---
name: nestjs-real-time-36804197328d
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

## Selection Strategy (Critical)

- **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.


---
Source attribution: ngxtm
Original project: https://github.com/ngxtm/devkit/blob/9439f27ce3fa7fe53ff9278a1ee9a44bffa70eba/rules/nestjs/real-time/SKILL.md
License: MIT
License information: https://opensource.org/license/mit
Original upstream instructions; downloaded from Mega AI Hub.

## Upstream license notice

MIT License

Copyright (c) 2024

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.



# Mega AI Hub resource metadata

Creator / publisher: ngxtm
Original project: https://github.com/ngxtm/devkit/blob/9439f27ce3fa7fe53ff9278a1ee9a44bffa70eba/rules/nestjs/real-time/SKILL.md
Attribution basis: Source repository record

Fields of work: Development & Engineering, Product Management
Roles: Backend Developer, DevOps / SRE, Product Manager
Specialties: Backend
Tools: Redis, Socket.Io
Country applicability: Country not specified
Classification basis: Source catalog labels
Instruction SHA-256: 9d614a301daf13ff80a24e21739fcf74dbfdec731c15c49b10fdb8a9a34d4dff