System designHardsd-g511

Subject Presence systemsLevel Mid–Staff~45 minCommon in Distributed systems interviewsIndustries Technology, Software development

Question

Design a presence and typing-indicator system for very large rooms. Some rooms have 500K concurrent members; presence (online/away/offline) and 'X is typing…' must feel real-time (<1s) but you cannot broadcast every keystroke or every member's status change to all 500K clients — that's a fan-out explosion. Members join/leave constantly and connections are flaky mobile sockets.

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.