I had an interview with Amazon for SDE2 position and I was being asked to build a CDN service from scratch. When I prep for a system design interview question I usually utilized CDN service as part of the high level architecture but I never thought about building it my own.

I approached it like there will be some kind of S3 storage across other multiple regions and there is some sort of an API that runs to sync contents by comparing the S3 buckets.

The interviewer seemed like he was not satisfied with my approach. Could anyone please guide me how to build a CDN service from scratch?