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Problem Understanding
Restate the problem in your own words.
Design a Chat System (WhatsApp-class)
Design a WhatsApp- or Messenger-class chat system: 1:1 and small group conversations (≤ 256 members), end-to-end encryption, message ordering preserved per chat, presence (online / typing / last-seen), delivery + read receipts, and offline backfill. The hard parts are scaling persistent connections — the WebSocket gateway pool sized for 10s of millions of concurrent sockets — preserving causal order across replicas, and fan-out for read receipts in groups (a ‘read’ from one user updates everyone else’s UI).
- WhatsApp~2B users, E2E by default (Signal protocol). Famous for handling > 100B messages/day on a small ops team.
- Messenger (Meta)Tightly coupled with the FB social graph; recently shipped E2E by default.
- TelegramCloud-first chats by default, optional E2E ‘secret chats’. Different storage trade-off than WhatsApp.
- Slack DMsWorkplace chat — same gateway + ordering problem, different durability and search expectations.
Your task: read the problem above, then write what the system is, who uses it, the rough scale, and the headline UX expectation — in your own words. Submit for AI review when you're ready.
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