Design a notification and fan-out system that delivers to millions of users across push, email, and SMS.

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.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.