest product


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Monday, September 7,1:48:54 PMRisk factors might involve

development cost overruns,

operating-cost miscalculation,customer's judgment, less-than-satisfactory final product.The existing prototype isevaluated in the same manner as was the previous prototype,

and, if necessary, another prototype is developed from itaccording to the fourfold

procedure outlined above.


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The preceding steps are iterated until thecustomer is satisfied that the refined

prototype represents the final productdesired.The final system is constructed, basedon the refined prototype .The final system is thoroughly evaluatedand tested. Routine maintenance iscarried out on a continuing basis to

prevent large-scale failures and tominimize downtime


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Spiral Model Strengths

Provides early indication of insurmountablerisks, without much costUsers see the system early because of

rapid prototyping toolsCritical high-risk functions are developedfirstThe design does not have to be perfect

Users can be closely tied to all lifecyclestepsEarly and frequent feedback from usersCumulative costs assessed frequently


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Spiral Model Weaknesses

Time spent for evaluating risks too large for small or low-risk projectsTime spent planning, resetting objectives,doing risk analysis and prototyping may beexcessiveThe model is complexRisk assessment expertise is requiredSpiral may continue indefinitelyDevelopers must be reassigned during non-development phase activitiesMay be hard to define objective, verifiablemilestones that indicate readiness toproceed through the next iteration


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When to use Spiral Model

When creation of a prototype isappropriateWhen costs and risk evaluation isimportantFor medium to high-risk projectsLong-term project commitmentunwise because of potential changesto economic prioritiesUsers are unsure of their needsRequirements are complexNew product lineSignificant changes are expected(research and exploration)


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Applications

The spiral model is used most oftenin large projects.


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Advantages

Estimates (i.e. budget, schedule, etc.)become more realistic as workprogresses, because important issuesare discovered earlier.

It is more able to cope with thechanges that software developmentgenerally entails .Software engineers can get their hands in and start working on thecore of a project earlier .


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Incremental Model


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Monday, September 7,1:48:54 PMIncremental Model (IM)

More specifically, the model isdesigned, implemented andtested as a series of incrementalbuilds until the product isfinished. A build consists of pieces of code from variousmodules that interact together toprovide a specific function.

At each stage of the IM a newbuild is coded and thenintegrated into the structure,which is tested as a whole . Notethat the product is only definedas finished when it satisfies allof its requirements.


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Monday, September 7,1:48:54 PMThe Incremental Model (IM)


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Monday, September 7,1:48:54 PMThis model combines

the elements of thewaterfall model with theiterative philosophy of

prototyping. IM focuses on the

delivery of anoperational product atthe end of eachincrement.


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Incremental Model Strengths

Develop high-risk or major functionsfirstEach release delivers an operationalproduct

Customer can respond to each buildUses divide and conquer breakdownof tasksLowers initial delivery cost

Initial product delivery is faster Customers get important functionalityearlyRisk of changing requirements isreduced


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Incremental Model Weaknesses

Requires good planning and designRequires early definition of acomplete and fully functional

system to allow for the definition of incrementsWell-defined module interfaces are

required (some will be developedlong before others)Total cost of the complete system is

not lower


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When to use the IncrementalModel

Risk, funding, schedule, programcomplexity, or need for earlyrealization of benefits.Most of the requirements are knownup-front but are expected to evolveover time

A need to get basic functionality tothe market earlyOn projects which have lengthydevelopment schedules

On a project with new technology


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The End

Documents

SAD8 Spiral Model and Incremental Model