Real-Time Features with WebSockets and Redis

The scaling problem

One server handles thousands of socket connections. But as soon as you scale horizontally, you need state coordination across instances.

Redis pub/sub as the backbone

TypeScript
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const pub = redis.createClient();
const sub = pub.duplicate();
io.adapter(createAdapter(pub, sub));

Presence and typing indicators

I store presence in Redis with short TTL keys and refresh on activity. It keeps the data lightweight and resilient to disconnects.

Watch out for fan-out

Broadcasting to large rooms can blow up memory usage fast. Partition large rooms by region or use targeted channels for updates.