© 2010 University of California, Irvine – André van der Hoek 1April 18, 2023 – 20:56:21

Informatics 121Software Design IInformatics 121

Software Design I

Lecture 11

André van der Hoek & Alex Baker

Duplication of course material for any commercial purpose without the explicit written permission of the professor is prohibited.

Today’s LectureToday’s Lecture

Design rationale

In-class design exercise

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A Simple QuestionA Simple Question

Why is a given design as it is?

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A Difficult AnswerA Difficult Answer

Any design involves 100s if not 1000s of decisions that are tough to all remember

Worse yet, any design involves 100s if not 1000s of discussions, considerations of alternatives, and reasons why certain choices are made, which are even tougher to all remember

© 2010 University of California, Irvine – André van der Hoek 4April 18, 2023 – 20:56:21

A Difficult AnswerA Difficult Answer

Any design involves 100s if not 1000s of decisions that are tough to all remember

Worse yet, any design involves 100s if not 1000s of discussions, considerations of alternatives, and reasons why certain choices are made, which are even tougher to all remember

Especially three months later, a year later, two years later…

© 2010 University of California, Irvine – André van der Hoek 5April 18, 2023 – 20:56:21

Knowing Why Is A Crucial NeedKnowing Why Is A Crucial Need

Helps explain particular features to the customer

Helps assist with coding

Helps build a shared understanding of the design among a team of designers

Helps guide maintenance and other design changes– evolution– roadblocks

Helps reflect on one’s own design practices

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SketchbooksSketchbooks

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SketchbooksSketchbooks

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SketchbooksSketchbooks

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Software “Sketchbooks”Software “Sketchbooks”

If they exist, they tend to be text, text, text, text, text, and the occasional picture of a whiteboard…

Maybe an architecture diagram

Maybe a UML diagram

…

© 2010 University of California, Irvine – André van der Hoek 10April 18, 2023 – 20:56:21

Design DecisionsDesign Decisions

Explicitly recognizing design decisions helps in documenting the “design space”, and at least articulating the available choices

Some of your homeworks even discuss tradeoffs, and motivate the choices you make for your final design– though typically ignoring any intermediate decisions that

may have been made to which you later may need to return

The actual rationale, though, tends to be quite small in comparison to design decisions– and reconstructing it is difficult

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An ExampleAn Example

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Design RationaleDesign Rationale

Design rationale is focused on capturing a historical record of the analysis that led to the choice of a particular artifact or feature in question, and in the most general sense is an explanation of why something is designed the way it is

Design rationale involves explicit capture of the design decisions that shaped a particular software system and the reasons for those decisions (‘know-why’ knowledge)

© 2010 University of California, Irvine – André van der Hoek 13April 18, 2023 – 20:56:21

Design RationaleDesign Rationale

Descriptive– retroactive– aims to communicate a design’s intent

Prescriptive– in-the-moment– aims to improve the design process

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IBISIBIS

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gIBISgIBIS

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Modern VariantModern Variant

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Design HistoriesDesign Histories

Expand on design rationale by capturing design processes as they occur over time, as most design does not take place in a single meeting

More focused on description than prescription to date

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ADDSSADDSS

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Alex’ HomegrownAlex’ Homegrown

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In-Class Design ExerciseIn-Class Design Exercise

Collectively, using the tablets and Calico, design a history representation that you could have used for the previous assignment (or use for the next assignment)

Use your tablets to support your verbal contributions, annotate our design in progress, develop another idea in parallel, etc.

Remember stakeholders, goals, audience, uses, …

Remember techniques and notations available

© 2010 University of California, Irvine – André van der Hoek 21April 18, 2023 – 20:56:21