Fan-out on write (push)
Alice tweets. For each of Alice's followers, tweet appended to their pre-built timeline in Redis. Read-time = single fetch.
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Fan-out on write (push)
Alice tweets. For each of Alice's followers, tweet appended to their pre-built timeline in Redis. Read-time = single fetch.
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Fan-out on read (pull) for celebs
Celeb with 100M followers → push fan-out too expensive. Their tweets pulled at read time + merged into follower's timeline.