Fan-out on write.
also: fan-out-on-write · push-based feedPre-computing feeds when someone posts: the post is pushed into every follower's feed, making reads cheap and writes expensive.
the two-minute refresher
- Post once → write to N follower feeds (the fan-out).
- Reading a feed is one cheap lookup of a ready list.
- Write amplification: one post becomes millions of writes for celebrities.
- Wasted work for inactive followers.