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Waterfall Development Model
Many traditional software development projects follow the waterfall model. In this case projects are similar to the Software Development Life Cycle (SDLC) which includes 5 main phases: Requirements Phase, Design Phase, Development Phase, System Testing, Release to Production.
The problem with waterfall models is when using this model each phase relies on the preceeding phase that needs to be completed begore moving on.
Once the analysis phase is finished, the whole project system will be design and documented. This phase is followed by project planning and budget creation. Once it's approved the system is developed, tested and release to live.
In real development environment this approach leads to great degree of inflexibility especially when complex proects are under development. Also the nature of this model makes it more difficult to change or modify passed tasks or modules once they are completed. You can imagine that let's say once requirements are writen this aproach doesn't allow changes during execution of the project.
When we can use Waterfall Model?
A waterfall model with feedback can be very effective choice to design a system that is clearly described. In this case only a few minor changes can be made in system design or its requirements. Also Waterfall model creates completed results in each phase being done and forever.