The Spiral Model

4 Mar

Following my last blog, I am now going to explain another traditional method of software development: The Spiral Model.

The  software project passes through phases called “spirals” repeatedly, hence the name of the model.

There are four phases in the spiral model:

1) Planning

2)Risk Analysis

3)Engineering

4)Evaluation/review

Phases

1)Planning:

Identify: The objectives, the alternative means of implementing this portion of the product and the constraints imposed on the application of the alternatives.

2) Risk Analysis:

Evaluate: the alternatives relative to objectives and constraints and the risks involved with each of these alternatives.

Conduct a risk resolution.

3)Engineering:

Develop and verify the product. ie the software requirements specification/ the design specification etc.

4) Evaluation/Review:

Plan the next phase. Depending on what this is, this plan could be a requirements plan, an integration plan and an integration and test plan.

Strengths of the Spiral Model:

  • It provides early indication of insurmountable risks.
  • Users get to see the system early due to rapid prototyping tools.
  • Users can be closely tied to all lifecycle steps.
  • Early and frequent feedback is obtained from users.

Weaknesses of the Spiral Model:

  • The time spent for evaluating risks is too large for small or low-risk projects.
  • The time spent planning, resetting objectives,  doing risk analysis and prototyping may be too excessive.
  • The spiral model is very complex.
  • Risk assessment expertise is required.
  • It can be hard to define objective, verifiable milestones.
  • The spiral can continue indefinitely.

When should I user the Spiral Model?

  • When creation of the prototype is appropriate.
  • When the cost and risk evaluation is appropriate.
  • For medium to high risk projects.
  • When users are unsure of their needs or when the requirements are complex.

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