How to Implement SSE for Notifications in .NET in a Production-Ready Way?

This is not a demo – this is the same pattern used in real chat/notification systems (WhatsApp-like browser notifications, dashboards, etc.). 1. When to Use SSE (Production Decision) Use SSE when: One-way server → browser notifications Chat message alerts (not full chat transport) Live counters, alerts, status updates Browser-based apps (no mobile socket requirement) Do NOT use SSE for: Binary streaming High-frequency bidirectional chat typing events Mobile background connections (WebSocket better) 2. Architecture (Production) Browser (EventSource) ↓ SSE Controller (long-lived HTTP) ↓ ConcurrentDictionary<UserId, SseClient> ↓ Notification Publisher (Chat / Job / API) Key rules: One SSE connection per user No DB polling No blocking threads Heartbeat required Auto-reconnect handled by browser 3. Core SSE Infrastructure (IMPORTANT) 3.1 SSE Client Model public