HAProxy concurrent HTTP query limit based on header

[Olivier D]

Hello everyone,
I would like to implement a "max concurrent connection" in HAProxy. This is
easy to do at TCP level :

    stick-table  type ipv6 size 100k  expire 30s  store conn_cur
    http-request track-sc0 src
    http-request deny deny_status 429 if { src_conn_cur ge 20 }

But now, I want to do the same for concurrent HTTP queries, based on header
'X-Forwarded-For'. For example, I want to send a 429 error code if someone
is sending an HTTP query when he already have 20 ongoing.

My first tries are based on something like this :
   stick-table type ipv6 size 100k  expire 30s  store http_req_rate(10s)
   http-request track-sc0 req.hdr( X-Forwarded-For )
   http-request deny deny_status 429 if { sc0_conn_cur ge 20 }

but it doesn't seem to work the way I want ...

Now I'm a bit lost, but maybe someone already implemented this ?

Thank you  !

Olivier