Chat (WhatsApp / Messenger)

One-to-one and group messaging with delivery receipts. The core problems are keeping millions of connections open, storing messages once even when clients retry, and delivering them in order to devices that come and go.

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Duration 45:00

Requirements checklist

Send a message to a user or a group (up to a few hundred members).Deliver in real time to online devices; store and deliver later to offline ones (with a push notification).Per-conversation message order; sent / delivered / read receipts.Fetch conversation history, paginated.End-to-end delivery latency under ~200 ms p99 between online users in a region.No message lost once the sender sees 'sent'; no message shown twice.Hundreds of millions of concurrent connections.

Clarifying questions

1:1 only, or groups — how large?
Do we store history on the server, or only until delivered?
Multiple devices per user?
End-to-end encrypted?
Which receipts are needed?

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requirements

Suggested time: about 6 minutes. Defines 'sent/delivered/read' precisely and what 'no duplicates' means.

Notes for requirements