Вот частичная конфигурация HAProxy, которая может сделать это за вас:
frontend webserver
bind :80
mode http
acl is_post method POST
use_backend post_app if is_post
default_backend get_app
backend post_app
mode http
option forwardfor
balance source
option httpclose
option httpchk HEAD / HTTP/1.0
server post_app1 172.16.0.11:80 weight 1 check inter 1000 rise 5 fall 1
server post_app2 172.16.0.12:80 weight 1 check inter 1000 rise 5 fall 1
server post_app3 172.16.0.13:80 weight 1 check inter 1000 rise 5 fall 1 backup
backend get_app
mode http
option forwardfor
balance source
option httpclose
option httpchk HEAD / HTTP/1.0
server get_app1 172.16.0.21:80 weight 1 check inter 1000 rise 5 fall 1
server get_app2 172.16.0.22:80 weight 1 check inter 1000 rise 5 fall 1
server get_app3 172.16.0.23:80 weight 1 check inter 1000 rise 5 fall 1 backup