Spiral Model

Spiral Software Process Model 

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Each loop in a spiral represents a development phase (and we can have any number of loops according to the project). Each loop has four sections or quadrants :

  1. To determine the objectives, alternatives and constraints. We try to understand the product objectives, alternatives in design and constraints imposed because of cost, technology, schedule, etc.
  2. Risk analysis and evaluation of alternatives. Here we try to find which other approaches can be implemented in order to fulfill the identified constraints. Operational and technical issues are addressed here. Risk mitigation is in focus in this phase. And evaluation of all these factors determines future action.
  3. Execution of that phase of development. In this phase we develop the planned product. Testing is also done. In order to do development, waterfall or incremental approach can be implemented.

  4. Planning the next phase
    . Here we review the progress and judge it considering all parameters. Issues which need to be resolved are identified in this phase and necessary steps are taken.
  • The spiral model is similar to the incremental model, with more emphases placed on risk analysis.
  • The spiral model has four phases:


◦Risk Analysis



  • A software project repeatedly passes through these phases in iterations (called Spirals in this model).
  • The baseline spiral, starting in the planning phase, requirements are gathered and risk is assessed.
  • Each subsequent spiral builds on the baseline spiral. Requirements are gathered during the planning phase.
  • In the risk analysis phase, a process is undertaken to identify risk and alternate solutions. A prototype is produced at the end of the risk analysis phase.
  • Software is produced in the engineering phase, along with testing at the end of the phase. The evaluation phase allows the customer to evaluate the output of the project to date before the project continues to the next spiral.
  • In the spiral model, the angular component represents progress, and the radius of the spiral represents cost.

Spiral Model sectors

  • Objective setting: Specific objectives for the phase are identified.
  • Risk assessment and reduction: Risks are assessed and activities are put in place to reduce the key risks.
  • Development and validation: A development model for the system is chosen which can be any of the general models.
  • Planning: The project is reviewed and the next phase of the spiral is planned.

Spiral Model Advantages

  • High amount of risk analysis.
  • Good for large and mission-critical projects.
  • Software is produced early in the software life cycle.

Spiral Model Disadvantages

  • Can be a costly model to use.
  • Risk analysis requires highly specific expertise.
  • Project’s success is highly dependent on the risk analysis phase.
  • Doesn’t work well for smaller projects

In waterfall model ,a working model of the software is only available at a very later stage therefore there is no room for correction/changes .it is mainly used when the developers have clear idea of the software and they have built similar software in the past.

In increment process model ,similar steps to waterfall model are applied but after each cycle a new version of the product is released. This process is continued till we develop the desired software. This model is used when the concept of the software is not that clear at the starting.

Spiral model is like an evolutionary model of waterfall model, in spiral model project risk factor is taken into consideration and the steps followed are planning ,risk analysis, development .


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