I read http://blog.stackoverflow.com/2010/01/stack-overflow-network-configuration. The post says that fault tolerance was achieved for the two HAProxy instances by using Heartbeat/Linux-HA which creates a virtual IP address shared by the instances. Does client traffic get sent to the shared IP address (the "virtual") and through the magic of changing the mac address associated with the IP address, traffic goes to one of the HAProxy’s until that box fails or is taken down?
This configuration wouldn't achieve load balancing if that was a goal. How would you have done that?
Finally, are clients still connecting directly to HAProxy as shown in the post or did you front it with nginx or something?