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CHAPTER 3

CHAPTER 3. SOFTWARE LIFE-CYCLE MODELS. 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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CHAPTER 3

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  1. CHAPTER 3 SOFTWARE LIFE-CYCLE MODELS

  2. 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

  3. 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

  4. Build and Fix Model • Problems • No specifications • No design • Totally unsatisfactory for any reasonable size software • Need life-cycle model • “Game plan” • Phases • Milestones

  5. Waterfall Model • The only widely-used model until the early 80’s • 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)

  6. 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

  7. Three Key Points • Do not turn a rapid prototype into product • Rapid prototyping may replace specification phase—never the design phase • Comparison: • Waterfall model—try to get it right first time • Rapid prototyping—frequent changes until the client is satisfied, then discard

  8. 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

  9. 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

  10. 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 architecture—maintenance implications

  11. Concurrent Incremental Model • More risky version—pieces may not fit • CABTAB (code a bit and test a bit) and its dangers

  12. 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

  13. 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

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

  15. Synchronize and Stabilize Model • Microsoft’s life-cycle model • Also based on the incremental model • Requirements analysis—interview potential customers • Draw up specifications • Divide project into 3 or 4 builds • Each build is carried out by small teams working in parallel

  16. Synchronize and Stabilize Model (contd) • At the end of the day—synchronize (test and debug) • At the end of each build—stabilize (freeze build) • Components always work together • Get early insights into operation of product

  17. 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

  18. 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

  19. Full Spiral Model • Radial dimension: cumulative cost to date • Angular dimension: progress through the spiral

  20. 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

  21. 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)

  22. Fountain Model • Circles (phases) Overlap (parallelism) • Arrows (iteration) • Smaller maintenance circle

  23. 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

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