In all use cases I could find, HAProxy is running on a different server than the content server, e.g.:
Instance 1 - Load Balancer Hostname: haproxy IP: 192.168.205.15
Instance 2 - Web Server 1 Hostname: webser01 IP: 192.168.205.16
Instance 2 - Web Server 2 Hostname: webserver02 IP: 192.168.205.17
Is it possible to run HAProxy (together with nginx e.g.) on one of the content server and how would the configuration be?