Download - Edu+Presentation
Slide 3.1
CHAPTER 3
SOFTWARE LIFE-CYCLE
MODELS
Slide 3.2
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
Slide 3.3
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
Slide 3.4
Build and Fix Model
Problems– No specifications– No design
Totally unsatisfactory for any reasonable size software
Need life-cycle model– “Game plan” – Phases– Milestones
Slide 3.5
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)
Slide 3.6
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
Slide 3.7
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
Slide 3.8
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
Slide 3.9
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
Slide 3.10
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
Slide 3.11
Concurrent Incremental Model
More risky version—pieces may not fit– CABTAB (code a bit and test a bit) and its dangers
Slide 3.12
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
Slide 3.13
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
Slide 3.14
Evaluating XP
XP has had some successes Good when requirements are vague or
changing Too soon to evaluate XP
Slide 3.15
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
Slide 3.16
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
Slide 3.17
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
Slide 3.18
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
Slide 3.19
Full Spiral Model
Radial dimension: cumulative cost to date Angular dimension: progress through the spiral
Slide 3.20
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
Slide 3.21
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)
Slide 3.22
Fountain Model
Circles (phases) Overlap (parallelism)
Arrows (iteration) Smaller
maintenance circle
Slide 3.23
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