The Software Process

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CHAPTER 3
SOFTWARE LIFE-CYCLE MODELS
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Overview

• Build-and-fix model
• Waterfall model
• Rapid prototyping model
• Incremental model
• Extreme programming
• Synchronize-and-stabilize model
• Spiral model
• Object-oriented life-cycle models
• Comparison of life-cycle models

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Software Life-Cycle Models

• Life-cycle model
• The steps through which the product progresses
• Requirements phase
• Specification phase
• Design phase
• Implementation phase
• Integration phase
• Maintenance phase
• Retirement

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Build and Fix Model

• Problems
• No specifications
• No design
• Totally unsatisfactory for any reasonable size
  software
• Need life-cycle model
• Game plan
• Phases
• Milestones

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Waterfall Model

• The only widely-used model until the early 80s
• Characterized by
• Feedback loops
• Documentation-driven
• Each phase needs to be approved by SQA
• Advantages
• Enforced disciplined approach
• Documentation
• Maintenance easier
• Disadvantages
• Specifications not easily understood by clients
• Example stories (read textbook)
• Joe and Jane Johnson (house)
• Mark Marberry (suit)

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Rapid Prototyping Model

• Rapid prototype a working model functionally
  equivalent to a subset of the product
• Determine what the client needs
• When developed, the client and users try using it
• When they are satisfied, the process moves to the
  next phase
• Linear model
• Specifications are drawn from the rapid prototype
• Feedback loops are not used
• Rapid is the key

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Three Key Points

• Do not turn a rapid prototype into product
• Rapid prototyping may replace specification
  phasenever the design phase
• Comparison
• Waterfall modeltry to get it right first time
• Rapid prototypingfrequent changes until the
  client is satisfied, then discard

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Integrating Waterfall and Rapid Prototyping Models

• Waterfall model
• Many successes
• Client needs may not be met
• Rapid prototyping model
• Some success but not really proven
• Has own problems
• Solution
• Rapid prototyping for requirements phase
• Waterfall for rest of life cycle

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Incremental Model

• The product is designed, implemented, integrated
  and tested as a series of builds
• A build consists of code pieces from various
  modules interacting to provide a specific
  functionality
• Too few builds can lead to build-and-fix model
• Too many builds can lead to inefficient
  development

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Incremental Model (contd)

• Waterfall, rapid prototyping models
• Operational quality complete product at end
• Incremental model
• Operational quality portion of product within
  weeks
• Less traumatic
• Smaller capital outlay, rapid return on
  investment
• Needs open architecturemaintenance implications

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Concurrent Incremental Model

• More risky versionpieces may not fit
• CABTAB (code a bit and test a bit) and its dangers

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Extreme Programming

• Somewhat controversial new approach based on the
  incremental model
• Development team determines stories (features
  client wants)
• Estimate duration and cost of each story
• Select stories for next build
• Each build is divided into tasks
• Test cases for task are drawn up first
• Pair programming
• Continuous integration of tasks

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Unusual Features of XP

• Computers are put in center of a large room lined
  with cubicles
• Client representative is always present
• Cannot work overtime for 2 successive weeks
• No specialization
• All members of the team work on specification,
  design, coding and testing
• Refactoring
• No overall design
• The design is modified while the product is being
  developed

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Evaluating XP

• XP has had some successes
• Good when requirements are vague or changing
• Too soon to evaluate XP

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Synchronize and Stabilize Model

• Microsofts life-cycle model
• Also based on the incremental model
• Requirements analysisinterview potential
  customers
• Draw up specifications
• Divide project into 3 or 4 builds
• Each build is carried out by small teams working
  in parallel

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Synchronize and Stabilize Model (contd)

• At the end of the daysynchronize (test and
  debug)
• At the end of each buildstabilize (freeze build)
• Components always work together
• Get early insights into operation of product

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Spiral Model

• Simplified Waterfall model plus risk analysis
• Uses rapid prototypes
• Precede each phase by
• Alternatives
• Risk analysis
• Follow each phase by
• Evaluation
• Planning of next phase

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Simplified Spiral Model

• If risks cannot be resolved, project is
  immediately terminated
• Potential risks
• Timing constraints
• Lack of personnel
• Competence of team
• Dependency on hardware delivery

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Full Spiral Model

• Radial dimension cumulative cost to date
• Angular dimension progress through the spiral

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Analysis of Spiral Model

• Strengths
• Easy to judge how much to test
• No distinction between development, maintenance
• Weaknesses
• For large-scale software only
• For internal (in-house) software only

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Object-Oriented Life-Cycle Models

• Need for iteration within and between phases
• Fountain model Henderson-Sellers and Edwards,
  1990
• Recursive/parallel life cycle Berard, 1993
• Unified software development process
• Jacobson, Booch, and Rumbaugh, 1999
• All incorporate some form of
• Iteration
• Parallelism
• Incremental development
• Danger
• CABTAB (undisciplined approach of s/w
  development, pg. 84)

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Fountain Model

• Circles (phases) Overlap (parallelism)
• Arrows (iteration)
• Smaller maintenance circle

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Conclusions

• Different life-cycle models
• Each with own strengths
• Each with own weaknesses
• Criteria for deciding on a model include
• The organization
• Its management
• Skills of the employees
• The nature of the product
• Best suggestion
• Mix-and-match life-cycle model