Maintenance and Evolution

Question 1

Describe how Canning’s iceberg diagram represents the cost of software engineering

Answer 1

The tip of the iceberg represents the acquisition cost - research, design, testing, etc. Below the surface lies many other costs that may not be accounted for:
- operation cost
- product distribution cost
- maintenance cost
- training cost
- retirement and disposal cost
and others

Question 2

Why is change in software inevitable?

Answer 2

Software doesn’t wear out like physical objects but it still needs to be
- fixed (corrective maintenance)
- adapted to changing need (adaptive maintenance)
- improved in performance (perfective maintenance)
- improved by fixing bugs (preventative maintenance)

Question 3

List the 3 R’s of SW evolution (3 R’s)

Answer 3

1. Reverse engineering - analyze an existing system
2. Refactoring - cleaning up code w/o modifying functionality
3. Reengineering - creating a maintainable system out of an unmaintainable one (at a larger scale than refactoring)

Question 4

List the 3 laws of science that the 3 R’s of SW evolution are based off of

Answer 4

1. Reductionism
2. Repeatability
3. Refutation

Question 5

Explain what a “legacy system” is

Answer 5

A system that is hard to evolve, for any reason

Question 6

Define “software evolution”

Answer 6

Software evolution refers to the process of developing software initially, then repeatedly updating it for various reasons.

Question 7

Discuss the impact of Moore’s law on software

Answer 7

Software tends to follow the exponential expansion of hardware, but developer’s capabilities do not. Creating new software paradigms yields discontinuous jumps in productivity, but at the expense of existing code bases.

Question 8

Describe what an S type program is

Answer 8

S (specified) type program - all requirements that are important to stakeholders are formally and completely defined. Correctness of the program w.r.t its specification are the only criterion of the acceptability of the program.
Example - calculation of the LCM of two integers

Question 9

Describe what a P type program is

Answer 9

P (Problem) type program - based on a practical abstraction of the problem instead of relying on completely defined specification
Example - a program that plays chess

Question 10

Describe what an E type program is

Answer 10

E (Evolving) type program - one that is embedded in the real world and changes as the world does. These programs mechanize a human activity and make simplifying assumptions. The acceptance of an E type program depends on the stakeholders’ opinion and judgment of the solution.

Question 11

What is meant by Lehman’s first law - continuing change

Answer 11

An E type program must continually be adapted, or else it becomes progressively less satisfactory.

Question 12

What is meant by Lehman’s second law - Increasing complexity

Answer 12

As an E type program evolves, its complexity increases unless work is done to maintain or reduce it.

Question 13

What is meant by Lehman’s third law - Self Regulation

Answer 13

The evolution process of an E type program is self regulating.

Question 14

What is meant by Lehman’s fourth law - organizational stability

Answer 14

The average effective global activity rate in an evolving E program is invariant over the program’s lifetime

Question 15

What is meant by Lehman’s 5th law - Conservation of familiarity

Answer 15

The average content of successive releases is constant during the life cycle of an E type program

Question 16

What is meant by Lehman’s 6th law - continuous growth

Answer 16

To maintain user satisfaction, the content of an E program must be continually increased.

Question 17

What is meant by Lehman’s 7th law - declining quality

Answer 17

The quality of an E type program will appear to decline unless rigorously maintained

Question 18

What is meant by Lehman’s 8th law - Feedback System

Answer 18

E type evolution processes constitute multi-agent feedback systems and must be treated as such to achieve significant improvement

Question 19

Explain Wirth’s law

Answer 19

Wirth’s law states that software is getting slower more rapidly than hardware is becoming faster

Question 20

Why does Wirth believe software is getting slower?

Answer 20

1. Rapidly growing hardware performance.
2. Customers’ ignorance of what features are essential and which are “nice to have”
   - non essential features are referred to as “fluff”

Question 21

Define software entropy

Answer 21

As a system is modified, its disorder or entropy tends to increase. This is known as software entropy.

Question 22

List the 3 types of software maintenance

Answer 22

1. Bug fixing
2. Modifying software to work in a new environment
3. Implementing new or changed requirements