1. CS 1120: Computer Science II Software Life Cycle
Slides courtesy of:
Prof. Ajay Gupta and Prof. James Yang
(format and other minor modifications by by Prof. L. Lilien)
Last updated on 1/10/10

2. Software engineering = production of quality software External quality
Software Quality
Software engineering = production of quality software
External quality
May be detected by its users
Internal quality
Perceptible only to computer professionals who have access to the actual code
What matters is the external quality
But it can only be achieved through internal quality

3. External Quality Criteria
Correctness
Perform the exact tasks as specified
Robustness
Ability to react appropriately to abnormal conditions
Extendibility
Ease of adapting program to changes of specification
Reusability
Ability of software components to serve for construction of many different applications

4. External Quality Criteria – Cont.
Other Quality Criteria
Security, Compatibility, Portability, Ease of Use, Efficiency, Timeliness (of delivery), …
Some Tradeoffs
Security vs. Ease of Use
Efficiency vs. Portability
Timeliness vs. Extendibility

5. Software Life Cycle – A Model

6. Software Life Cycle Phase 1: Specification
Answers the question:
“What do we build?”
Define clearly all aspects of the problem
What is input (valid/invalid) data?
What assumptions are possible?
Are there special cases?
What future enhancements are likely?

7. Software Life Cycle Phase 2: Design
Divide into manageable parts – modules
Classes & methods are (some of) the modules in C#
For each module specify:
purpose
assumptions
input
output
Develop algorithms for each module
Look for existing software components
Reuse them
Do not reinvent the wheel

8. Software Life Cycle Phase 3: Risk Analysis
Attempts to answer “What can go wrong, and how bad can it be?”
Predict and manage what risks you can
Consider risks to:
timetable,
cost,
human health …
Risks can greatly influence the
direction of a project
E.g. reduce software portability

9. Software Life Cycle Phase 4: Verification
Answers the question:
“Are the algorithms correct?”
Some algorithms can be proven correct
By using:
assertion: condition at a certain point
invariant: condition that is always true

10. Software Life Cycle Phase 5: Coding
Translate the algorithms into code
In a particular programming language
Coding is a minor phase in the software life cycle
Relatively easy
Developing algorithm (to be coded) is typically much more challenging

11. Software Life Cycle Phase 6: Testing
Answers the question:
“Did we build it correctly?”
Try to make the software fail-safe (or fault-tolerant)
Develop as many test cases as possible
Typically, as time allows
Testing is both a science and an art

12. Software Life Cycle Phase 7: Refining a Solution
Add bells and whistles
Retest after any changes are made

13. Software Life Cycle Phase 8: Production
Distribute software
Install it

14. Software Life Cycle Phase 9: Maintenance
Performed as long as software in use
Activities during maintenance:
Fix previously undiscovered bugs
Add new features
Enhance old features
Generally the most costly phase
80% of total cost by some estimates

15. Software Life Cycle During Phases 1-9: Documentation
Performed extensively in every phase
Hence “Documentation” is the axis of this “wheel model’
Different people working on each phase of the software life cycle
For all but very small projects
Novice programmers usually undervalue documentation

16. Continuing Effort
Next generation software being developed while the previous generation still in use
So Maintenance for Generation N overlapped by development for Generation N+1
Example:
Maintenance for
Windows XP overlapped by
development for Windows
Vista
Maintenance for Windows XP
and Windows Vista
overlapped by development
for Windows 7

17. The End