Chat System Architecture
Senior-level chat system design with WebSocket real-time messaging, presence tracking, group management, push notifications, Cassandra message storage, Redis caching, and media CDN with object storage.
Requirements
Functional
- Users can send 1:1 and group messages in real-time
- Online/offline presence indicators for contacts
- Push notifications for offline users
- Users can share media files in chat
- Message history is persistent and searchable
Non-Functional
- Message delivery latency under 100ms
- System supports 10M concurrent connections
- Messages stored with at-least-once delivery
- 99.99% availability for core messaging
Author
Published
February 24, 2026
Last updated February 24, 2026