Global CDN cache hierarchy

Design a global CDN with a multi-tier cache hierarchy serving static assets and video segments to hundreds of millions of users. Constraints: hundreds of edge PoPs, a target origin offload above 95%, p95 edge cache-hit latency under 30 ms, mixed object sizes (small CSS/JS up to multi-MB video chunks), and the ability to purge a customer's asset globally within seconds. Cover the cache tiers, fill path, and invalidation.