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Fastly is basically a varnish-based CDN. They even let you provide VCL code to program it.

They forked Varnish (around 2.0, judging by the VCL syntax) quite early and I suspect there isn't much left of the original codebase.

I'm very impressed how they managed to retrofit multitenancy.

Last I heard they were moving away from Varnish (but still supporting VCL for customers that were using it)