Pub/sub layer such as Redis or NATS ensures cross-instance message delivery

In-process broadcast with io.to(channel).emit() only reaches clients connected to the same server instance. The moment you deploy two instances — behind a load balancer or via auto-scaling — users on different instances stop receiving each other's messages. This is a latent failure: the app works perfectly in development and single-instance staging, then silently breaks in production the first time the process count exceeds one. ISO 25010 reliability requires the system to function correctly under expected deployment configurations, including horizontal scaling.

Route all broadcasts through a shared pub/sub layer — Redis is the standard choice for Socket.IO via the @socket.io/redis-adapter package.

import { createAdapter } from '@socket.io/redis-adapter';
import { createClient } from 'redis';

const pub = createClient();
const sub = pub.duplicate();
await Promise.all([pub.connect(), sub.connect()]);

io.adapter(createAdapter(pub, sub));

// io.to(channel).emit() now fans out across all instances

Alternatively, use NATS or Kafka if your infrastructure already includes them. The skip condition applies only when the deployment is permanently single-process — document that constraint explicitly if you take the skip.