Custom load balancing in HAProxy - load-balancing

Question

I am using HAProxy for load balancing my HTTP requests. I would like to know if there is any way to customize the selection of backend server based on the responses returned by each server. I have a servlet which can return the responses (number of clients connected to it). I would like to use this information and route the request to the backend server which has the lowest number.
My HAProxy configuration looks like:
listen http_front xx.xx.xx.xx:8080
mode http
option httpchk GET /servlet/GetClientCountServlet
server app1 xx.xx.xx.xx:8080 check port 8080
server app2 xx.xx.xx.xx:8080 check port 8080
server app3 xx.xx.xx.xx:8080 check port 8080

Answer 1

Would not leastconn balance mode work for your use case? Otherwise I you can use Lua scripts to customize the way load balancing is done using HAProxy

Answer 2

As I am searching for a solution in the same direction, maye this helps as a base:
Loadbalancing via custom lua script
Create a file called least_sessions.lua and add the following code:
local function backend_with_least_sessions(txn)
-- Get the frontend that was used
local fe_name = txn.f:fe_name()
local least_sessions_backend = ""
local least_sessions = 99999999999
-- Loop through all the backends. You could change this
-- so that the backend names are passed into the function too.
for _, backend in pairs(core.backends) do
-- Look at only backends that have names that start with
-- the name of the frontend, e.g. "www_" prefix for "www" frontend.
if backend and backend.name:sub(1, #fe_name + 1) == fe_name .. '_' then
local total_sessions = 0
-- Using the backend, loop through each of its servers
for _, server in pairs(backend.servers) do
-- Get server's stats
local stats = server:get_stats()
-- Get the backend's total number of current sessions
if stats['status'] == 'UP' then
total_sessions = total_sessions + stats['scur']
core.Debug(backend.name .. ": " .. total_sessions)
end
end
if least_sessions > total_sessions then
least_sessions = total_sessions
least_sessions_backend = backend.name
end
end
end
-- Return the name of the backend that has the fewest sessions
core.Debug("Returning: " .. least_sessions_backend)
return least_sessions_backend
end
core.register_fetches('leastsess_backend', backend_with_least_sessions)
This code will loop through all of the backends that start with the same letters as the current frontend, for example finding the backends www_dc1 and www_dc2 for the frontend www. It will then find the backend that currently has the fewest sessions and return its name.
Use a lua-load directive to load the file into HAProxy. Then, add a use_backend line to your frontend to route traffic to the backend that has the fewest, active sessions.
global
lua-load /path/to/least_sessions.lua
frontend www
bind :80
use_backend %[lua.leastsess_backend]
backend www_dc1
balance roundrobin
server server1 192.168.10.5:8080 check maxconn 30
backend www_dc2
balance roundrobin
server server1 192.168.11.5:8080 check maxconn 30
More details:
https://www.haproxy.com/de/blog/5-ways-to-extend-haproxy-with-lua/