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Design a real-time communication system supporting 5M concurrent connections with sub-200ms message delivery, covering WebSocket vs SSE vs long-polling, connection management, heartbeats, reconnection, message ordering, and fan-out at scale.
## Problem
You are designing a real-time communication system for a platform that supports live messaging, presence indicators, typing notifications, and real-time updates. The system must maintain persistent connections with millions of concurrent users, deliver messages with low latency, and handle the messy realities of mobile networks — intermittent connectivity, network switches, and variable latency. Your design must scale horizontally and handle the fan-out problem of delivering a single message to thousands of subscribers.
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Design canvas, rubric, hints, and model solutions.