Tomcat Clustering and Load Balancing

The easiest way of doing this if you currently have a load balancer in place is to decrypt the data on the load balancer and look at a cookie. At that point you can either send the request to the backend server un-ecnrypted or you can re-encrypt it and send it on.

Most setups I know of consider the network connection between the load balancer and the backend server secure and don't bother to re-encrypt the traffic for multiple reasons. One reason is that hardware based load balancers also act as SSL accelerators and this is another reason the HTTPS traffic ends at their door. Another is that it allows the traffic to be inspected for attacks.

HAProxy uses a single process event based model whereas with something like Apache I believe you have thread based (mpm_worked) or process based (prefork). Assuming I am correct in that mod_proxy will still be based off threads or processes for each connection (I could be totally wrong about this one), that is going to take a lot more system resources when you get to higher load levels.

Also, from my personal experience something that is built to do something particular often excels at doing that one thing than more of a swiss-army knife solution. So for example with HAProxy you have various options about how keep-alive connections are handled between the clients and the load balancer and between the load balancer and the servers. I am not sure if something like mod_proxy will offer this sort of flexibility.