37. Design a News Feed
1
The interview prompt2
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 yourself13
Variations to practise14
Sources15
Code lab37. Design a News Feed
37.9 How do the feed cache and pagination work?
3 min read+10 XP
The feed cache holds, per user, the latest 800 post IDs (newest first), in a Redis list or sorted set by post ID, sharded by user. About 0.64 TB for 100 million users. Trim on every insert; older items come from the posts store if someone scrolls that far.
Pagination with a ranked feed is tricky. Offset pagination ("items 11-20") breaks when new posts arrive and the ranking shifts between pages:
Figure: rank once per session; page through the snapshot with a cursor.
In the chapter's code, three new high-scoring posts arrive between page 1 and page 2; re-ranking and taking items 11-20 shows 3 posts twice. The fix:
- Rank once when the session opens (or on pull-to-refresh) and keep the ordered IDs as a snapshot (in a short-lived cache, keyed by session).
- Page through it with a cursor ("after this post ID"), as in Chapter 24.
- New posts show as a "3 new posts" banner; tapping it refreshes and re-ranks.
Memory code: "Rank once per visit, scroll with a cursor, announce new posts at the top."