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Code RoomWebsocket fan-out gateway
HardPrep Room Coding #3759

Websocket fan-out gateway

System designNetworking & APIsDistributed systemsSenior–Staff~50 min

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.

Clarify5:30 left
Estimate5:30 planned
Design16:30 planned
Deep dive13:30 planned
Failure9:00 planned
0:00
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.