What is Redis Pub/Sub, and how would a Node.js application use it to broadcast real-time events across multiple server instances?

Redis Pub/Sub lets clients subscribe to named channels and receive messages published to those channels in real time -- useful for broadcasting an event (like a chat message or a live notification) to every connected server instance simultaneously, which matters for something like a WebSocket-based chat application running multiple instances behind a load balancer, since a message published by one instance needs to reach clients connected to every other instance too.

const subscriber = client.duplicate();
await subscriber.connect();
await subscriber.subscribe('chat-room-1', (message) => {
  broadcastToLocalWebSocketClients(JSON.parse(message));
});

// Any instance can publish, and all subscribed instances receive it
await client.publish('chat-room-1', JSON.stringify({ user: 'Alice', text: 'Hello!' }));