Software Life Cycle Models

1
Software Life Cycle Models
Alan M. Davis University Of Colorado

• EEL 6887 Software Engineering Life Cycle Control
• Presented by
• MOHAMMED HAROON SHARIFF
• 02/15/06

2
References

• Software Life Cycle Models Alan M. Davis,
  University of Colorado
• A Strategy for Comparing Alternative Software
  Development Life Cycle Models Davis, A M, E H
  Bersoff, and E R Comer, IEEE Trans. Software Eng.
• Alternative Software Life cycle Models , in
  Aerospace Eng. Comer E
• Software Engineering Principles and Practice
  Van Vliet
• Capability Maturity Model for Software Mark
  C P, Bill C, Mary B C, Charles V W.
• Lecture 2 Dr. Workman.
• www.images.google.com

3
Overview

• Introduction
• Definition of a Life Cycle Model
• Choices for a Life Cycle Model
• Different Life Cycle Models
• Selecting Life Cycle Models
• Summary
• Conclusion

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Introduction

• In the 1970s the first Software Life Cycle
  Model was developed by Winston Royce, it was
  called the Waterfall Model . This was used as the
  primary life cycle in the industry for over
  twenty years.
• As time progressed increasing demands, budget
  and time constraints led to the invent of newer
  life cycles. Many other life cycle models have
  been proposed in the last two decades which claim
  to be better than the Waterfall model.
• The new life cycles have their own way to
  develop software more rapidly, or incrementally ,
  or in a evolutionary manner, or they precede
  full-scale development with some kind of rapid
  prototype.
• This paper gives an insight into the different
  software life cycle models that have been
  developed. It gives an approach to evaluate and
  contrast these different life cycle models, but
  emphasizes that all of them just provide another
  view to the basic software development process.

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What is a Life Cycle Model ?

• Definition A software life cycle model is a
  view of the activities
• that occur during the software development
• To elaborate on the definition a software life
  cycle model is a
• model that helps us understand the various
  activities and characteristics
• while developing a software. It is not an actual
  method in which one can
• develop a software but it is a procedure which
  helps us look at the on
• going software development . In short

• It illustrates all the important development
  phases
• It defines the main functions to be
  accomplished in each phase
• Helps the management keep a track of
  development progress
• It gives reference model to define the software
  development process

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Choices for a Life Cycle Model

• Using different life cycle models is just a
  different way of looking at the manner in which
  the software can be developed. There can be many
  such views in which the same software project can
  be looked at i.e.. the software can be modeled
  using various life cycles such as the waterfall
  model, incremental development, evolutionary
  development, prototyping, spiral, etc
• Although each life cycle model has its own
  views to develop software , the models discussed
  in this paper are more complementary than
  exclusionary.
• Thus it is not wise decision to assign a single
  life cycle model to which a software project
  should adhere to.

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Different Life Cycle Models

• The different life cycle models are
• Waterfall Model
• Incremental Development Model
• Evolutionary Development Model
• Requirements Prototyping Model
• Spiral Model
• Concurrent Model

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Waterfall Model

• The waterfall model is one of the first models
  developed . It serves as a building block for
  most of the later life cycle models.
• It has a very simple concept where it views the
  entire software development process as a
  sequence of phases (figure 1) .
• Each phase has a set of goals and activities that
  need to be accomplished in that phase .
• Development progress can be seen as a flow from
  one phase to the next represented by the forward
  arrows. The backward arrows represent feedback to
  the previous phases.
• There is no constraints that activities from the
  current phase only should be done in that phase.
  
• Also a complete cycle of execution of all the
  phases might lead either to the complete system
  or just a part of it.
• The primary advantage of the waterfall model is
  that it helps the management to keep track of the
  overall project progress.

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Waterfall Model

• Figure 1

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Waterfall Model

• To summarize the waterfall model emphasizes and
  recommends
• the following practices

• Planning of a project before starting to
  developing it.
• Specifying a systems desired external behavior
  before proceeding to its internal architecture.
• Documentation at the end of each activity or
  goal.
• Have a complete design of the system before
  moving onto the coding phase.
• System should be tested on completion to verify
  that it meets the requirements.

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Incremental Development Model

• Building large complex systems leads to a lot of
  risks, to reduce these risks only a subset of the
  system is built .
• Each increment of development results in
  capturing an increasing subset of requirements.
• One method to implement incremental development
  is to maintain a separate requirements document
  for each increment of development.
• Another way to implement incremental development
  is to maintain one document with all requirements
  and the increment in which that requirement will
  be implemented.
• The incremental model can be used in combination
  with the waterfall model as it does not specify a
  particular way in which each increment should be
  completed.

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Incremental Development Model

• Figure 2

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Incremental Development and Waterfall models

• Figure 3

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Incremental Development Model

• Important benefits of this model are

• As you build the system in small increments the
  risk is lowered.
• Implementing system functionality in parts
  ensures that the requirements planned for next
  increment are accurate.
• Reduction in time to develop each increment
  reduces the possibility of change in user
  requirements.
• If at any stage a major error is found,
  development can be continued just by moving back
  to the previous stage.
• Development errors made in one increment can be
  rectified before beginning the next increment.

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Evolutionary Development Model

• Here we build the system in increments but it is
  assumed that the complete requirements are not
  know initially.
• All requirements are examined and only those
  which are completely understood and selected.
• An increment of the system is built using only
  the above selected complete requirements.
• This system is given to the user to deploy and
  provide feedback.
• The requirements are updated based on the user
  feedback and the second increment is developed
  and given to the user.
• This process is repeated indefinitely to get the
  complete system.
• The Evolutionary Model can be used in combination
  with the waterfall and the incremental models.

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Evolutionary Development Model

• Figure 4

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Requirements Prototyping Model

• In this life cycle a prototype of the system is
  built to learn more details about the systems
  requirements.
• A quick prototype is built and handed over to the
  user requesting for the prototypes pros and cons.
• The developer updates this feedback into the
  actual requirements specification document of the
  real system.
• This process is continued till the complete
  system is developed.
• It is useful to develop complex systems where the
  requirements are difficult to arrive at.
• It can be used along with the other life cycle
  models, but its use should precede the other
  models.
• In contrast to evolutionary prototyping here a
  prototype is built using the least understood
  requirements.

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Requirements Prototyping Model

• Figure 5

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Spiral Model

• This is also called a meta-life cycle model as
  the development process is iterative.
• The first step here is to define the required
  goals.
• Next all the possible methods to accomplish these
  goals are analyzed and the best method is
  selected.
• The above selected method is deployed.
• After completion of the method , the results are
  reviewed to complete the first spiral.
• Now the next spiral is started, and this process
  goes on till the systems is complete.
• The spiral model is compatible with the other
  life cycle models.

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Spiral Model

• Figure 6

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Spiral Model

• To summarize the Spiral Model recommends the
  following practices

• Identify the problem you want to solve before
  attempting to solve it.
• Find all the possible solutions to the problem.
• Choose the optimal solution to the problem and
  use it to solve the problem.
• Review results from the above solution.
• Never make an assumption that the system being
  built is the exact system the user needs.
• Calculate to amount of risk you are going to
  tolerate.

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Concurrent Model

• The Concurrent Model also provides a
  meta-description of the model like the Spiral
  model.
• Here multiply software development activities are
  done simultaneously and are represented by Harel
  statecharts.
• When most of the requirements are done, we move
  on to give the design phase more time.
• During design any changes in requirements are
  re-baselined which in turn might or might not
  affect the design.
• If any component is well defined the detailed
  design of that component can be done. Further
  even coding, integration and testing also can be
  done even if design of other components is not
  complete.
• Occasionally multiple versions of the same
  product can be developed concurrently.

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Selecting Life Cycle Models

• All the above mentioned life cycles are
  compatible with each other,
• but every project must identify which view is
  beneficiary in its unique
• situation. A study was done by Linda
  AlexanderALE91 after which she
• proposed 23 criteria based on the project and its
  application. She created
• Ideal Profiles of projects compatible with
  each life cycle. Project managers
• had to create an ideal profile for their project
  and select the life cycle that
• closely matches it. A few of the criteria are
• Maturity of the application
• Problem and the solutions complexity
• Usefulness of early functionality
• Fuzziness and frequency of changes in user
  requirements
• Availability of time and money
• Risk tolerance of project
• Importance of meeting deadlines and budget

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Summary

• The waterfall model is the basic model from which
  most of the other models were derived.
• All the life cycle models are compatible with
  each other and can be deployed together in the
  same project.
• Hence using multiple software development models
  during the lifetime of the project can increase
  its efficiency.
• Project profiles can be created and its matching
  ideal profile can be chosen to help decide which
  life cycle model is more fruitful.

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Conclusion

• The author emphasizes that each model is not
  particularly different, but they provide
  different views of the same basic software
  development process.
• All the software development life cycles serve
  as tools to monitor the activities and overall
  progress of software development. Each life cycle
  has an extent to which it reflects the software
  development. On one hand the waterfall model
  gives a gross idea of development progress,
  whereas the concurrent model gives an actual idea
  of development progress.
• These models are used as tools by the project
  manager and the management to keep track of
  development.
• Most of the life cycle models help to achieve
  CMM level 2. Repeated use of these models will
  help to achieve CMM level 3.

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Questions ?