fan out

The real-time messaging classic. Interviewers want the delivery path (persistent connections, not polling), the fan-out strategy that does not collapse on large groups, and an ordering and delivery-guarantee story. Here is that design with the fan-out tradeoff that separates seniors.

A celebrity posts and 50M followers need a notification. The hard parts are not sending one message, they are fan-out strategy, deduplication, channel routing, and not flooding users. Here is the architecture that handles both the long tail and the viral spike.