Pick one mechanism per flow — don't confuse SSE with WebSocket

Use SSE for one-way LLM token streaming — reserve WebSocket for bidirectional chat.

NestJS·Realtime & Streaming·High·Since 2026-05-01

nestjs-realtime-transport-sse-vs-websocket

Why it matters

Failure modes if this rule is ignored

Stake	If ignored
Can't scale	WebSocket is operationally more expensive (sticky sessions, scaling, connections that don't close). Proxies and load balancers treat long-lived sockets differently from plain HTTP/SSE.

How to fix

• SSE (Server-Sent Events) — one-way stream from server to client. Ideal for LLM token streaming, job progress, notifications.
• WebSocket — bidirectional. Required only when the client sends frequent unsolicited messages (collaborative editing, user-to-user chat).
• Long polling — fallback, not default.

Examples

Bad

@WebSocketGateway()
export class ChatGateway {
  @SubscribeMessage('prompt')
  async handlePrompt(@MessageBody() data: any, @ConnectedSocket() client: Socket) {
    for await (const token of this.llm.stream(data.prompt)) {
      client.emit('token', token);
    }
  }
}

Good

@Controller('chat')
export class ChatController {
  @Post('stream')
  @Sse()
  async stream(@Body() dto: PromptDto): Promise<Observable<MessageEvent>> {
    return new Observable((subscriber) => {
      (async () => {
        for await (const token of this.llm.stream(dto.prompt)) {
          subscriber.next({ data: { token } });
        }
        subscriber.complete();
      })();
    });
  }
}

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