Tower Defense: Cache Control

> stale-while-revalidate is a relatively recent cache-control option

Varnish Cache have supported that since version 4.1, it was released 2015-09-30:

> Varnish will now use the stale-while-revalidate defined in RFC5861 to set object grace time.

https://varnish-cache.org/docs/trunk/whats-new/changes-4.1.h...

You can even do neat things like having the revalidate fetch tell your backend “I served this object out of cache 1100 times since last fetch, maybe consider putting a few extra cpu cycles into the compression on this one”