CDN

When the same service is deployed in many places, the usual approach is to give each node a different address and let DNS, a configuration center, or an upper-layer scheduler decide which one the user should connect to. That can work, but it has a clear cost: the access decision happens outside the network, so very often the name has already been resolved before the user discovers that they were sent somewhere suboptimal.

What is hard to understand about a CDN is not “it can cache files.” It is that it moves user access points, cache layers, and origin protection out to the network edge. As a result, the first-packet time for the same URL may differ a lot across regions, the origin may be healthy while some users still see old content, and when image or video traffic rises, the first thing to saturate is often not the CPU but the cross-region bandwidth and origin fetch path.