Cache-Control directives: max-age, no-store, private and stale-while-revalidate

What it is

RFC 9111 defines the response directives a cache MUST obey:

• max-age=N: the response is stale once its age exceeds N seconds. s-maxage=N overrides it for shared caches and also requires them to revalidate once stale.
• no-cache: the response MUST NOT be used to satisfy another request without forwarding it for validation first. It does not forbid storing; a stored copy plus a validator makes revalidation cheap.
• no-store: the cache MUST NOT store any part of the request or response. RFC 9111 states that this is not a reliable or sufficient mechanism for ensuring privacy.
• private: a shared cache MUST NOT store the response; public explicitly marks it cacheable, for example a response to a request that carried Authorization.
• must-revalidate: once stale, the response MUST NOT be reused without successful validation; a disconnected cache has to return an error (504 is suggested).
• immutable (RFC 8246): clients should not send conditional requests during the freshness lifetime, even on reload.
• stale-while-revalidate=N and stale-if-error=N (RFC 5861): caches may serve the stale response for N further seconds while revalidating in the background, or while the origin returns errors.

With no explicit freshness at all, a cache MAY compute a heuristic lifetime for heuristically cacheable status codes; RFC 9111 mentions a fraction of the interval since Last-Modified, typically 10%.

Why it matters

Silence is not "do not cache": a plain 200 with Last-Modified may be reused for a heuristic period. no-cache is routinely misread as no-store, and public on a personalised response leaks it through a CDN. The directives are the contract between origin, CDN and browser; a wrong one is hard to withdraw because the cached copy outlives the fix.

How to apply

• Versioned static assets: public, max-age=31536000, immutable; change the URL when the content changes.
• HTML and API responses that change: no-cache (or max-age=0, must-revalidate) plus an ETag, so revalidation costs a 304.
• Per-user responses: private with a short max-age, or no-store where storing on shared machines is a concern.
• Slowly changing public data: max-age=60, stale-while-revalidate=600, stale-if-error=86400, combined with a validator.
• Always send explicit directives and Vary for negotiated content; read the Age header on CDN responses to see how old a copy is.

Pitfalls

Caches MUST ignore unknown directives, so stale-while-revalidate degrades to normal behaviour on caches that predate it. The qualified forms no-cache="Set-Cookie" and private="..." are described by RFC 9111 as not widely implemented. max-age counts from the origin's generation time as accumulated in Age, not from receipt. Request directives (max-stale, only-if-cached) come from the client and are a separate list.

Personalised responses and shared caches

no-cache allows storage; it only forces revalidation before reuse. A shared cache may therefore store a cookie-authenticated response and, on the next user's request, revalidate it with the first user's ETag; if the origin answers 304 by comparing the tag alone, the cache serves one user's body to another. Only responses to requests carrying Authorization are excluded from shared caches by default. Mark every response that depends on who asks as private (or no-store where storing on the device is also unwanted), keep no-cache and the ETag for cheap browser revalidation, and make the 304 decision depend on the authenticated principal as well as on the tag. Use no-cache alone only for content that is identical for every requester.