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Spiral Methodology:

While the waterfall methodology offers an orderly structure for software
development, demands for reduced time-to-market make its series steps inappropriate. The
next evolutionary step from the waterfall is where the various steps are staged for
multiple deliveries or handoffs.  The ultimate evolution from the water fall is the
spiral, taking advantage of the fact that development projects work best when they are
both incremental and iterative, where the team is able to start small and benefit from
enlightened trial and error along the way. 

The spiral methodology reflects the relationship of tasks with rapid
prototyping, increased parallelism, and concurrency in design and build activities. The
spiral method should still be planned methodically, with tasks and deliverables identified
for each step in the spiral.

Waterfall Methodology

All projects can be managed better when segmented into a hierarchy of
chunks such as phases, stages, activities, tasks and
steps.  In system development projects, the simplist rendition of this is called the
"waterfall" methodology, as shown in the following figure:

In looking at this graphic, which was for major defense systems
developments, please note this presumes that the system requirement have already been
defined and scrubbed exhaustively, which is probably the most important step towards
project success.  Nevertheless, the graphic illustrates a few critical principles of
a good methodology:

• Work is done in stages,


• Content reviews are conducted between stages, and


• Reviews represent quality gates and decision points for continuing.

The waterfall provides an orderly sequence of development
steps and helps ensure the adequacy of documentation and design reviews to ensure the
quality, reliability, and maintainability of the developed software.  While
almost everyone these days disparages the "waterfall methodology" as being
needlessly slow and cumbersome, it does illustrate a few sound principles of life cycle
development. 

Pure
Waterfall

This
is the classical system development model. It consists of discontinuous
phases:

1. Concept


2. Requirements


3. Architectural
   design


4. Detailed
   design


5. Coding
   and development


6. Testing
   and implementation

Spiral

The
spiral is a risk-reduction oriented model that breaks a software
project up into mini-projects, each addressing one or more major
risks. After major risks have been addressed, the spiral model terminates
as a waterfall model. Spiral iterations involve six steps:

1. Determine
   objectives, alternatives and constraints.


2. Identify
   and resolve risks.


3. Evaluate
   alternatives.


4. Develop
   the deliverables for that iteration and verify that they are
   correct.


5. Plan
   the next iteration.


6. Commit
   to an approach for the next iteration.

In the water fall model the process flows from top to 
bottom like a flow of water . But any new changes can not 
be incorporated in the middle of the project development.

Whereas the spiral model is best suted for projects 
associated with risks.

Overview

Waterfall development isn't new -- it's been around since
1970 -- but most developers still only have a vague idea of what it means.
Essentially, it's a framework for software development in which development
proceeds sequentially through a series of phases, starting with system
requirements analysis and leading up to product release and maintenance.
Feedback loops exist between each phase, so that as new information is
uncovered or problems are discovered, it is possible to "go back" a
phase and make appropriate modification. Progress "flows" from one
stage to the next, much like the waterfall that gives the model its name.

The waterfall model, as described above, offers numerous
advantages for software developers. First, the staged development cycle
enforces discipline: every phase has a defined start and end point, and
progress can be conclusively identified (through the use of milestones) by both
vendor and client. The emphasis on requirements and design before writing a
single line of code ensures minimal wastage of time and effort and reduces the
risk of schedule slippage, or of customer expectations not being met.

Getting the requirements and design out of the way first
also improves quality; it's much easier to catch and correct possible flaws at
the design stage than at the testing stage, after all the components have been
integrated and tracking down specific errors is more complex. Finally, because
the first two phases end in the production of a formal specification, the
waterfall model can aid efficient knowledge transfer when team members are
dispersed in different locations.

Criticisms

Customer needs

While the model does have critics, it still remains useful
for certain types of projects and can, when properly implemented, produce
significant cost and time savings. Whether you should use it or not depends
largely on how well you believe you understand your customer's needs, and how
much volatility you expect in those needs as the project progresses. It's worth
noting that for more volatile projects, other frameworks exists for thinking
about project management, notably the so-called spiral model...but that's a
story for another day!