System designMediumsd-g700

Subject Sse longpoll fanout protocolLevel Mid–Senior~40 minCommon in Networking & APIs · Distributed systems interviewsIndustries Technology, Software development

Question

Design a server-push fan-out service using Server-Sent Events (with HTTP long-poll fallback for old proxies) that streams live updates — order status, scores, dashboard metrics — to 3M concurrently-connected browsers. Updates are one-directional server→client, must arrive within ~1 s, and clients must not miss events across reconnects. Why SSE over WebSockets here, and how do you fan out, hold connections, and handle reconnect gaps?

What a strong answer looks like

Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts — data model, bottlenecks, consistency, failure modes — and name the trade-offs you are making.

Learn the concepts

Narrate your design

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Run or narrate your approach, then ask the coach.