Using Nginx as a load balancer

original document at http://mickeyben.com/2009/12/30/using-nginx-as-a-load-balancer.html

Here’s a look at how nginx does basic load balancing :

upstream  yoursite  {
   server   yoursite1.yoursite.com;
   server   yoursite2.yoursite.com;
}

server {
   server_name www.yoursite.com;
   location / {
      proxy_pass  http://yoursite;
   }
}

This configuration will send 50% of the requests for www.yoursite.com to yoursite1.yoursite.com and the other 50% to yoursite2.yoursite.com.

You can specify the ip_hash directive that guarantees the client request will always be transferred to the same server. If this server is considered inoperative, then the request of this client will be transferred to another server.

upstream  yoursite  {
   ip_hash;
   server   yoursite1.yoursite.com;
   server   yoursite2.yoursite.com;
}

If one of the servers must be removed for some time, you must mark that server as down.

upstream  yoursite  {
   ip_hash;
   server   yoursite1.yoursite.com down;
   server   yoursite2.yoursite.com;
}

If you add a weight tag onto the end of the server definition you can modify the percentages of the requests send to the servers. When there’s no weight set, the weight is equal to one.

upstream  yoursite  {
   server   yoursite1.yoursite.com weight=4;
   server   yoursite2.yoursite.com;
}

This configuration will send 80% of the requests to yoursite1.yoursite.com and the other 20% to yoursite2.yoursite.com.

max_fails is a directive defining the number of unsuccessful attempts in the time period defined by fail_timeout before the server is considered inoperative. If not set, the number of attempts is one. A value of 0 turns off this check. If fail_timeout is not set the time is 10 seconds.

upstream  yoursite  {
   server   yoursite1.yoursite.com;
   server   yoursite2.yoursite.com max_fails=3  fail_timeout=30s;
}

In this configuration nginx will consider yoursite2.yoursite.com as inoperative if a request fails 3 times with a 30s timeout.

If the non-backup servers are all down or busy, the server(s) with the backup directive will be used.

upstream  yoursite  {
   server   yoursite1.yoursite.com max_fails=3;
   server   yoursite2.yoursite.com max_fails=3;
   server   yoursite3.yoursite.com backup;
}

This configuration will send 50% of the requests for www.yoursite.com to yoursite1.yoursite.com and the other 50% to yoursite2.yoursite.com. If yoursite1.yoursite.com and yoursite2.yoursite.com both fails 3 times the requests will be send to yoursite3.yoursite.com.