There are a couple of things that are different with Varnish Cache, as opposed to other programs. One thing you’ve already seen - VCL. In this section we provide a very quick tour of other peculiarities you need to know about to get the most out of Varnish.
The Varnish Configuration is written in VCL. When Varnish is ran this configuration is transformed into C code and then fed into a C compiler, loaded and executed.
So, as opposed to switching various settings on or off, you write polices on how the incoming traffic should be handled.
Varnish Cache has an admin console. You can connect it through the varnishadm command. In order to connect the user needs to be able to read /etc/varnish/secret in order to authenticate.
Once you’ve started the console you can do quite a few operations on Varnish, like stopping and starting the cache process, load VCL, adjust the built in load balancer and invalidate cached content.
It has a built in command “help” which will give you some hints on what it does.