Typing indicators and read receipts
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.
Clarify3:30 left
Estimate3:30 planned
Design10:00 planned
Deep dive8:00 planned
Failure5:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.