Two ways to build a feed.
Fan-out on write: when you post, copy it into every follower's timeline immediately. Reads are instant — the feed is already built.
Fan-out on read: store the post once. Assemble each timeline when someone opens the app. Writes are cheap, reads do the work.
Now someone with 50 million followers posts.
Fan-out on write means 50 million inserts. From one tap. Your queue is on fire and the last follower gets it forty minutes late.
So nobody picks one:
| Account type | Strategy |
|---|---|
| Normal user | Fan-out on write — pushed to followers |
| Celebrity | Fan-out on read — pulled in at open time |
Your feed is a merge of pre-built rows plus a live fetch of the handful of huge accounts you follow.