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
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