Design a Real-Time Messaging System (1:1 and Group)

Context

You are designing a consumer-scale messaging platform supporting both direct and group conversations across multiple user devices. The system must provide low-latency real-time delivery, offline sync, and strong privacy controls while operating at large scale.

• Scale targets: 100M MAU, 10M DAU
• Latency SLO: p99 send-to-receive under 150 ms (same-region)

Requirements

1. Features
   • 1:1 and group chats
   • Multi-device sync (multiple active devices per user)
   • Presence and typing indicators
   • Read receipts and delivery receipts
   • Media attachments (images/video/docs) with previews
   • Message search
2. Delivery Semantics and Ordering
   • At-least-once delivery with deduplication
   • Message IDs and ordering guarantees per conversation
3. Offline and Sync
   • Offline storage and background sync for clients
   • Server-side backfill and out-of-order handling
4. APIs (define schemas and behavior)
   • SendMessage
   • Ack (delivery/read/typing)
   • Sync (incremental state)
   • FetchHistory (backfill)
   • SubscribePresence (presence/typing)
5. Architecture
   • Edge gateways and real-time fanout (e.g., WebSockets)
   • Message persistence and indexing for search
   • Cold storage tiering
   • Partitioning by user/conversation
6. Non-Functional Concerns
   • Exactly-once UX despite at-least-once transport
   • End-to-end encryption options
   • Abuse/spam mitigation, rate limits/quotas
   • Data retention/TTL and GDPR deletion
   • Observability (metrics, logs, tracing, SLOs)

Describe the end-to-end design and trade-offs. Include key data flows and how you meet latency and scale targets.