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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:

he weekness of the Waterfall Model is at hand:





It is very important to gather all possible requirements during the first phase of requirements collection and analysis. If not all requirements are obtained at once the subsequent phases will suffer from it. Reality is cruel. Usually only a part of the requirements is known at the beginning and a good deal will be gathered during the complete development time. Iterations are only meant to happen within the same phase or at best from the start of the subsequent phase back to the previous phase. If the process is kept according to the school book this tends to shift the solution of problems into later phases which eventually results in a bad system design. Instead of solving the root causes the tendency is to patch problems with inadequate measures. There may be a very big "Maintenance" phase at the end. The process only allows for a single run through the waterfall. Eventually this could be only a first sample phase which means that the further development is squeezed into the last never ending maintenance phase and virtually run without a proper process.

Pure
Waterfall




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



• Concept
• Requirements
• Architectural
  design
• Detailed
  design
• Coding
  and development
• Testing
  and implementation
• Minimizes
  planning overhead since it can be done up front.
• Structure
  minimizes wasted effort, so it works well for technically
  weak or inexperienced staff.
• Inflexible
• Only
  the final phase produces a non-documentation deliverable.
• Backing
  up to address mistakes is difficult.






Pure Waterfall Summary The pure waterfall performs well for products with clearly understood requirements or when working with well understood technical tools, architectures and infrastructures. It's weaknesses frequently make it inadvisable when rapid development is needed. In those cases, modified models may be more effective.

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:



• Determine
  objectives, alternatives and constraints.
• Identify
  and resolve risks.
• Evaluate
  alternatives.
• Develop
  the deliverables for that iteration and verify that they are
  correct.
• Plan
  the next iteration.
• Commit
  to an approach for the next iteration.
• Early
  iterations of the project are the cheapest, enabling the
  highest risks to be addressed at the lowest total cost.
  This ensures that as costs increase, risks decrease.
• Each
  iteration of the spiral can be tailored to suit the needs
  of the project.
• It
  is complicated and requires attentive and knowledgeable
  management to pull it off.






Spiral Summary For projects with risky elements, it's beneficial to run a series of risk-reduction iterations which can be followed by a waterfall or other non-risk-based lifecycle.

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.

Web Development usage




In the Web development field, three-tier is often used to refer to Websites, commonly Electronic commerce websites, which are built using three tiers:





1. A front end Web server serving static content
2. A middle dynamic content processing and generation level Application server, for example Java EE platform.
3. A back end Database, comprising both data sets and the Database management system or RDBMS software that manages and provides access to the data.





Three Tier Architecture (Linux journal) [1]