Twitter (News Feed) design must handle a massive imbalance: a celebrity tweet must be visible to millions instantly.
Fan-out on Write (Push)
For most users, when you tweet, we push it to the pre-computed "Home Feed" caches of all your followers. Optimized for blazing-fast reads.
Fan-out on Read (Pull)
For celebrities (millions of followers), pushing is too slow. Instead, their tweets are stored in a separate table, and followers pull them and merge into their feed only when they log in.