Art of Codeartofcode
Lesson 14 of 15

37. Design a News Feed

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The interview prompt
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37.1 What do you ask first?
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37.2 How big is this, really?
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37.3 What are the doors?
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37.4 What do we store?
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37.5 What does the whole system look like?
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37.6 Fan-out on write or on read?
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37.7 How do you solve the celebrity problem?
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37.8 How does ranking work?
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37.9 How do the feed cache and pagination work?
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37.10 What would you say in the last five minutes?
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Score yourself
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Variations to practise
14
Sources
15
Code lab
37. Design a News Feed

Sources

3 min read+10 XP

Source notes

  • Fan-out on write with special handling for high-follower accounts: Raffi Krikorian (Twitter), "Timelines at Scale", QCon San Francisco 2012 (see Chapter 23).
  • Early public description of feed ranking as affinity × weight × time decay: Facebook's "EdgeRank", as presented at f8 in 2010 and widely reported. Modern feeds use learned models.
  • Time-sortable IDs: see Chapter 10.
  • All user, follower, post and read numbers are interview assumptions agreed in Step 1, not figures for any real app. Arithmetic computed 2026-10-08.
  • Fan-out arithmetic, ranking order and snapshot pagination: tested 2026-10-08 (code/ch37_news_feed.py, all checks pass).