System designMediumsd-g704

Subject Presence status service networkingLevel Mid–Senior~40 minCommon in Databases & SQL · Networking & APIs · Distributed systems interviewsIndustries Technology, Software development

Question

Design a presence / online-status service (the 'green dot' — online / away / offline / typing) for a social or chat app with 100M users, where each user's status must propagate to their friends/contacts within ~2 s. Status changes are extremely high-volume (every connect/disconnect/idle), reads are even higher (rendering buddy lists). How do you track who's online, detect disconnects, and fan status changes out to watchers efficiently?

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.