Basic HAProxy config that supports Engine.io 1.x and Socket.io 1.x using both polling and websockets and sticky sessions based on session cookie.

haproxy.engineio.conf

global
  maxconn 32768
  quiet

  # Performance turning
  tune.maxaccept -1

  stats socket /tmp/haproxy.sock mode 0600 level admin
  log 127.0.0.1 local0 notice
  log 127.0.0.1 local0 debug
  daemon

defaults
  mode http

  log global
  option httplog
  option dontlognull

  # Add x-forwarded-for header.
  option forwardfor
  option redispatch

  timeout connect 5s
  timeout client 30s
  timeout server 30s

  # Long timeout for WebSocket connections.
  timeout tunnel 2h

  #cookie AIOSERVERID insert indirect nocache
  balance url_param key
  option httpchk GET /status-check
  http-check expect string ok


listen stats *:1936
  stats enable
  stats uri /
  stats hide-version
  stats auth someuser:password

frontend public
  # HTTPS
  bind *:443 ssl crt /home/ec2-user/etc/wildcard.articulate.io.pem

  # HTTP
  bind *:4000

  maxconn 6000

  # At most 10 concurrent connections from a client
  acl too_fast fe_sess_rate ge 10

  # Matches any path beginning with a given prefix
  #acl bursts_inclined path_beg -i /

  # Effectively working as a delay mechanism for clients that are too fast
  tcp-request inspect-delay 1000ms

  # Fast-path - accept connection if it's not this troublesome client
  #tcp-request content accept unless bursts_inclined too_fast
  tcp-request content accept unless too_fast

  # The very fast client gets here meaning they have to wait full inspect-delay
  tcp-request content accept if WAIT_END

  acl is_websocket hdr(Upgrade) -i WebSocket

  # default to the websocket backend
  default_backend polling
  use_backend websocket if is_websocket

backend polling
  timeout server 30s
  option forwardfor
  server test01 test.articulate.io:8000 maxconn 2000 check
  server test02 test.articulate.io:7900 maxconn 2000 check

backend websocket
  timeout queue 5000
  timeout server 86400000
  timeout connect 86400000
  timeout check 1s
  option forwardfor

  # Do not use httpclose (= client and server
  # connections get closed), since it will close
  # Websockets connections
  no option httpclose
  # Use "option http-server-close" to preserve
  # client persistent connections while handling
  # every incoming request individually, dispatching
  # them one after another to servers, in HTTP close mode
  option http-server-close
  option forceclose

  server test01 test.articulate.io:8000 maxconn 2000 check
  server test02 test.articulate.io:7900 maxconn 2000 check

adapted from balancerbattle/haproxy.cfg