CDNs & Edge Caching

Light in fibre covers roughly 200 km per millisecond, and real routes are never straight. A round trip from Sydney to a server in Virginia is around 200ms before your application does a single thing. No amount of tuning changes that — the only fix is to not make the trip.

A CDN is a distributed network of caches. Routing tries to place a user on a nearby healthy edge, shortening the long-haul portion even when the selected site is not in the same city.

The second benefit is often larger at scale: cache hits at the edge never reach your origin. For the cacheable request set, a 95% edge hit rate leaves roughly 5% reaching origin — a different order of magnitude of origin traffic.