site.conf

upstream unicorn_proxy_api {
  server proxy_api_1 fail_timeout=0;
  server proxy_api_2 fail_timeout=0;
  server proxy_api_3 fail_timeout=0;
  server proxy_api_4 fail_timeout=0;
  server proxy_api_5 fail_timeout=0;
  server proxy_api_6 fail_timeout=0;
  server proxy_api_7 fail_timeout=0;
  server proxy_api_8 fail_timeout=0;
  server proxy_api_9 fail_timeout=0;
}

upstream unicorn_proxy_app {
  server proxy_app_1 fail_timeout=0;
  server proxy_app_2 fail_timeout=0;
  server proxy_app_3 fail_timeout=0;
}

server {
  listen 80;
  root /path/to/public;
  server_name site.com;

  location ^~ /api/ {
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Real-IP  $remote_addr;
    proxy_set_header Host $http_host;
    proxy_redirect off;

    if (!-f $request_filename) {
      proxy_pass http://unicorn_proxy_api;
      break;
    }
  }

  try_files $uri/index.html $uri @unicorn;

  location @unicorn {
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Real-IP  $remote_addr;
    proxy_set_header Host $http_host;
    proxy_redirect off;

    if (!-f $request_filename) {
      proxy_pass http://unicorn_proxy_app;
      break;
    }
  }
}