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Design a chat / messaging service

1:1 and small groups: real-time delivery, history, presence — common follow-up to distributed systems fundamentals.

~26 minDesign interview
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Interview script (outline)
  1. Clarify: 1:1 vs groups, max group size, attachments, search, E2E encryption scope.
  2. Estimate: DAU, messages/user/day, peak connection churn, storage growth for history.
  3. Real-time: WebSockets vs long-polling; connection management; sticky routing optional.
  4. Storage: message log per conversation, pagination, read receipts optional table.
  5. Scale: fan-out for large groups (different from 1:1); presence service.
  6. Failure: offline queue, idempotent send, push notifications path.
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