I don't know how troubleshoot this. I get "503 Service Unavailable" http error for all "nginx upstreams" proxy passing calls to haproxy fast_thin and slow_thin ( server 127.0.0.1:3100 and server 127.0.0.1:3200 ), which loadbalance on 6 Thin servers ( 127.0.0.1:3000 .. 3005 ). Static files like /blog are currently fine. The falldown is: nginx on port 80 - haproxy on 3100 and 3200 - thin on 3000 .. 3005 and then Rails. Here it is /etc/nginx/nginx.conf :

user  nginx;
worker_processes  2;

pid        /var/run/nginx.pid;

events {
    worker_connections  1024;
}

http {
    include       /etc/nginx/mime.types;
    default_type  application/octet-stream;

    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    sendfile        on;
    tcp_nopush     on;

    keepalive_timeout  65;
    tcp_nodelay        on;

    include /etc/nginx/conf.d/*.conf;
}

then /etc/nginx/conf.d/default.conf

upstream fast_thin {
    server 127.0.0.1:3100;
}
upstream slow_thin {
    server 127.0.0.1:3200;
}
server {
            listen   80;
            server_name  www.gitwatcher.com;
            rewrite ^/(.*) http://gitwatcher.com/$1 permanent;
}
server {
    listen   80;
    server_name gitwatcher.com;

    access_log /var/www/gitwatcher/log/access.log;
    error_log  /var/www/gitwatcher/log/error.log;

    root       /var/www/gitwatcher/public;
    # index      index.html;

            location /about {
              proxy_pass http://fast_thin;
              break;
        }

            location /trends {
              proxy_pass http://slow_thin;
              break;
        }
            location /categories {
                          proxy_pass http://slow_thin;
                          break;
            }
            location /signout {
                          proxy_pass http://slow_thin;
                          break;
            }
            location /auth/github {
                          proxy_pass http://slow_thin;
                          break;
            }             


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

                          if (-f $request_filename/index.html) {
                                           rewrite (.*) $1/index.html break;
                          }

                          if (-f $request_filename.html) {
                                           rewrite (.*) $1.html break;
                          }

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

then haproxy config file /etc/haproxy/haproxy.cfg :

global

  log 127.0.0.1   local0
  log 127.0.0.1   local1 notice
  #log loghost    local0 info
  maxconn 4096
  #chroot /usr/share/haproxy
  user haproxy
  group haproxy
  daemon
  #debug
  #quiet
  nbproc        1         # number of processing cores

defaults

        log     global
        retries 3
        maxconn 2000
        contimeout      5000

  mode              http
  clitimeout        60000       # maximum inactivity time on the client side
  srvtimeout        30000       # maximum inactivity time on the server side
  timeout connect   4000        # maximum time to wait for a connection attempt to a server to succeed

        option  httplog
        option  dontlognull
        option redispatch

  option            httpclose     # disable keepalive (HAProxy does not yet support the HTTP keep-alive mode)
  option            abortonclose  # enable early dropping of aborted requests from pending queue
  option            httpchk       # enable HTTP protocol to check on servers health
  option            forwardfor    # enable insert of X-Forwarded-For headers

  balance roundrobin            # each server is used in turns, according to assigned weight

  stats enable                  # enable web-stats at /haproxy?stats
  stats auth        haproxy:pr0xystats  # force HTTP Auth to view stats
  stats refresh     5s      # refresh rate of stats page

listen rails_proxy 127.0.0.1:3100

  # - equal weights on all servers
  # - maxconn will queue requests at HAProxy if limit is reached
  # - minconn dynamically scales the connection concurrency (bound my maxconn) depending on size of HAProxy queue
  # - check health every 20000 microseconds

  server web1 127.0.0.1:3000 weight 1 minconn 3 maxconn 6 check inter 20000
  server web1 127.0.0.1:3001 weight 1 minconn 3 maxconn 6 check inter 20000
  server web1 127.0.0.1:3002 weight 1 minconn 3 maxconn 6 check inter 20000

listen slow_proxy 127.0.0.1:3200

  # cluster for slow requests, lower the queues, check less frequently

  server slow1 127.0.0.1:3003 weight 1 minconn 1 maxconn 3 check inter 40000
  server slow2 127.0.0.1:3004 weight 1 minconn 1 maxconn 3 check inter 40000
  server slow3 127.0.0.1:3005 weight 1 minconn 1 maxconn 3 check inter 40000

and the Thin config file /etc/thin/gitwatcher.yml :

---
chdir: /var/www/gitwatcher
environment: production
address: 0.0.0.0
port: 3000
timeout: 30
log: log/thin.log
pid: tmp/pids/thin.pid
max_conns: 1024
max_persistent_conns: 100
require: []
wait: 30
servers: 6
daemonize: true

if I look into open listen ports, I got the following :

root@fullness:/var/www/gitwatcher# lsof | grep TCP | egrep "nginx|haproxy|thin"
nginx      834       root    8u     IPv4                921        0t0        TCP *:http (LISTEN)
nginx      835      nginx    8u     IPv4                921        0t0        TCP *:http (LISTEN)
nginx      837      nginx    8u     IPv4                921        0t0        TCP *:http (LISTEN)
haproxy   1908    haproxy    4u     IPv4              11699        0t0        TCP localhost:3100 (LISTEN)
haproxy   1908    haproxy    6u     IPv4              11701        0t0        TCP localhost:3200 (LISTEN)
root@fullness:/var/www/gitwatcher# 

iptables -L get me the following :

Chain INPUT (policy DROP)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     all  --  anywhere             anywhere             state RELATED,ESTABLISHED
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:22222
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:http
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:https
ACCEPT     all  --  anywhere             anywhere            
DROP       all  --  anywhere             anywhere            

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     all  --  anywhere             anywhere            

Any help ?