HTTP private cache

An HTTP private cache is dedicated to a single user. It is often implemented as part of a user agent, but its defining property is the single-user scope rather than a particular machine or storage medium. An HTTP shared cache can instead reuse responses for multiple users.^[ref]

Suppose a user's response carries Cache-Control: private, max-age=60. The unqualified private directive permits a private cache to store the response subject to the normal storage rules, while forbidding a shared cache from storing it. The max-age directive supplies its explicit HTTP freshness lifetime; it does not cancel the single-user storage restriction.^[ref]

If the correctly calculated HTTP current age is twenty seconds, sixty seconds exceeds that age and forty seconds remain until the freshness boundary. Reuse still requires matching the relevant request and satisfying other applicable constraints. At current age sixty, the strict freshness comparison fails; HTTP cache validation may then establish whether the stored representation remains suitable.^[ref]

Being private does not mean that every response must be stored or may be reused indefinitely. A cache can discard an entry, and an otherwise applicable restrictive directive remains significant. In particular, unqualified no-cache requires successful validation before subsequent reuse, whereas no-store prohibits the corresponding storage and reuse under its defined rules.^[ref]

The word private is also a cache-control distinction, not an encryption mechanism or a promise that the message is inaccessible to everyone else. The standard explicitly separates those meanings. Correct HTTP cache key selection and response handling still matter inside a single-user cache.^[ref]

For negotiated content, the stored representation can depend on request fields as well as the target. A private cache must therefore apply the same relevant matching rules rather than treating every request by its one user as interchangeable.