Art of Codeartofcode
Lesson 7 of 21

1. How to Approach a Design Question

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The picture to keep in your head
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1.1 What is a system design interview, and what isn't it?
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1.2 What is the interviewer actually scoring?
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1.3 What's the step-by-step framework?
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1.4 How do you manage a 45-minute round?
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1.5 What questions should you ask first?
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1.6 What's the difference between functional and non-functional requirements?
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1.7 What's an API, and why define it before drawing boxes?
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1.8 How do you sketch the data model?
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1.9 What does a good high-level design look like?
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1.10 How do you choose what to deep dive into?
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1.11 How do you talk about trade-offs?
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1.12 What if you get stuck or don't know something?
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1.13 What are the most common mistakes?
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1.14 What does the bar look like: fresher vs SDE-2?
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Memory card and where the analogy breaks
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Practice: five levels
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Interview questions
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Solutions
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Sources
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Code lab
1. How to Approach a Design Question

1.6 What's the difference between functional and non-functional requirements?

3 min read+10 XP

At the architect's desk: "Three bedrooms, a kitchen, a balcony" is a list of rooms: what the house has. "Earthquake-safe, cool in summer, ready in six months" is about how good the house is. Both matter, but they're different lists.

In the interview:

  • Functional requirements = what the system does. Features a user could press a button for.
  • Non-functional requirements = how well it does it. Qualities like speed, uptime and durability.

The quick test: functional requirements are verbs, non-functional ones are adjectives.

functional requirements are verbs, non-functional requirements are adjectives.

Figure: functional requirements are verbs (what it does). Non-functional requirements are adjectives (how well it does it).

For the URL shortener:

  • Functional: shorten a long URL; redirect a short URL to the original.
  • Non-functional: fast (a redirect feels instant), available (links keep working when a server dies), durable (a saved link is never lost), scalable (billions of links without a redesign).

Why the non-functional list matters more than beginners think: the verbs tell you what to build, but the adjectives decide how. "Shorten a URL" for 100 users fits on one laptop. "Shorten a URL, always available, for billions of links" needs replication, caching and careful ID generation. The adjectives choose the architecture.

Memory code: "Verbs vs adjectives." Functional = what it does (verbs). Non-functional = how well (adjectives). The adjectives pick the architecture.