HardReal-time45 min

Design a Chat System (WhatsApp)

Design a real-time messaging system similar to WhatsApp supporting 1:1 and group chats with message delivery guarantees.

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Functional Requirements

1:1 messaging and group chats (up to 500 members)
Online/offline status and read receipts
Message persistence and history
Support text, images, and file sharing
End-to-end encryption
Push notifications for offline users

Steps (0/6)

Step 1: Clarify Requirements

Scale: 500M daily active users, 50B messages/day. 1:1 and group chat. Message types: text, images, files. Delivery guarantees: sent, delivered, read. Message history stored for 30 days? End-to-end encryption?

Key Points

500M DAU, 50B messages/day → ~580K messages/sec
Average message size: ~100 bytes text, media varies
Storage: 50B × 100 bytes × 30 days ≈ 15 PB for text
Real-time with <100ms latency when both users online

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