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.

Anti-Patterns

• No HTTP guards for WebSocket auth: Validate JWT in

  handleConnection()

  ; HTTP guards don't trigger on WS.
• No WebSocket at scale without Redis adapter: Without

  @socket.io/redis-adapter

  , cross-pod events are lost.
• No SSE over HTTP/1.1 at scale: Use HTTP/2 to avoid the 6-connection-per-domain browser limit.