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CS101 Introduction to Computing

Lecture 24Design Heuristics

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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

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

Aristotle

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efficiency ∝ 1universality

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focus!

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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

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Heuristic?

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Heuristic

Rule of thumb learned through trial & error

Common sense lesson drawn from experience

Qualitative principle, guideline, general judgement

Natural language description of experience

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?Heuristic

Wisdom

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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

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system?

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System

A collection of elements which

working togetherproduces a result not achieved by the things alone

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System Architecture?

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System Architecture

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

of a product or a process

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Design == Architecture?

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Heuristics for system architecting

Rules and lessons learnt by system architects

after long experiences

which when followed result in sound, stable, practical systems

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My favorite system architecting(and other relevant) heuristics

--- in no particular order ---

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

do the hard part 1st1st

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# 3All the serious

mistakes are made on the very first day

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# 4

Simplify, simplify, simplify!

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

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CautionCaution!!Everything should be as

simple as possiblebut not simpler

Al Einstein

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

you don’t understand it or it does not work

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# 6A system will develop &

evolve much more rapidly if there are stable intermediate

forms than if there are not

Build iteratively; add features gradually

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# 7Success is defined by

the user, not the builder

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Customer is always

right?

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It depends!# 8

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

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# 9If you think that your

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

--- Get your designs reviewed ---

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# 10

A good solution to a problem

somehow looks nice & elegant

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#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

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

7±2 chunks emerges

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Life is the art of drawing sufficient conclusions

from insufficient premises

Samuel Butler

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#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

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#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

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# 15Don’t just remove the defect; ...

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

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

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

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#17 Programmers deliver the same number of

LOC/day regardless of the language they are writing in

Use the highest-level language

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There are many more!

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

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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

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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