In 'The Odyssey,' Greek poet Homer describes the 'wine-dark sea’. Today we would probably say “dark blue sea”, but at the time Greeks, and other ancient populations, didn’t have a name to describe what we today call “blue”. Blue, though, has always existed.
When using a traditional CDN or caching system, creating user-centric security and access policies can be a complex and performance challenging undertaking. Not only do you have to pull user data from a backend, but you then have to apply the security policies from that data onto the request. VCL is an excellent candidate for the latter problem of applying security policies to requests. This leaves the problem of how you get user data (JSON) into VCL? How do you do that on a user-by-user basis? And how do you do that in a way where you keep backend communication to an absolute minimum, or put another way, serve as much data from cache as possible?