The spiral model

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CFICSE 99
The spiral model SP-06 Lt(N) Claude Bernard
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The Spiral Model

• Spiral Model elaborates and generalizes Waterfall
  Model by iterating these steps
• Planning, identifying and evaluating alternatives
• Resolving risks
• Developing a product (document, specification,
  prototype)

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The Spiral Model
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The Spiral Model

• Solid curve - angular dimension - represents
  progress (from initial requirements analysis
  through OM)
• Radial dimension represents the cumulative cost
  of the project (a function of calendar time,
  person hours and dollars)
• Spiral shaped because
• project continually progresses (angle increases)
• cost starts at zero and always increases (radius
  increases)

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The Spiral Model

• Each cycle goes through same series of steps
  (represented as four quadrants)
• Identifying the objectives, alternatives and
  constraints on each alternatives
• Evaluating each alternative with respect to its
  constraints, assigning levels of risk to each,
  and prototyping where required
• Developing and validating the next cycles
  specifications
• Developing a plan for the next cycle

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The Spiral Model

• Three dotted arcs in second quadrant represent
  option of using prototypes to bypass
• Concept of operations
• Requirements
• Design
• Final prototype may be used as basis for
• Requirements
• Design
• Detailed design

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Strengths of Spiral

• Relies on evaluation of risks, alternatives and
  constraints,
• At real core of creation design
• Attacks high risk reqts first, defers easy
  stuff
• Defers documentation
• Which is a peripheral, post-development activity,
  reduced cost and spoilage
• Encourages prototyping
• Allows leaps forward when prototype is highly
  successful
• Allows further prototyping when prototype is
  unsuccessful

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Strengths of Spiral

• Allows evolutionary development
• Early release of partial solutions
• Avoids the bid bang approach to release
• Easily de-scoped (important to DND)
• Realistically admits that development is
• Iterative, with roll-back
• Refinement of ideas
• Trial and error

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Weaknesses of Spiral

• No powerful milestone and auditing features like
  Waterfall Model (less appropriate for contracts)
• Does not require extensive documentation like
  Waterfall
• Needed to ensure continuity over long term
  project thus inappropriate for high turnover
  organizations
• Documentation likely to be cut when budget
  overruns or is reduced which is needed for
  economy and maintenance

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Weaknesses of Spiral

• Depends heavily on experts to correctly assess
  risks
• Errors in risk management can be just as fatal as
  design errors in the Waterfall Model

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Conclusion

• More general than the Waterfall Model
• Not yet popular because
• New
• Has problems with contracted software