Fan-out-on-write
A strategy for feeds where, on publishing, a post is copied out to all followers' precomputed "inbox" — so reading becomes cheap (fetch your ready-made list). The opposite, fan-out-on-read, builds the feed at each read instead.
Trade-off: fan-out-on-write makes the write expensive (a post from someone with 1 million followers → 1 million inserts) but reading becomes lightning-fast (your timeline is already assembled). Fan-out-on-read is cheap to write but expensive to read (merge all your followees' posts on each page load). Classic solution (Twitter/Instagram): a hybrid — fan-out-on-write for regular users, but "celebrities" with huge followings are handled with fan-out-on-read (merged in at read time) to avoid million-row writes. Related: the "thundering herd" when a celebrity posts. One of the most famous scalability problems in social-media architecture. The choice is driven by the read/write ratio and follower distribution.