Nginx Load Balancing Basics

Nginx is a powerful high performance web server with a lot of features that can help high load projects to overcome their problems.
Here you can see a particular feature of Nginx that allows you to load balance the traffic across multiple external and internal(on same hardware) servers.

Load Balancing can come in handy when your only server can no longer handle all the incoming requests and you need to offload some of the load to an other server.
The load balancing is implemented by the HttpUpstreamModule that most of the times is included in Nginx by default.

Basic Config

Lets say you have deployed your website/webapp on multiple servers in the same network with the IPs 10.0.1.1,10.0.1.2,10.0.1.3 (of course it doesn't matter where the servers are. They can all be on the same physical machine, in the same network or even somewhere else in the internet).
All of the servers/backends will be combined into one upstream link by Nginx and then used as a single server in the rest of the configuration. 
This is an oversimplified principal of a personal "cloud" so lets call our upstream myCloud:

upstream myCloud{
  server 10.0.1.1;
  server 10.0.1.2;
  server 10.0.1.3;
} 

You can add as many servers as you want in there, including localhost,LAN and Online servers.

upstream myCloud{
  server s1.domain.com;
  server 10.0.1.2;
  server unix:/tmp/backend;
  server 127.0.0.1:8080;
} 

"myCloud" is now a registered upstream server that can be used as a parameter with the proxy_pass option. Now, wherever we use the myCloud upstream, Nginx, with the help of Round-Robin algorithm, will select the server to be used and proxy the received request to it.

Now you can use the new upstream link in your vhost config:

 

server {
  listen domain.com:80;
  access_log /var/log/nginx/proxy.log;
 
  location / {
    proxy_pass http://myCloud;
  }
}

Bind clients to servers

Often, it will be necessary for a single client to be served by a single server, not having different servers answering each request. For this, Nginx has the option ip_hash.
When ip_hash is turned on, the proxy server will remember the client's IP address hash and will use the same server every time.

upstream myCloud{
  ip_hash;
  server 10.0.1.1;
  server 10.0.1.2;
  server 10.0.1.3;
} 

Exclude Servers

If for some reason you need to temporary exclude one or more servers from being proxied by Nginx, you can use the "down" parameter:

upstream myCloud{
  server 10.0.1.1;
  server 10.0.1.2 down;
  server 10.0.1.3;
} 

Define priorities

You can define priorities by using a "weight" option for each server. The weight of a server roughly describes how often he will be used.
For example let's say the weight of the 10.0.1.1 server is 3, of the 10.0.1.2 is 1 and the weight of 10.0.1.3 is 2. 
In this case the first 3 requests will be proxied to server 10.0.1.1, the 4th to server 10.0.1.2 and the 5th and 6th requests will be proxied to server 10.0.1.3, after which the cycle will start again from the beginning.
The weight for all servers by default is equal to 1, but you change it as you wish.

upstream myCloud{
  server 10.0.1.1 weight=3;
  server 10.0.1.2;
  server 10.0.1.3 weight=2;
} 

Automatic Failover

If any of the upstream servers stops responding, then Nginx won't be able to connect to it and will serve the next available server from the "cloud".
The client won't actually experience any downtime but rather a long response from the server, which is not good. 
The next problem is that Nginx will try to connect to the non responsive server over and over again on each cycle, losing valuable time.
But with the parameter max_fails you can set a maximum amount of connection failures before Nginx marks the server as down and stops trying to connect there.
By default this option equals to 1, which means that after a connection failure Nginx will stop trying to connect for a certain amount of time. This is defined by the option fail_timeout and by default is 10 seconds.

upstream myCloud{
  server 10.0.1.1 max_fails=3 fail_timeout=120;
  server 10.0.1.2;
  server 10.0.1.3;
} 

Backup Servers

Backup Servers are used only when all of the normal upstream servers stop responding to requests. They are marked with the backup parameter.

upstream myCloud{
  server 10.0.1.1;
  server 10.0.1.2;
  server 10.0.1.3;
  server 10.0.1.8 backup;
  server 10.0.1.9 backup;
} 

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