HAProxy Change Farm and Soft Stop Server on current farm - haproxy

Question

Im pretty new to load balancing and using HAProxy.
After doing a lot of reading and setting up my own development environment, I am unable to work out the following:
I have a bank of web servers which traffic is being balanced across, how do I soft stop these servers whilst i fail over to a new bank?
My test congif looks like this:
frontend front-end-test
maxconn 2000
bind 192.168.0.10:8666
use_backend back-end-test-a
backend back-end-test-a
mode http
balance roundrobin
option forwardfor
server machine1 10.1.1.92:44410 weight 1 maxconn 2000 check
server machine2 10.1.1.92:44420 weight 1 maxconn 2000 check
backend back-end-test-b
mode http
balance roundrobin
server machine3 10.1.1.92:44430 weight 1 maxconn 2000 check
server machine4 10.1.1.92:44440 weight 1 maxconn 2000 check
To change banks the config would be updated and the hot restart used:
frontend front-end-test
maxconn 2000
bind 192.168.0.10:8666
use_backend back-end-test-a
To
frontend front-end-test
maxconn 2000
bind 192.168.0.10:8666
use_backend back-end-test-b
So to recap, I need to update the config so my backend farm changes but any requests for the original servers still need to be served by them.
Is this possible?
Adam

Answer 1

Normally, if I want to change backends, I would switch it right away (as in change to use_backend back-end-test-b) and do a graceful reload by delaying SYN packets (new incoming connections) before restarting the haproxy service. Ex:
iptables -I INPUT -p tcp --dport 8666 --syn -j DROP
sleep 1
service haproxy restart
iptables -D INPUT -p tcp --dport 8666 --syn -j DROP
The delay should allow running requests to finish before that service restarts and applies the new config.
This may not work for long running transactions though.