nginx config

Попробуйте http://nginx.org/en/docs/http/ngx_http_upstream_module.html#least_conn
Ну или haproxy с "balance leastconn".

Конфиги типа таких 


на сервере, где стоит nginx, который распределяет запросы

upstream myserv_production {
ip_hash;
server 127.0.0.1:1157 max_fails=5 fail_timeout=10s;
server 55.99.33.222:9091 max_fails=5 fail_timeout=10s;
}

server {
listen 80;
server_name
i0.myserv.ru
i1.myserv.ru
i2.myserv.ru
i3.myserv.ru
;

access_log off;
error_log /var/log/nginx/static.myserv.ru.error.log;
log_not_found off;

gzip on;
gzip_min_length 1000;
gzip_disable "msie6";
gzip_comp_level 4;
gzip_types text/plain application/xhtml+xml text/xml application/xml application/xml+rss
text/json application/json
text/javascript application/x-javascript text/css;

if ($request_method !~ ^(GET|POST|PUT|DELETE|HEAD)$ ) {
return 444;
}

root /var/www/myserv/current/public;

location ~* \.(ico|css|js|gif|jpg|jpeg|png|mp4|flv|f4v)(\?[0-9]+)?$ {
expires max;
break;
}


location / {
expires max;
try_files $uri @myserv_production;
rewrite ^ http://www.myserv.ru$request_uri? permanent; #301 redirect
}

location @myserv_production {
include proxy_params;
proxy_pass http://myserv_production;
}
}


location / {
rewrite ^/(.*)/$ /$1 permanent;
root /var/www/myserv/current/public;
rewrite ^/?page=8&tab=contest / permanent;
if (-f $document_root/system/maintenance.html) {
return 503;
}
try_files $uri $uri/index.html $uri.html @myserv_production;
}

location @myserv_production {
access_log /var/log/nginx/myserv.ru.access.log;
include proxy_params;
proxy_pass http://myserv_production;
}


И второй, сервер 

server {
  listen 80;
  server_name
    i0.myserv.ru
    i1.myserv.ru
    i2.myserv.ru
    i3.myserv.ru
  ;

  access_log off;
  error_log /var/log/nginx/static.myserv.ru.error.log;
  log_not_found off;

  proxy_buffering off;
  gzip on;
  gzip_min_length 1000;
  gzip_disable "msie6";
  gzip_comp_level 4;
  gzip_types text/plain application/xhtml+xml text/xml application/xml application/xml+rss
             text/json application/json
             text/javascript application/x-javascript text/css;

  if ($request_method !~ ^(GET|POST|PUT|DELETE|HEAD)$ ) {
    return 444;
  }

  location ~* \.(ico|gif|jpg|jpeg|png|mp4|flv|f4v)(\?[0-9]+)?$ {
    add_header Cache-Control public;
    root /media/storage00/backups/current/storage00/var-www/myserv/shared;
    expires max;
    try_files $uri @web_static;
    break;
  }

  location / {
    add_header Cache-Control public;
    expires max;
    root /media/storage00/backups/current/storage00/var-www/myserv/shared;
    try_files $uri @web1_unicorn;
    rewrite ^ http://www.myserv.ru$request_uri? permanent; #301 redirect
  }

  location @web1_unicorn {
    include proxy_params;
    proxy_pass http://127.0.0.1:9090;
  }

  location @web_static {
    include proxy_params;
    proxy_pass http://176.9.50.182;
  }

}

http://wiki.summercode.com/rails_deployment_step_by_step_to_ubuntu
http://wiki.summercode.com/setting_up_unicorn_with_nginx