When your location changes, every friend who has you in their Nearby Friends list needs to know. At 10 million active users each with 200 friends, a single location update could trigger 200 downstream notifications. At 333,000 updates per second, that’s 66 million fan-out operations per second. This is location fan-out: propagating position changes to all interested subscribers efficiently. Why This Is Different From Chat Fan-Out Chat fan-out sends one message to N subscribers.

A popular Twitch stream has 200,000 concurrent viewers. Every one of them can send chat messages. A message from one viewer needs to reach all other viewers within a second. That’s fundamentally different from the fan-out problem in a news feed, and the solutions are different too. Why News Feed Fan-Out Doesn’t Apply News feed fan-out is about delivering content to followers: write to each follower’s timeline, or read and merge at request time.