HTTP shared cache

An HTTP shared cache stores HTTP responses for reuse by more than one user. It is often part of an intermediary, though that deployment is not required by the definition. An HTTP private cache is instead dedicated to a single user.^[ref]

For an own example, suppose four users request the same public manual through one shared cache. The first request obtains a complete, cacheable response with a $1200$-octet body. Assume the following three requests match the required HTTP cache key inputs, arrive while the stored response remains reusable, and have no directives requiring forwarding. They can be satisfied from that stored response. The origin then sends the body once instead of four times, avoiding $3(1200)=3600$ origin-body octets in this stipulated sequence. This does not count headers, validation traffic or delivery from the cache to the users.

Matching the target identifier alone need not be sufficient. If the response uses Vary: Accept, the relevant request-field matching rules also matter; one user's negotiated HTTP representation is not automatically suitable for another request's preferences.^[ref] HTTP content negotiation and cache selection therefore interact.

Storage and reuse are separate decisions. The unqualified Cache-Control: private response directive forbids storage by a shared cache. Other responses must still satisfy the ordinary storage constraints, and the cache must consider HTTP freshness or successful HTTP cache validation when constructing a later response.^[ref]

A shared cache also interprets s-maxage when present. For Cache-Control: max-age=600, s-maxage=60, its explicit freshness lifetime is sixty seconds rather than six hundred. This gives the origin a way to specify a shared reuse interval independently of the corresponding private-cache interval.^[ref]

The benefit comes from reusing an eligible response across users, not from assuming all users receive identical content. State selection inputs and cache controls before extrapolating the simple manual example to personalized or negotiated resources.