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

Software Life-Cycle Models The steps through which the product progresses Requirements phase Specification phase Design phase Implementation phase Integration phase Maintenance phase Retirement

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

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