1

CS101 Introduction to Computing

Lecture 24Design Heuristics

2

During the last lecture …

• We became familiar with the various phases of the process that developers follow to develop SW systems of reasonable complexity

• We looked at a couple of problems related to the Waterfall SW development model

3

In order to understand anything, you must not try to understand everything

Aristotle

4

efficiency 1universality

5

focus!

6

Today’s LectureHeuristics for System Architecting

• We will try to understand the role of heuristics in architectural (or high-level) design

• We will become familiar with a few popular design heuristics

7

Heuristic?

8

Heuristic

Rule of thumb learned through trial & error

Common sense lesson drawn from experience

Qualitative principle, guideline, general judgement

Natural language description of experience

9

?Heuristic

Wisdom

10

CautionCaution!! CautionCaution!! Heuristics don’t always lead to the best results

At times they even lead to the wrong ones, but mostly to results that are good-enough

11

system?

12

System

A collection of elements which

working togetherproduces a result not achieved by the things alone

13

System Architecture?

14

System Architecture

The structure(in terms of components, connections, constraints)

of a product or a process

15

Design == Architecture?

16

Heuristics for system architecting

Rules and lessons learnt by system architects

after long experiences

which when followed result in sound, stable, practical systems

17

My favorite system architecting(and other relevant) heuristics

--- in no particular order ---

18

19

Given many parts of a system to be designed/built,

do the hard part 1st1st

20

# 3All the serious

mistakes are made on the very first day

21

# 4

Simplify, simplify, simplify!

Probably the most useful heuristics for increasing reliability while decreasing cost & time-to-build

22

CautionCaution!!Everything should be as

simple as possiblebut not simpler

Al Einstein

23

# 5If you can’t explain it in 5 minutes, either

you don’t understand it or it does not work

24

# 6A system will develop &

evolve much more rapidly if there are stable intermediate

forms than if there are not

Build iteratively; add features gradually

25

# 7Success is defined by

the user, not the builder

26

Customer is always

right?

27

It depends!# 8

It’s more important to know what the customerneeds instead of what he says he wants

28

# 9If you think that your

design is perfect, it is only because you have not shown to anyone else

--- Get your designs reviewed ---

29

# 10

A good solution to a problem

somehow looks nice & elegant

30

#11 In partitioning, choose the chunks so that they are as independent as possible

Chunks should have low external complexity & high internal complexity

Organize personal tasks to minimize the time individuals face interfacing

31

2

6

5

4

31

32

2

6

5

4

31

33

2

6

5

4

31

34

12

3

5

6

7

84

9

35

#12Partition/repartition the problem until a model consisting of

7±2 chunks emerges

36

Life is the art of drawing sufficient conclusions

from insufficient premises

Samuel Butler

37

#13 When choices must be made with unavoidably inadequate info:

Choose the best available & then watch to see:

whether further solutions appear faster than future problems

If so, the choice was at least adequate

If not, go back & choose again

38

#14 The Triage

1. Let the dying die

2. Ignore who’ll recover on their own

3. Treat only those who’ll die without your help

39

# 15Don’t just remove the defect; ...

40

# 15Don’t just remove the defect; correct the process that caused it

41

#16 The number of defects remaining in a system aftera given level of tests is proportional to ...

42

#16 The number of defects remaining in a system aftera given level of tests is proportional to the number found during the test

43

#17 Programmers deliver the same number of

LOC/day regardless of the language they are writing in

Use the highest-level language

44

There are many more!

Please post any that are familiar to you on the message board

45

In Today’s Lecture

• We became familiar with the the role of heuristics in design

• We also discussed a few well-known design heuristics for architectural design

46

Next Lecture:Web Design for Usability

• To become able to appreciate the role of usability in Web design

• To become able to identify some of the factors affecting the usability of a Web page