Art of Codeartofcode
Lesson 14 of 15

31. Design a URL Shortener

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The interview prompt
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31.1 What do you ask first?
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31.2 How big is this, really?
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31.3 What are the doors?
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31.4 What do we store?
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31.5 What does the whole system look like?
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31.6 How do you generate the short code?
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31.7 301 or 302?
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31.8 How do you keep redirects fast at 12,000 a second?
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31.9 How do you count clicks without slowing redirects?
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31.10 What would you say in the last five minutes?
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Score yourself
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Variations to practise
14
Sources
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Code lab
31. Design a URL Shortener

Sources

3 min read+10 XP

Source notes

  • 301 (Moved Permanently) responses are cacheable by default; 302 (Found) only with explicit freshness information; 307 and 308 keep the request method: HTTP Semantics, RFC 9110, sections 15.4.2, 15.4.3, 15.4.8 and 15.4.9.
  • All traffic numbers are interview assumptions agreed in Step 1, not figures for any real service.
  • Arithmetic (62⁶ = 56,800,235,584; 62⁷ = 3,521,614,606,208; about 333 million redirects a day; about 33 GB of hot cache; 6 billion ÷ 62⁷ ≈ 0.17% clash chance per new random code at full size) computed 2026-10-05.
  • ID blocks, scrambling and hash-with-retry code: tested 2026-10-05 (code/ch31_url_shortener.py, all checks pass).