CS101 Introduction to Computing Lecture 24 Design Heuristics

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

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

1 universality efficiency 

focus!

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

Heuristic?

Heuristic Rule of thumb learned through trial & error Common sense lesson drawn from experience Qualitative principle, guideline, general judgement Natural language description of experience

Heuristic Wisdom ?

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

system?

working together A collection of elements which System produces a result not achieved by the things alone

System Architecture?

The structure of a product or a process System Architecture (in terms of components, connections, constraints) of a product or a process

Design == Architecture ? Design == Architecture

Heuristics for system architecting Rules and lessons learnt by system architects after long experiences which when followed result in sound, stable, practical systems

My favorite system architecting (and other relevant) heuristics --- in no particular order ---

Given many parts of a system to be designed/built, do the hard part 1st

# 3 All the serious mistakes are made on the very first day

# 4 Simplify, simplify, simplify # 4 Simplify, simplify, simplify! Probably the most useful heuristics for increasing reliability while decreasing cost & time-to-build

Caution! Everything should be as simple as possible but not simpler Al Einstein

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

# 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

# 7 Success is defined by the user, not the builder

Customer is always right?

It depends! # 8 It’s more important to know what the customer needs instead of what he says he wants

# 9 If you think that your design is perfect, it is only because you have not shown to anyone else --- Get your designs reviewed ---

# 10 A good solution to a problem somehow looks nice & elegant

#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

5 1 3 6 2 4

5 1 3 6 2 4

5 1 3 6 2 4

8 4 1 2 5 7 9 3 6

#12 Partition/repartition the problem until a model consisting of 7±2 chunks emerges

Life is the art of drawing sufficient conclusions from insufficient premises Samuel Butler

#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

#14 The Triage 1. Let the dying die 2 #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

# 15 Don’t just remove the defect; ...

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

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

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

#17 Programmers deliver the same number of LOC/day regardless of the language they are writing in Use the highest-level language

There are many more! Please post any that are familiar to you on the message board

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

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