"Session ID unknown" after handshake on high server load [Socket.io 1.0.6]

I am running a multi-node server (16 workers running Socket.io 1.0.6; accessed via Nginx, configured as a reverse proxy supporting sticky sessions) for ~ 5k users. While the load of the server is low (2~3 on a 20 core server / 2k users), everyone is able to connect instantly. When the load of the server gets higher (5~6 / 5k users), new users are not able to connect and receive data instantly. In this case, it takes 2~4 handshakes for the users to connect succesfully.

This is what happens (high load):

• User opens the website; receives HTML and JS
• User’s browser attempts to initialize a socket.io connection to the server (io.connect(...))
• A handshake request is sent to the server, the server responds with a SID and other information ({"sid":"f-re6ABU3Si4pmyWADCx","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":60000})
• The client initiates a polling-request, including this SID: GET .../socket.io/?EIO=2&transport=polling&t=1408648886249-1&sid=f-re6ABU3Si4pmyWADCx
• Instead of sending data, the server responds with 400 Bad Request: {"code":1,"message":"Session ID unknown"}
• The client performs a new handshake (GET .../socket.io/?EIO=2&transport=polling&t=1408648888050-3, notice the previously received SID is omitted)
• The server responds with new connection data, including a new SID: ({"sid":"DdRxn2gv6vrtZOBiAEAS","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":60000})
• The client performs a new polling request, including the new SID: GET .../socket.io/?EIO=2&transport=polling&t=1408648888097-4&sid=DdRxn2gv6vrtZOBiAEAS
• The server responds with the data that is emitted in the worker source code.

Depending on the load of the server, it may happen 1~3 times that the server responds with "Session ID unknown" and the client needs to perform a new handshake before data is actually received.

[p3x-robot]

for me, it was with nginx ssl http2, and it was polling, so the good config is:

 const ioSocket = io('', {
      // Send auth token on connection, you will need to DI the Auth service above
      // 'query': 'token=' + Auth.getToken()
      path: '/socket.io',
      transports: ['websocket'],
      secure: true,
    });

[jaykumarthaker]

DON’T FORGET TO CONFIGURE CLIENT AS WELL

Making just nodejs backend to use transport as websocket protocol won’t do much. socket.io clients are also required to set with the same configuration. So, in my onion below should work:

in nodejs:

  const ioSocket = io('',  {
      transports: ['websocket',  'polling']
    });

and in js client:

  socket = io.connect(SocketServerRootURL, {
        transports:['websocket', 'polling']
    });

['websocket', 'polling'] will force socket.io to try webscoket as the first protocol to connect, otherwise fall back to polling (just in case some browsers/clients may not support websockets). For cluster environment, better to use ['websocket'] only.