Using varnish-counters MAIN.* in VCL - varnish

Question

Is it possible to use varnish-counters in VCL in conditions?
I would like to write a condition based on current value of MAIN.backend_conn but I cannot figure out, whether can I use statistics in VCL even in inline C.
Current Solution
Now I am using configuration like this:
backend default {
.host = "192.168.122.11";
.probe = {
.url = "/check-connections.php";
.interval = 1s;
.threshold = 4;
}
}
backend sessionWorker {
.host = "192.168.122.11";
.probe = {
.url = "/other-probe";
.interval = 5s;
.threshold = 2;
}
}
sub vcl_recv {
if (req.http.cookie ~ "(^|;\s*)(SESS=)" || std.healthy(req.backend_hint)) {
set req.backend_hint = sessionWorker;
} else {
return (synth(503, "Server overloaded"));
}
}
check-connections.php reads nginx status - active connections and triggers an error, if there are more active connections:
if ($active > 10) {
http_response_code(502);
} else {
http_response_code(200);
}
I would like to find a solution how to replace std.healty(req.backend_hint) with current connections (VBE.conn) to the backend directly in the VCL.

Answer 1

There are some counters implemented already. Like bereq.retries:
sub vcl_backend_response {
if (beresp.status == 503 && bereq.retries < 5 ) {
return(retry);
}
}
sub vcl_backend_error {
if (beresp.status == 503 && bereq.retries == 5) {
synthetic(std.fileread("/path/to/my/file/varnish503.html"));
return(deliver);
}
}
Maybe this is already what you need. Otherwise here is a list of other build in counters.