We've recently configured a new Cluster with the following configuration:

     Load Balancer
      /           \
  Tomcat1      Tomcat2

All the requests are made through HTTPS and the two tomcat instances are using SessionReplication via DeltaManager.

The cluster and all the wep-apps are working just fine.

Now one of these webapps requires the use of WebSockets with WSS:// protocol and i'm getting this exception

java.lang.UnsupportedOperationException: HTTP upgrade is not supported by this protocol

Has anyone ever a clue on what is causing this ?


You need to install mod_proxy_wstunnel apache module.

  • so now we are using mod_jk and should migrate to a mod_proxy clustering solution ? Will mod_proxy handle be able to load balance also the WSS:// requests ? – Medioman92 Oct 16 '17 at 12:40
  • moved configuration to mod_proxy and all seems to work fine, thanks – Medioman92 Mar 16 '18 at 10:32

As discussed on the tomcat-users mailing list: "[...] the AJP protocol does not support upgrading to other protocols (e.g. websocket, HTTP/2)."

This means you'd have to use some other method of proxying these connections to your Tomcat instances. As Tux_DEV_NULL mentioned you could use mod_proxy_wstunnel for this.

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