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# Advanced Backend configuration

At some point you might need Varnish to cache content from several servers. You might want Varnish to map all the URL into one single host or not. There are lot of options.

Lets say we need to introduce a Java application into out PHP web site. Lets say our Java application should handle URL beginning with /java/.

We manage to get the thing up and running on port 8000. Now, lets have a look a default.vcl.:

    backend default {
        .host = "127.0.0.1";
        .port = "8080";
    }

We add a new backend.:

    backend java {
        .host = "127.0.0.1";
        .port = "8000";
    }

Now we need tell where to send the difference URL. Lets look at vcl_recv.:

    sub vcl_recv {
        if (req.url ~ "^/java/") {
            set req.backend = java;
        } else {
            set req.backend = default.
        }
    }

It’s quite simple, really. Lets stop and think about this for a moment. As you can see you can define how you choose backends based on really arbitrary data. You want to send mobile devices to a different backend? No problem. if (req.User-agent ~ /mobile/) .... should do the trick.

# Directors

You can also group several backend into a group of backends. These groups are called directors. This will give you increased performance and resilience. You can define several backends and group them together in a director.:

    backend server1 {
        .host = "192.168.0.10";
    }
    backend server2{
        .host = "192.168.0.10";
    }

Now we create the director.:

    director example_director round-robin {
    {
            .backend = server1;
    }
    # server2
    {
            .backend = server2;
    }
    # foo
    }

This director is a round-robin director. This means the director will distribute the incoming requests on a round-robin basis. There is also a random director which distributes requests in a, you guessed it, random fashion.

But what if one of your servers goes down? Can Varnish direct all the requests to the healthy server? Sure it can. This is where the Health Checks come into play.

# Health checks

Lets set up a director with two backends and health checks. First lets define the backends.:

    backend server1 {
      .host = "server1.example.com";
      .probe = {
             .url = "/";
             .interval = 5s;
             .timeout = 1 s;
             .window = 5;
             .threshold = 3;
        }
      }
    backend server2 {
       .host = "server2.example.com";
       .probe = {
             .url = "/";
             .interval = 5s;
             .timeout = 1 s;
             .window = 5;
             .threshold = 3;
       }
     }

Whats new here is the probe. Varnish will check the health of each backend with a probe. The options are

url
  What URL should varnish request.
interval
  How often should we poll
timeout
  What is the timeout of the probe
window
  Varnish will maintain a sliding window of the results. Here the window has five checks.
threshold
  How many of the .window last polls must be good for the backend to be declared healthy.
initial
  How many of the of the probes a good when Varnish starts - defaults to the same amount as the threshold.

Now we define the director.:

    director example_director round-robin {
          {
                  .backend = server1;
          }
          # server2
          {
                  .backend = server2;
          }

          }

You use this director just as you would use any other director or backend. Varnish will not send traffic to hosts that are marked as unhealthy. Varnish can also serve stale content if all the backends are down. See Misbehaving servers for more information on how to enable this.

Please note that Varnish will keep probes active for all loaded VCLs. Varnish will coalesce probes that seem identical - so be careful not to change the probe config if you do a lot of VCL loading. Unloading the VCL will discard the probes.

### Navigation

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