Comment by rogerdonut

IMO you would use OpenResty as a web server and HAProxy as a load balancer in front of it. HAProxy focuses on being a high-performance load balancer. OpenResty is built ontop of NGINX and many of the load balancing features in NGINX are locked behind their "Plus" product, all of which are provided by default within HAProxy. In order to take advantage of these features in OpenResty (such as active health checking and DNS service discovery) you would need to use Lua scripts where in HAProxy they are implemented in the core.