fan out

The real-time messaging staple. Interviewers look for the delivery path (persistent connections, not polling), a fan-out strategy that survives large groups, and a clear story on ordering and delivery guarantees. This lays out that design, including the fan-out tradeoff that marks senior candidates.

A celebrity posts and 50M followers need a notification. The tricky 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.