1 CS101 Introduction to Computing Lecture 24 Design 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  1 universality

5 focus!

6 Today’s Lecture Heuristics 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 Caution ! Caution ! Caution ! Caution ! 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 together produces 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 1st Given many parts of a system to be designed/built, do the hard part 1st

20 # 3 All 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 Caution ! Caution ! Everything should be as simple as possible but not simpler Al Einstein

23 # 5 If you can’t explain it in 5 minutes, either you don’t understand it or it does not work

24 # 6 A system will develop & evolve much more rapidly if there are stable intermediate forms than if there are not Build iteratively; add features gradually

25 # 7 Success 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 customer needs instead of what he says he wants

28 # 9 If 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

35 #12 Partition/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 # 15 Don’t just remove the defect;...

40 # 15 Don’t just remove the defect; correct the process that caused it

41 #16 The number of defects remaining in a system after a given level of tests is proportional to...

42 #16 The number of defects remaining in a system after a 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