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Chat System Design
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Scenario
A product launch spikes 30K messages per second while 5M users sit with WebSockets open. One user's "typing…" indicator should not fan out as a full message; offline users must catch up without duplicate bubbles when the network retries. The interview is real-time delivery, ordering, and connection scale—not a single "message queue" box.
Design a real-time chat system supporting one-on-one and group messaging with delivery and read receipts, presence, and searchable history. Production systems separate low-latency signaling from media blobs and assume networks retry—your design must handle duplicates and ordering without confusing users.
You should support text and attachments, online/typing presence, offline catch-up, and ordered delivery at scale. Be ready to explain connection server sharding, per-chat sequence numbers, group fan-out policies, and storage for 2.5B+ messages/day. Latency budgets and degradation when the ranker or fan-out queue lags are fair game.
Constraints
1:1 and group chat, text/images/files, delivered/read receipts, presence, message history with pagination
< 200 ms p95 delivery for online users, 99.99% availability, ordered per-chat delivery, durable history
500M users, 50M DAU; ~2.5B messages/day (~30K/s peak); ~1.25 TB/day; millions of concurrent connections
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