Users publish 5,000 posts a second, and an author has 200 followers on average. With fan-out on write, how many feed-cache inserts per second does that cause?

Answer

About 1,000,000 feed inserts/s. In review, anything from 500,000 to 2,000,000 counts as right.

The worked estimate

Each post is copied into every follower's feed: 5,000 × 200 = 1,000,000 inserts/s. A modest write rate becomes the busiest path in the system, which is why the fan-out runs on workers behind a queue.

Numbers: Numbers to know.

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