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name: NestJS Real-Time description: WebSocket and SSE selection strategies and scaling. metadata: labels: [nestjs, websockets, sse, socket.io] triggers: files: ['/*.gateway.ts', '/*.controller.ts'] keywords: [WebSocketGateway, SubscribeMessage, Sse, Socket.io]

Real-Time & WebSockets

Priority: P1 (OPERATIONAL)

WebSocket and real-time communication patterns with NestJS.

• WebSockets (Bi-directional): Use for Chat, Multiplayer Games, Collaborative Editing.
  • High Complexity: Requires custom scaling (Redis Adapter) and sticky sessions (sometimes).
• Server-Sent Events (SSE) (Uni-directional): Use for Notifications, Live Feeds, Tickers, CI Log streaming.
  • Low Complexity: Standard HTTP. Works with standard Load Balancers. Easy to secure.
  • NestJS: Use @Sse('route') returning Observable<MessageEvent>.
• Long Polling: Use only as a fallback or for extremely low-frequency updates (e.g., job status check every 10m).
  • Impact: High header overhead. Blocks threads if not handled carefully.

WebSockets Implementation

• Socket.io: Default choice. Features "Rooms", "Namespaces", and automatic reconnection. Heavy protocol.
• Fastify/WS: Use ws adapter if performance is critical (e.g., high-frequency trading updates) and you don't need "Rooms" logic.

Scaling (Critical)

• WebSockets: In K8s, a client connects to Pod A. If Pod B emits an event, the client won't receive it.
  • Solution: Redis Adapter (@socket.io/redis-adapter). Every pod publishes to Redis; Redis distributes to all other pods.
• SSE: Stateless. No special adapter needed, but be aware of Connection Limits (6 concurrent connections per domain in HTTP/1.1; virtually unlimited in HTTP/2).
  • Rule: Must use HTTP/2 for SSE at scale.

Security

• Handshake Auth: Standard HTTP Guards don't trigger on Ws connection efficiently.
  • Pattern: Validate JWT during the handleConnection() lifecycle method. Disconnect immediately if invalid.
• Rate Limiting: Sockets are expensive. Apply strict throttling on "Message" events to prevent flooding.

Architecture

• Gateway != Service: The WebSocketGateway should only handle client comms (Join Room, Ack message).
  • Rule: Delegate business logic to a Service or Command Bus.
• Events: Use AsyncApi or SocketApi decorators (from community packages) to document WS events similarly to OpenAPI.