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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

Strengths	Weaknesses
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:

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.

Strengths	Weaknesses
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.

# 2

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.

The waterfall model the process goes to the next step after
completion of the previous step as first requirement then
design then coding then implementation then maintenance but
here is no end user feedback taken to consideration any
change in SRS will result to start work fro first step and
goes step by step again.
But in case of Spiral model for each and every step there
is testing for that step carry on simultaneously after
finishing that step so that it will easy to recover any
error and fix it there. In this model we don?t have to
start work from beginning

The Waterfall model is the most simple and widely accepted/followed software development model, but like any other system, Waterfall model does have its own pros and cons. Spiral Model for software development was designed in order to overcome the disadvantages of the Waterfall Model.

Waterfall Model" has its own disadvantages such as there is no fair division of phases in the life cycle, not all the errors/problems related to a phase are resolved during the same phase, instead all those problems related to one phase are carried out in the next phase and are needed to be resolved in the next phase, this takes much of time of the next phase to solve them. The risk factor is the most important part, which affects the success rate of the software developed by following "The Waterfall Model".

In order to overcome the cons of "The Waterfall Model", it was necessary to develop a new Software Development Model, which could help in ensuring the success of software project. One such model was developed which incorporated the common methodologies followed in "The Waterfall Model", but it also eliminated almost every possible/known risk factors from it. This model is referred as "The Spiral Model" or "Boehm’s Model".

four phases in the "Spiral Model" which are:

Plan: In this phase, the objectives, alternatives and constraints of the project are determined and are documented. The objectives and other specifications are fixed in order to decide which strategies/approaches to follow during the project life cycle.



Risk Analysis: This phase is the most important part of "Spiral Model". In this phase all possible (and available) alternatives, which can help in developing a cost effective project are analyzed and strategies are decided to use them. This phase has been added specially in order to identify and resolve all the possible risks in the project development. If risks indicate any kind of uncertainty in requirements, prototyping may be used to proceed with the available data and find out possible solution in order to deal with the potential changes in the requirements.



Engineering: In this phase, the actual development of the project is carried out. The output of this phase is passed through all the phases iteratively in order to obtain improvements in the same.



Customer Evaluation: In this phase, developed product is passed on to the customer in order to receive customer’s comments and suggestions which can help in identifying and resolving potential problems/errors in the software developed. This phase is very much similar to TESTING phase.

process progresses in spiral sense to indicate iterative path followed, progressively more complete software is built as we go on iterating through all four phases. The first iteration in this model is considered to be most important, as in the first iteration almost all possible risk factors, constraints, requirements are identified

The Spiral Model" does have its pros and cons too. As this model is developed to overcome the disadvantages of the "Waterfall Model", to follow "Spiral Model", highly skilled people in the area of planning, risk analysis and mitigation, development, customer relation etc. are required. This along with the fact that the process needs to be iterated more than once demands more time and is somehow expensive task.