How to cache intelligently

Now that you understand all of the above, it's time to look at the basic state machine which Varnish uses to serve responses. In this sense, VCL is simply a DSL that drives the caching state machine, but not much more than that.

<aside> 💡 NOTE: To be completely explicit — the below diagram only describes clustering at the POP level and the interactions between edge/cluster nodes. Shielding is completely described above.

</aside>

Fastly Varnish, Mark McDonnell, 2017

The above diagram comes from an informative article by Mark McDonnell, Fastly Varnish, which I strongly suggest you read (probably the most accurate technical overview you can get short of being a Fastly employee, or watching all the talks at Fastly's Altitude conference.)