Do I need sticky sessions for WebSocket load balancing?
Yes. Use ip_hash in Nginx upstream to ensure the same client always connects to the same Node.js instance. This is important for WebSocket connections because they're persistent. Without sticky sessions, the initial HTTP upgrade and subsequent WebSocket frames might go to different instances.
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More FAQs in Scaling Socket.io with Redis Adapter Multi-Server WebSocket Support
When running multiple Node.js instances, each only knows about its own connected clients. Without the Redis adapter, a message broadcast on one instance won't reach clients on other instances. The Redis adapter uses Redis pub/sub to broadcast across all instances.
Install @socket.io/redis-adapter and redis. Create a Redis pub client and a sub client (duplicate). Connect both, then apply io.adapter(createAdapter(pubClient, subClient)). Now messages broadcast on one instance are delivered to clients on all instances.
When a message is broadcast, the instance publishes it to a Redis channel. All other instances subscribe to that channel and receive the message. Each instance then delivers the message to its own connected clients that are in the target room. This ensures all clients receive the message regardless of which instance they're on.
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