Spiral Model

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Spiral Model Spiral Model in Software
Engineering
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Spiral Model The spiral model is a combination
of waterfall and iterative development process
with emphasizing on more risk analysis. Risk is
essentially any adverse circumstance that might
hamper the successful completion of a software
project. For example, the risk involved in
accessing data from a remote database can be that
the data access rate might be too slow. The risk
can be resolved by building a prototype of the
data access subsystem.  Thus, this model provides
direct support for coping with the project risks.
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Spiral Model in Software Engineering It has
planning, risk analysis, engineering and
evaluation phase. Each phase in the spiral model
begins with a design goal and ends with the
client reviewing the progress. The development
team in Spiral-SDLC Model starts with a small set
of requirements and goes through each development
phase for those set of requirements. The
development team adds functionality for the
additional requirement in every-increasing
spirals until the application is ready for the
production phase.
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The diagrammatic representation of this model
appears like a spiral with many loops. The exact
number of loops in the spiral is not fixed. Each
loop of the spiral represents a phase of the
software process.
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For example, the innermost loop might be
concerned with the feasibility study, the next
loop with requirement specification, the next one
with design, and so on. Each phase in this
model is split into four sectors (or
quadrants) The following activities are carried
out during each phase of a spiral model.
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• First quadrant (Objective Setting-Planning)
• During the first quadrant, it is needed to
  identify the objectives of the phase.
• Examine the risks associated with these
  objectives.
• Second Quadrant (Risk Assessment and Reduction
  (Risk Analysis))
• A detailed analysis is carried out for each
  identified project risk.
• Steps are taken to reduce the risks. For example,
  if there is a risk that the requirements are
  inappropriate, a prototype system may be
  developed.

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• Third Quadrant (Development and Validation
  (Engineering))
• Develop and validate the next level of the
  product after resolving the identified risks.
• Fourth Quadrant (Review and Planning
  (Evaluation))
• Review the results achieved so far with the
  customer and plan the next iteration around the
  spiral.
• Progressively more complete version of the
  software gets built with each iteration around
  the spiral.

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With each iteration around the spiral (beginning
at the center and moving outwards), progressively
a more complete version of the software gets
built. Usually, after several iterations along
the spiral, all risks are resolved, and software
is ready for the development. At this point, a
waterfall model of software development is
adopted. The radius of the spiral at any point
represents the progress made in the current phase.
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In the Spiral model of development, the project
team must decide how exactly to structure the
project into phases. Normally, projects start by
using some generic model and then adding extra
phases whenever some special risks are identified
later during the project. Possibly the most
distinguishing feature of this model is its
ability to handle risks. Unlike other models,
risk handling in each phase of development is
specifically included in this model. During each
iteration, risk analysis through prototype
construction allows weighing different
alternatives available to handle the risks.
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Comparing the Spiral model with other important
process models

• The classical waterfall model can be considered
  as the basic model and all other life cycle
  models are embellishments of this model. However,
  the classical waterfall model cannot be used in
  practical development projects, since this model
  supports no mechanism to handle the errors
  committed during any of the phases. This problem
  is overcome in the iterative waterfall model.
• The iterative waterfall model is probably the
  most widely used software development model
  evolved so far. This model is simple to
  understand and use. 

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However, this model is suitable only for
well-understood problems, it is not suitable foe
very large projects and for projects that are
subject to many risks. The prototyping model is
suitable for projects for which either the user
requirements or the underlying technical aspects
are not well understood. This model is especially
popular for the development of the user interface
part of the project.
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The evolutionary approach is suitable for large
projects which can be decomposed into a set of
modules for incremental development and delivery.
This model is also widely used for
object-oriented development projects. Of course,
this model can be used if the incremental
delivery of the system is acceptable to the
customer.
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The Spiral model can be viewed as a Meta-model
since it subsumes all the initial models. For
example, a single loop spiral represents the
waterfall model. The Spiral model uses a
prototyping approach by first building a
prototype before embarking on the actual product
development effort. Also, Spiral model can be
considered as supporting the evolutionary
model-the iterations along the spiral model can
be considered as the evolutionary levels through
which the complete system is built.
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• This enables the developer to understand and
  resolve the risks at each evolutionary level
  (i.e. iteration along the spiral). The spiral
  model uses prototyping as a risk reduction
  mechanism and also retains the systematic
  stepwise approach of the waterfall model.
• Advantages of the Spiral Model
• Best for a high-risk project
• Good for large and mission-critical projects

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• Strong approval and documentation control
• Continuous or repeated development helps in risk
  management
• Development is fast and features are added in a
  systematic approach
• Additional functionality or change can be done at
  a later stage.
• Cost estimation becomes easy as the prototype
  building happens in small fragments.
• There is always space for Customer feedback.

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• very hard to properly monitor and maintain
• works for large products only, Not suitable for
  small scale projects
• Risk analysis expert is required and could be
  costly
• For its smooth operation, the Spiral Models
  protocols need to be followed strictly

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Circumstances to use Spiral Model

• The spiral model is called a meta model since it
  encompasses all other life cycle models. Risk
  handling is inherently built into this model. The
  spiral model is suitable for the development of
  technically challenging software products that
  are prone to several kinds of risks. However,
  this model is much more complex than the other
  models this is probably a factor deterring its
  use in ordinary projects. So, this model is used

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• When the project is large
• When releases are required to be frequent
• When the creation of a prototype is applicable
• When risk and costs evaluation is important
• For medium to high-risk projects
• When requirements are unclear and complex
• When changes may require at any time
• When long term project commitment is not feasible
  due to changes in economic priorities.

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