Hard Design: Chat System (WhatsApp/Slack)

What mechanism prevents a message from being delivered twice if the sender retries due to network timeout?

— Tests your understanding of this concept.

Answer Options

A Rate limiting

B Idempotency keys (unique message_id per message)

C Database transactions

D Client-side deduplication only

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This question is from the Design: Chat System (WhatsApp/Slack) topic (System Design Cases).

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