Implement a rate-limit relating to the healthy servers count using haproxy - haproxy

Question

I want to implement a rate-limit system using the sticky table of HAProxy. Consider that I have 100 servers, and a limit of 10 requests per server, the ACL would be
http-request track-sc0 int(1) table GlobalRequestsTracker
http-request deny deny_status 429 if { sc0_http_req_rate(GlobalRequestsTracker),div(100) gt 10 }
Now if I want to make this dynamic depending on the healthy servers count, I need to replace the hardcoded 100 per the nbsrv converter.
http-request track-sc0 int(1) table GlobalRequestsTracker
http-request deny deny_status 429 if { sc0_http_req_rate(GlobalRequestsTracker),div(nbsrv(MyBackend)) gt 10 }
But I'm getting the error:
error detected while parsing an 'http-request deny' condition : invalid args in converter 'div' : expects an integer or a variable name in ACL expression 'sc0_http_req_rate(GlobalRequestsTracker),div(nbsrv(MyBackend))'.
Is there a way to use nbsrv as a variable inside the div operator?

Answer 1

HAProxy does no allow for nested function calls as far as I know. But you could store the number of backend servers in a variable and use it in the division (see http-request set-var in the HAProxy documentation). I have not tested it or used personaly, but I guess it could look like:
frontend <fe>
http-request track-sc0 int(1) table <tbl>
http-request set-var(req.<var>) nbsrv(<be>)
http-request deny deny_status <code> if { sc0_http_req_rate(<tbl>),div(req.<var>) gt <val> }
See the HAProxy documentation.