Design a News Feed

Design a News Feed

Step 1 of 11

read : write50 : 1

Ten million users, 500 posts a second, and fifty reads for every write. The ratio decides everything.

The Idea

Show each user recent posts from the accounts they follow, ranked. Assume 10 million users, 500 posts a second, and fifty reads per write — the ratio decides whether you pay when a post is published or when a feed is opened.

Real-World Example

A delivery round versus a newsstand. Dropping a copy through every subscriber's door is fan-out on write; making everyone walk to the stand and pick their titles is fan-out on read.

The Tradeoff

Fan-out on write gives an O(1) read and turns a celebrity's single post into tens of millions of inbox writes. Fan-out on read publishes instantly and makes every open a large merge. Real systems split it: write for ordinary accounts, read for the few enormous ones.

Your turn

Put the steps in the right order.

1. 1Merge the prebuilt inbox with freshly pulled celebrity posts↑↓
2. 2Write the post once to the author's own timeline↑↓
3. 3Rank the merged set and return one page↑↓
4. 4Fan the post id into each ordinary follower's inbox↑↓

Check order

Mini quiz

1 / 3

Fan-out on write makes opening the app cheap because:

• APosts are compressed
• BEach user's feed is already assembled and read once
• CFollowers are stored in memory
• DRanking is skipped