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