When browsers make a request, they include HTTP headers for the server to decide what to send back (Is this a mobile client? Can it handle compressed content? Does it need a certain language?).
That’s great for direct access, but modern networks use intermediate caches and CDNs. And there’s the problem: how does the cache use headers to decide what to send back? How can it replicate the server’s decision-making logic?
Vary to the rescue. The Vary header describes what information “uniquely” identifies a request — caches should only be used if the incoming request matches the Vary information in the cache.
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