Scaling Realtime with a Redis Pub/Sub Adapter

A single NestJS WebSocket gateway keeps every connected socket in the memory of one Node process. The moment you scale horizontally behind a load balancer, that assumption breaks.

• Client A connects to instance 1.
• Client B connects to instance 2.
• When instance 1 emits to a room, instance 2 never hears about it — so Client B misses the message.

To fix this we need a shared message bus that lets every instance broadcast events to every other instance. Redis Pub/Sub is the canonical choice, and Socket.IO ships an adapter for exactly this.

The @socket.io/redis-adapter replaces Socket.IO's in-memory adapter. Whenever your code calls server.to(room).emit(...), the adapter publishes that event to a Redis channel instead of only delivering it locally.

• Every instance subscribes to the same Redis channels.
• An emit on instance 1 is published to Redis, then every subscribed instance (including instance 2) receives it and delivers it to its own local sockets.
• Redis is only a relay — socket connections still live on each node; no socket state is stored in Redis.

This means room membership, broadcasts, and even server.emit() fan out correctly across the whole cluster.