What is the primary purpose of the ‘Vary’ HTTP header in a caching context?

Question

Web Development — Hard

What is the primary purpose of the ‘Vary’ HTTP header in a caching context?

Accepted Answer

The 'Vary' header tells caches that the response depends on the value of specific request headers (e.g., 'Accept-Encoding' or 'User-Agent'). Without 'Vary', a cache might serve a cached version of a resource to a client that cannot handle the specific encoding or format of that cached version. It effectively expands the cache key beyond just the URL.