How to Fix Socket.IO 400 Bad Request Errors on AWS Elastic Beanstalk with Multiple Instances?

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Asked By TechWhiz42 On

I'm running into some issues with Socket.IO connections on AWS Elastic Beanstalk. Everything works smoothly when deployed on a single instance, but once I scale up to multiple instances behind a load balancer, I keep getting 400 Bad Request errors.

Here's what's happening: in my client console, I see an error pattern like this:

`POST https://[redacted-domain].elasticbeanstalk.com/socket.io/?EIO=4&transport=polling&t=meh0duro&sid=WDHmjbJd7v5aE7mdAAeK 400 (Bad Request)`

And it mentions a connection error caused by an `xhr post error`.

I've enabled sticky sessions in the EC2 target groups as suggested in various forums, but that hasn't helped.

My setup includes a Node.js backend using Socket.IO and a React frontend, all hosted on AWS Elastic Beanstalk with an Application Load Balancer, currently running across two instances.

In my frontend code, I initiate the Socket.IO connection like this:

```javascript
socket = io(import.meta.env.VITE_SOCKET_SERVER, {
reconnection: true,
reconnectionDelay: 1000,
timeout: 10000
});
```

And my backend setup looks like this:

```javascript
const io = new Server(server, {
cors: {
origin: "*",
methods: ["GET", "POST"]
},
transports: ['websocket', 'polling']
});
```

Has anyone else faced a similar issue? What steps should I take to ensure that my Socket.IO connections operate correctly behind a load balancer with multiple instances? Thanks!

1 Answer

Answered By CloudGuru99 On

You might want to try removing the polling transport and only using websockets. Sometimes the polling can cause issues in multi-instance setups due to how the load balancer handles requests. It's worth a shot!

TechWhiz42 -

I tried that, but it still doesn't work.

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