step 1 .First will enable maintenance page

step 2 . We are going to add prodapp03 and remove prodapp02 in Haproxy 

## comment following line server prodapp02 10.166.152.19
## uncomment following line server prodapp03 10.166.152.14

 



backend learnexa
balance roundrobin
option httplog
option forwardfor
cookie SERVERID insert indirect
source 10.166.152.246
#timeout client 150000ms
timeout connect 35000ms
timeout server 8m
option httpchk HEAD /contact-us/new HTTP/1.1\r\nHost:\ www.learnexa.com\r\nUser-Agent:\ HaProxy
server prodapp01 10.166.152.11:80 check port 80  cookie app1 maxconn 31 inter 5000 rise 2 fall 3
#server prodapp02 10.166.152.19:80 check port 80 cookie app2 maxconn 31 inter 5000 rise 2 fall 3
server prodapp03 10.166.152.14:80 check port 80 cookie app3 maxconn 31 inter 5000 rise 2 fall 3

Step 3 haproxy reload

/etc/init.d/haproxy reload

Step 4 will give for validation
add below host entry in /etc/hosts file ##NOT REQUIRED VPN THIS IS PUBLIC IP
207.20.54.0 test.learnexa.com

step 5 once validated result positive

step 6 remove maintenance page

step 7 If test result negative will rollback haproxy changes

step 8 remove maintenance page