System design interview question
Design an Offline-First Mobile Sync Engine with Conflict Resolution like WhatsApp's Message Sync Across Devices at Scale
Design a system that synchronizes user data (messages, media metadata, read receipts, typing indicators) across 5+ mobile devices and web clients for a user, with offline-first capabilities and eventual consistency guarantees. The system must handle 2B monthly active users, 500M concurrent users, 1M messages/second ingestion, high-latency networks (2G/3G), intermittent connectivity, and device storage constraints (<500MB per app). Users expect <5s sync latency on reconnection, zero message loss, and deterministic conflict resolution when the same message is edited/deleted across devices simultaneously. Address on-device SQLite limitations, bandwidth optimization for poor networks, battery impact, and operational observability for mobile clients in the wild.
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