How Instagram handles celebrities with 500M followers without melting their servers

Taylor Swift has 300 million followers, how do we keep our system from falling over when she posts to all of them?

One solution the post gets pushed into every follower's feed cache immediately. They can then read from that cache almost instantly when they load the app. Great!

Errr ... until you have 300 million followers. A single post triggers 300 million cache writes at once. That write storm takes minutes to propagate and can spike server load to catastrophic levels.

So let’s use a hybrid approach. Regular users still get fan-out on write. [show a post going to a small number of inboxes) Their posts are pre-computed into follower feeds. Fast reads, one lookup.

Write load stays flat no matter what Taylor does. The tradeoff is slightly more complex read logic, but feed latency stays under 500ms because the celebrity merge is a small operation on top of an already-cached feed.

There's no universally correct fan-out strategy. The right answer depends on the data your system is handling. That's the kind of nuance that separates a good system design answer from a great one. #shorts

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