A fingerprinted asset puts a digest of its bytes into its URL. Changing a stylesheet produces a different address, so a page can request the new representation without depending on every existing cache expiring simultaneously. The useful guarantee comes from the publishing process: once an address is released, its contents must remain consistent with that digest. ...
Age is an estimate of elapsed seconds since an origin generated or successfully validated a response. It includes time in caches and in transit. It is not the object’s creation timestamp, a count of requests, or simply the time spent on the last proxy’s disk. Reading it correctly requires the response’s freshness policy and the request context. ...
Storage scope is not an identity system. Cache-Control: private tells a conforming shared cache not to store a response; it does not teach a browser which application account owns the bytes. This distinction matters when a person logs out, another account logs in, and the same browser requests the same URL again. ...
DNS can return useful information by saying that data is absent. NXDOMAIN reports a name error; NODATA describes a successful response without the requested record type. In a direct query without aliases, these distinguish a nonexistent name from an existing name that lacks the requested data. Caching those results avoids repeating the same unsuccessful upstream lookup for every client request. ...
One URL can return JSON to an API client and plain text to another reader. Content-Type identifies the selected representation; Vary identifies request fields relevant to selecting it. A correct body label cannot prevent a cache from choosing the wrong stored response when the request-matching information is missing. ...
A 304 is useful only when the old representation really remains an acceptable substitute. ETag supplies the origin’s version claim; it does not independently verify that claim. An incorrect tag can make conditional requests efficiently preserve wrong content. Test both unchanged data and meaningful changes before counting reduced transfers as a performance improvement. ...
A cache is only as coherent as the pair of conditions your headers make explicit.
An origin defines storage and freshness policy; a cache decides whether its stored response can satisfy the next request. When it cannot reuse that response directly, validators can make checking cheaper. These are separate decisions. Finding Cache-Control and ETag in one successful response does not prove that the next request follows the intended path. ...