Art of Codeartofcode
Lesson 11 of 15

37. Design a News Feed

1
The interview prompt
2
37.1 What do you ask first?
3
37.2 How big is this, really?
4
37.3 What are the doors?
5
37.4 What do we store?
6
37.5 What does the whole system look like?
7
37.6 Fan-out on write or on read?
8
37.7 How do you solve the celebrity problem?
9
37.8 How does ranking work?
10
37.9 How do the feed cache and pagination work?
11
37.10 What would you say in the last five minutes?
12
Score yourself
13
Variations to practise
14
Sources
15
Code lab
37. Design a News Feed

37.10 What would you say in the last five minutes?

3 min read+10 XP

Step 6: Wrap-up (5 minutes)

  • Bottlenecks: fan-out writes (hybrid, skip inactive users, priority lanes, Chapter 22) and feed reads (cache of IDs, hydration from caches).
  • Single points of failure: none central; feed cache and posts are sharded and replicated; feeds can always be rebuilt by pulling.
  • Freshness: async fan-out within seconds for most posts; celebrity posts are always fresh because they're pulled.
  • Next with more time: ads blended into the ranking, video, "stories", privacy settings (who can see a post affects fan-out), and deletion (remove from feeds lazily: filter deleted IDs at hydration).

Memory code: "Precompute what's cheap, pull what's huge, rank what's left."