Ch 2: Software Life-Cycle
Models
CSCI 4320
Ideal Software Development
Actual Software Development
• Software developers make mistakes
• Clients requirements or environment
changes while software is being
developed.
The Software Development Life-Cycle
involves many iterations (loops).
Bus Fare Machine Program
GOAL: Fare machines on bus must recognize real dollar bills.
• Episode 1: The first version is implemented
• Episode 2: A fault is found
– The product is too slow because of an implementation fault (double
precision instead of single precision used)
– Changes to the implementation are begun
• Episode 3: The requirements change
– A different/faster dollar recognition algorithm has been discovered
• Episode 4: A new design is adopted
– New design is more accurate.
- Development is complete
• Epilogue: A few years later, these problems recur
Teal Tractors Business Software
Goal : Handle sales, inventory and commissions for
business
• While the Teal Tractors software product is being
constructed, the requirements change
• The company is expanding into Canada
– Additional sales regions must be added
– Canadian taxes and other business aspects that are handled
differently
– Two different currencies, US and CANADA
Moving Target Problem
• Growing companies are always going to change
• Even if the reasons for the change are good, the
software product can be adversely impacted
– Dependencies will be induced
– Potentially cause a regression fault (A fault in an
apparently unrelated part of the software)
– feature creep
• There is no solution to the moving target problem
Life-Cycle Models
•
•
•
•
Waterfall Life
Code-and-Fix
Rapid-Prototyping
Extreme Programming and Agile
Processes
• Synchronize-and-Stabilize
• Spiral
• Iteration and Incrementation
Iteration and Incrementation
The basic software development process is
Iterative
– Each successive version is intended to be
closer to its target than its predecessor
An Incremental Process
– Miller’s Law : concentrate on ~ 7 chunks
– Use stepwise refinement
– handle in order of current importance
Iteration and Incrementation
• Idea:
– Project is divided into several mini projects
(increments)
– Each project goes through all phases,
extending existing artifacts
• Multiple opportunities to check for faults
• Always have a working version of
software
Waterfall
• No phase is complete until documentation for that phase
is complete.
• Relying solely on written specifications may lead to
products that don’t meet client’s needs.
Code-and Fix
• Developers simply throw code together
and rework it as many times as necessary
• No Requirements written
• May work well for small programs
• Maintenance Nightmare!
Rapid Prototyping
• Build a rapid-prototype and let client
interact and experiment with it.
• Developers draw up specifications from
rapid prototype and develop true system
• Key: Speed is of the essence
• The rapid prototype is discarded but
lessons learned are retained.
Open-Source Life-Cycle
• A single individual has an idea for a program and
makes it available free of charge
– SourceForge.net, FreshMeat.net
• Users become co-developers
– Report and correct defects
– Add additional functionality
– Port program to new environment
• Individuals are not paid to participate
• Release new versions after minimal testing
Open Source Software
• 100,000 open-source projects at SouceForge.net
and Freshmeat.net
• ½ have never attracted a team to work on the
project
• Majority have never been completed
• But there are cases where it has been benefical
– Operating Systems (Linux), web browsers (Firefox),
compilers (gcc), web servers(Apache)
Extreme Programming and Agile
Processes
New Approach: responsive to change, collaborate
with client
• Client picks features based on time and cost
estimates
• Build is broken down into tasks
• Pair programming
– Two programmers work on one computer
– Alternate typing every 15 or 20 minutes
– Team members change coding partners daily
• Client works with team at all times
Extreme Programming and Agile
Processes
• No individual can work overtime for 2 weeks
• No specialization: All members of team work on
analysis, design, coding
• No overall design step before the various builds
Agile Processes
• Less emphasis on analysis and design
• Goal is to deliver working software frequently
• Have short meetings often
• Successful for small-scale projects where
customer requirements are vague
Synchronized and Stabilize
•
•
•
•
Microsoft
Expected Features are prioritized
Work is divided into several builds
Each build is carried out by several teams
working in parallel
• At the end of the day all teams
synchronize and test
Spiral
• Waterfall method with each phase
preceded by risk analysis
• Uses prototypes to mitigate risks
• Advantage: Emphasis on alternatives and
constraints supports the reuse of existing
software
• Problem: When do you determine if a
phase has been sufficiently tested?
Comparison of Life-Cycle Models
Figure 2.12