Socket.io Websockets on a TCP configured Amazon Elastic Load Balancer

Question

I'm planning to set up a group of NodeJS application servers running Socket.io on EC2, and I'd like to use the Elastic Load Balancer to spread load between them. I know ELB doesn't support Websockets out of the box, but I can use the setup described here in Scenario 2.

As described in the blog post, though, I notice that this setup offers no session affinity or source IP info:

We can not have Session Affinity nor X-Forward headers with this setup because ELB is not parsing the HTTP messages, so its impossible to match the cookies to ensure Session Affinity nor Inject special X-Forward headers.

Will Socket.io still work under these circumstances? Or is there another way to have a set of Socket.io app servers behind a load balancer with SSL?

EDIT: Tim Caswell talks about doing this already here. Are there any posts explaining how to set this up? Again there's no session stickiness here, but things seem to be working fine.

As an aside, are sticky sessions actually necessary with websockets? Does information travel as new and separate requests or is there only one request + connection that all the information moves along?

Answer 1

Socket.io does not work out of the box even with a TCP ELB because it makes two HTTP requests before upgrading the connection to websockets.

The first connection is used to establish protocol, since socket.io supports more than just websockets.

GET /socket.io/1/?t=1360136617252 HTTP/1.1
User-Agent: node-XMLHttpRequest
Accept: */*
Host: localhost:9999
Connection: keep-alive

HTTP/1.1 200 OK
Content-Type: text/plain
Date: Wed, 06 Feb 2013 07:43:37 GMT
Connection: keep-alive
Transfer-Encoding: chunked

47
xX_HbcG1DN_nufWddblv:60:60:websocket,htmlfile,xhr-polling,jsonp-polling
0

The second request is used to actually upgrade the connection:

GET /socket.io/1/websocket/xX_HbcG1DN_nufWddblv HTTP/1.1
Connection: Upgrade
Upgrade: websocket
Sec-WebSocket-Version: 13
Sec-WebSocket-Key: MTMtMTM2MDEzNjYxNzMxOA==
Host: localhost:9999

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: 249I3zzVp0SzEn0Te2RLp0iS/z0=

You can see in the above example that xX_HbcG1DN_nufWddblv is a shared key between requests. This is the problem. ELBs do round-robin routing, meaning the upgrade request hits a server than did not participate in the initial negotiation. As such, the server has no idea who the client is.

In-memory stateful data is the enemy of load-balancing. Thankfully, socket.io supports using Redis to store the data instead. If you share your redis connection with multiple servers, they essentially share the sessions of all clients.

See the socket.io wiki page for details on setting up Redis.

Answer 2

You can now use the new application load balancer recently launched by AWS.

Just replace the ELB(now called Classic load balancer) with the ALB (Application load balancer) and enable sticky sessions.

ALB supports Web sockets. This should do the trick.

https://aws.amazon.com/blogs/aws/new-aws-application-load-balancer/

http://docs.aws.amazon.com/elasticloadbalancing/latest/application/introduction.html

Answer 3

As I mentioned in the post, we only use ELB to ssl terminate and load-balance across a cluster of http-proxy servers that do support websockets. ELB doesn't talk to the websocket servers directly. The HTTP proxy cluster handles looking up the right socket.io server to connect to ensuring session stickiness.