In HA mode, how to let participant go to the same jitsi meet server

I have enabled the HA mode for jitsi. OCTO is not enabled because this is just a local cluster.
one Nignx (as haproxy)
two jitsi meet server (each has its own nginx and two jvb)
the user first go to HA, then route to one of the jitsi server.

but encounter a problem. if two participant enter the same meeting room name, they are routing any different jitsi servers.

on, it can handle it well. participants goes to the same shard

is it possible to explain the logic ?
e.g. if I input
how does the server know this meeting room is non-existed or existed ?
if the first participant goes to jitis server 1, how does the haproxy route other participant goes to the same server ?

in other post , I see:
So the HAProxy needs to share a table of rooms. Every bosh connection has a ?room= parameter so you need to route calls to the same shard.

is there any sample code ?