The diagram at http://docs.engineyard.com/environment-cluster.html is confusing because it shows traffic (represented by arrows) to and from HAProxy on every application instance. I confirmed with support in the IRC channel that only the HAProxy on master handles traffic; HAProxy is on the slaves only for the situation in which a slave is promoted to master. It would be clearer if the arrows to and from HAProxy on the slaves were removed.
The diagram at http://docs.engineyard.com/environment-cluster.html is confusing because it shows traffic (represented by arrows) to and from HAProxy on every application instance. I confirmed with support in the IRC channel that only the HAProxy on master handles traffic; HAProxy is on the slaves only for the situation in which a slave is promoted to master. It would be clearer if the arrows to and from HAProxy on the slaves were removed.