System designMediumsd-g020

Subject Presence systemsLevel Mid–Senior~30 minCommon in Distributed systems interviewsIndustries Technology, Software development

Question

Design the typing-indicator and 'delivered/read' receipt subsystem for a large-scale messaging app (200M users) with group chats up to a few thousand members. Requirements: a typing bubble appears within ~500ms and disappears reliably even if the sender's app is force-killed, receipts must scale in big groups without melting the network, and none of this ephemeral signaling should bloat your durable message store. Walk through the design.

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.