How can Nginx not give the client a response from an asynchronous POST echo_subrequest_async?

Question

Nginx

Hello, there is an application in which it would be necessary to collect statistics when accessing
location / open and location / load
At the moment, the following has been done:

upstream app_server{
        server 127.0.0.1:7101; # основное приложение
    }
upstream stat_app_server{
  server 127.0.0.1:9292; # для сбора статистики
    }
server {
        listen 80 default_server;
        location / {
          proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
          proxy_set_header Host $http_host;
         ..........
          if (!-f $request_filename) {
            proxy_pass http://app_server;
            break;
          }
        }
        # Rails error pages
        error_page 500 502 503 504 /500.html;
        location = /500.html {
          root .....-major/current/public;
        }
location /open {
      echo_subrequest_async POST '/openstat' -q 'uri=$request_uri&refer=$http_referer&ip=$remote_addr';
      proxy_pass http://app_server/open;
  }
location /load {
      echo_subrequest_async POST '/loadstat' -q 'uri=$request_uri&refer=$http_referer&ip=$remote_addr';
      proxy_pass http://app_server/load;
  }
  location /openstat {
      proxy_pass http://stat_app_server/openstat;
  }
  location /loadstat {
      proxy_pass http://stat_app_server/loadstat;
  }
}

As a result, when asked

http://app_url/open

given a blank page from

proxy_pass http://stat_app_server/openstat;

but not from

proxy_pass http://app_server/open;

Help to force nginx not to return the result of an asynchronous request. Or is another config possible?

Answer 1

I'll post the solution just in case.
used the lua module, with requests like this in the required locations:

access_by_lua 'res = ngx.location.capture("/loadstat",
  { args = { uri = ngx.var.request_uri, refer = ngx.var.http_referer, ip = ngx.var.remote_addr }})';