Pub/Sub

When you have many Node servers and need to notify all of them — Redis pub/sub or a broker like NATS/RabbitMQ.

3 min read Level 2/5 #nodejs#pubsub#redis

What you'll learn

Use Redis pub/sub
Know when to reach for a broker
Compare with queues

You have multiple Node servers behind a load balancer. Server A needs to tell Server B “user 42 just logged out — invalidate their WebSocket connection.” How?

Pub/sub. One publisher, many subscribers, no point-to-point connection between them.

Redis Pub/Sub

// publisher (any server)
import { Redis } from "ioredis";
const pub = new Redis();
await pub.publish("user-events", JSON.stringify({ type: "logout", userId: 42 }));

// subscriber (on every server)
import { Redis } from "ioredis";
const sub = new Redis();
sub.subscribe("user-events");
sub.on("message", (channel, raw) => {
  const event = JSON.parse(raw);
  if (event.type === "logout") {
    disconnectUser(event.userId);
  }
});

Every subscribed server gets every message. No persistence — if a server is down, it misses messages.

Pub/Sub vs Queue

	Queue (BullMQ)	Pub/Sub (Redis)
Delivery	One worker handles each job	Every subscriber gets every message
Persistence	Survives restarts	Lost if no subscriber
Retries	Built-in	Up to you
Use for	Async work (emails, images)	Real-time notifications, fan-out

If “this job should be done once” — queue. If “every server should know about this event” — pub/sub.

When To Use a Broker

For larger systems where persistence and ordering matter, reach for a real message broker:

NATS — fast, JS-friendly, persistent streams (JetStream)
RabbitMQ — battle-tested AMQP broker
Apache Kafka — high-throughput event streaming, replay history

Each is a chapter unto itself. For most Node apps, Redis pub/sub is enough until it isn’t.

A Real Pattern

WebSocket fan-out across N servers:

Client connects to Server A → opens a WS to Server A
Client B connects to Server B
A wants to send a message to B.
  1. A publishes to Redis: "to-user-B: hi"
  2. Every server's subscriber sees it.
  3. Server B finds B's WS and sends.

Without pub/sub, A would need to know which server B is on.

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