Haproxy agent-check DRAIN(agent) status still accepts new connections - haproxy

Question

I am trying to perform a load balancing for my Node.js application which uses websockets. I need haproxy to stop load balance new connections on a server, which has reached its maximum number of connections, keeping the existing ones intact in the same time.
I do this by performing an agent-check for each of my servers. If a server cannot accept new connections it responds with "drain" to an agent-check. If a server is able to respond to a new connection, it responds with "ready" to an agent-check.
Here is my haproxy.cfg configuration file:
global
daemon
maxconn 240000
log /dev/log local0 debug
log /dev/log local1 notice
tune.ssl.default-dh-param 2048
defaults
mode http
log global
option httplog
option dontlognull
option dontlog-normal
option http-server-close
option redispatch
timeout connect 20000ms
timeout http-request 1m
timeout client 2100000ms
timeout server 2100000ms
timeout queue 30s
timeout check 5s
timeout http-keep-alive 180s
timeout tunnel 3600s
timeout tarpit 60s
frontend stats
bind *:8084
stats enable
stats uri /stats
stats refresh 10s
stats admin if TRUE
frontend test
mode http
bind *:5000
default_backend ws
backend ws
mode http
fullconn 100000
balance roundrobin
cookie SERVERID insert indirect nocache
server 1 backend1:9999 check agent-check agent-port 8080 cookie 1 inter 500 fall 1 rise 2
And here is how I respond to haproxy agent-check in my Node.js app:
const healthCheckServer = net.createServer((c) => {
let data = '';
if (currentConn < MAX_CONN) {
data += 'ready';
} else {
data += 'drain';
}
c.write(data + '\r\n');
c.destroy();
});
healthCheckServer.listen(8080, '0.0.0.0');
When number of connections to my application is reaching its maximum, haproxy correctly changes server status to DRAIN (agent) (I can observe this in haproxy web dashboard). The problem is, that new connections are still accepted by the application.
I am new to haproxy, so can someone point me to where I am wrong?

Answer 1

Found out that when server is drained by agent (status is set to DRAIN (agent)) and if server is the only one existing in the backend it will still accept new connections.
When there are multiple servers present and each server is drained the behavior is just like expected: haproxy returns HTTP 503.
UPDATE:
Turned out that I was looking in the wrong direction all the time.
First I had to mark my backend which processes WebSocket connections as a non-http (remove mode http line). My guess is haproxy incorrectly counts current sessions for http backend when using WebSockets. Removing mode http solved my problem.
Second, returning maxconn:<conn> in agent-check looks like much simple and more idiomatic way to limit the number of concurrent connections.
Sources:
https://cbonte.github.io/haproxy-dconv/1.8/configuration.html#5.2-agent-check
https://www.haproxy.com/blog/websockets-load-balancing-with-haproxy/
I hope this will help someone.