System designHardsd-g651

Subject Websocket gatewayLevel Senior–Staff~50 minCommon in Networking & APIs · Distributed systems interviewsIndustries Technology

Question

Design a websocket fan-out gateway for a live-events product that holds 10M concurrent persistent connections and must deliver a published message to all subscribers of a channel (some channels have 2M subscribers) with p99 delivery under 500ms. Connections are long-lived, clients reconnect constantly on flaky mobile networks, and the system must survive a gateway node dying without dropping the whole channel. Walk through the connection tier, the fan-out path, subscription/routing state, and the central trade-off.

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.