websocket-realtime

WebSocket & Real-Time

This skill covers the server and client patterns for WebSocket and SSE connections in production — beyond the happy path.

When to Use

• Implementing chat, live notifications, or collaborative editing
• Streaming LLM responses to the frontend (SSE is ideal for this)
• Building presence indicators (who is online, who is typing)
• Any feature where the server needs to push updates to clients without polling
• An existing WebSocket implementation is dropping connections or failing under load

When NOT to Use

• Simple polling for infrequent updates — polling is simpler and sufficient for < 1 update/minute
• Webhook delivery to other servers — use HTTP POST callbacks
• Background job status that is checked once — use a regular REST endpoint

Protocol Choice

SSE runs over HTTP — no special proxy config, works through load balancers out of the box. WebSocket requires Upgrade header support (most modern load balancers support this).

Server-Sent Events (SSE)

SSE is the right choice for LLM streaming and unidirectional server push:

NestJS / Express SSE endpoint:

@Get('stream/:id')
@Sse()
streamDocument(@Param('id') id: string, @Req() req: Request): Observable<MessageEvent> {
  return new Observable((observer) => {
    const stream = this.documentService.streamAnalysis(id);

    stream.on('data', (chunk: string) => {
      observer.next({ data: JSON.stringify({ content: chunk }) });
    });

    stream.on('end', () => {
      observer.next({ data: JSON.stringify({ done: true }) });
      observer.complete();
    });

    stream.on('error', (err) => {
      observer.next({ data: JSON.stringify({ error: err.message }) });
      observer.complete();
    });

    // Clean up on client disconnect
    req.on('close', () => stream.destroy());
  });
}