Question: Could HAProxy and Mod_Cluster and Tomcat be used together ?
Either Mod_Cluster+Tomcat or HAProxy+Tomcat but not HAProxy+Mod_Cluster+Tomcat unless we are setting up multiple load balancer correct?
You can chain mod_cluster Apache HTTP Server balancer behind HAProxy, both in TCP and HTTP mode, in front of Tomcat. You could also use mod_cluster Apache HTTP Server balancer and HAProxy side-by-side, having them both sending requests to your Tomcats. The latter makes much less sense though.
If you tell me what would you like to achieve in which environment, I could help you architect the right solution. For instance: Azure, multiple datacentres, VMs, clients are plain HTTP embedded devices or Docker containers on Tutum/Docker Cloud, clients use HTTPS and certificate for authentication. You can / cannot use AJP internally between mod_cluster and Tomcat etc. Help me help you :-) -K-