Design: news feed

System Design · lesson 14 of 32 · 4 min read

Compare fan-out on write and on read, and see why real feeds use both at once.

Key points

A feed is posts from the accounts you follow, newest first. The hard requirement is that a read must be fast for every user at once.
Fan-out on write: when someone posts, push the post id into each follower’s feed list in Redis. Reads become one lookup.
That breaks on celebrities. One post to 50M followers is 50M writes, and most of those feeds are never opened.
Fan-out on read: fetch the accounts a user follows and merge their recent posts at request time. Cheap writes, slow reads.
Hybrid is what real systems ship: fan out for normal accounts, pull for the few huge ones, merge the two at read time.
Keep only the newest ~1,000 ids per user; older pages fall back to the database. Ranking is a layer on top - get delivery right first.

Precompute for the many, compute on demand for the few, and merge the two at read time.

This is a reading copy. The full lesson — with the visual explainer, the interactive lab and a Run button for the code — lives in the System Design course, and every lesson in it is listed on the System Design contents page.