CDN

A CDN's entire value is serving a response without asking the origin, from a location physically closer to the user. Every problem in CDN configuration traces back to one question asked carelessly: is this response actually the same for the next request, or did it just look that way in testing? Get that wrong and you either serve one user's private data to another, or fail to cache anything and pay full origin cost with extra hops on top.

A cached response is a promise that this exact bytes-for-bytes answer is correct for anyone who asks the same question — verify that promise before you make it.

1. Define the cache key as precisely as the response varies

The cache key — usually URL plus a chosen set of headers/cookies/query params — determines what counts as "the same request." Too narrow a key (ignoring a header the response actually varies by, like Accept-Language or an auth cookie) causes one user's personalized or region-specific response to be served to everyone else. Too broad a key (including a header that doesn't affect the response, like a random request ID) destroys the cache hit rate by fragmenting it into singletons.