System Requirements
- Sub-second real-time message delivery over flaky mobile connections
- Delivery status receipts: Sent (✓), Delivered (✓✓), Read (blue ✓✓)
- Store-and-forward offline buffer for disconnected devices
- Group chats up to 1,024 members with decentralized fan-out
Graded Constraints
01Budget ceiling: $30,000/month
02Gateway cluster maintaining millions of concurrent WebSocket/TCP connections
03Fast session routing cache (Redis) mapping user IDs to active gateway servers
04Ephemeral message queue buffer (Kafka / Queue) for offline delivery
05Secure key store and user profile DB
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