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How to choose a safe cache TTL
First decide the maximum age at which the response is still useful and safe to show. Make that decision with the product or data owner: an old public article may be acceptable, while an outdated price, availability status, or account balance may not be.
- Set an age tolerance. Define how old the data may be when a user sees it, including during a refresh or an origin problem.
- Choose the fresh lifetime. Set
max-ageno higher than the period for which a cache may reuse the response without checking the origin. - Decide whether shared caches need a different lifetime. For responses safe to share,
s-maxagesets freshness for shared caches and overridesmax-agethere. Private caches continue to usemax-age. - Account for existing age. Freshness is measured from when the origin generated the response, not restarted when a cache receives or forwards it. Check the
Ageheader when estimating how much freshness remains. - Set stale allowances separately. Add only a bounded stale interval that fits the same age tolerance. Decide independently whether stale content may be used during refresh and whether it may be used after an eligible server error.
HTTP cache directives and their semantics are described in MDN’s Cache-Control reference; the protocol’s core caching rules are in RFC 9111.
What stale-while-revalidate does
stale-while-revalidate=N permits a cache to serve a stale stored response for a bounded interval while it revalidates that response in the background. It can reduce the delay a user sees during refresh, but it also means a user may receive older data. It is not a promise that a refresh happens on a timer: if no request arrives during the stale window, the next request validates normally.
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Think about the maximum possible age, not just the fresh TTL. A response near the end of its freshness period may then be served during the stale window before a successful refresh, depending on cache behavior and timing. RFC 5861 describes the stale response extensions, including stale-while-revalidate and stale-if-error: RFC 5861.
Example policy: fresh content with bounded background refresh
This is a template, not a set of recommended durations. Replace each placeholder with a value justified by the response’s age tolerance and the caches in your delivery path.
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Cache-Control: public, max-age=<fresh-seconds>, s-maxage=<shared-fresh-seconds>, stale-while-revalidate=<bounded-stale-seconds>
ETag: "<representation-version>"Use public only when the response is genuinely safe for shared caches to store and serve. Do not apply it casually to personalized or access-controlled responses. If shared and private caches should have the same freshness, determine whether a separate s-maxage is needed rather than assuming the example fits every deployment.
An ETag gives a cache a validator for the stored representation. When content changes, the origin should provide a validator that lets the cache recognize the new version. Test the actual browser, proxy, and CDN path: HTTP semantics do not guarantee identical behavior across every implementation.
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Choose between reuse, revalidation, and no storage
| Policy | What it permits | When it fits |
|---|---|---|
max-age (and, where appropriate, s-maxage) |
Reuse while the response is fresh, without checking the origin on each reuse. | Use when the chosen freshness period is within the tolerated age for unchecked data. |
stale-while-revalidate |
Reuse stale content for a bounded interval while validation proceeds in the background. | Use when lower visible latency is worth temporarily showing older data within an explicit age limit. |
no-cache with validators |
Storage is allowed, but the cache must validate with the origin before reuse. An unchanged response can be confirmed without transferring the full body. | Use when the stored response may be retained but must be checked each time it is reused. |
no-store |
Caches must not store the response. | Use when storage itself is inappropriate, rather than as a substitute for requiring validation. |
stale-if-error |
Allows bounded stale reuse for qualifying server errors, subject to the directive and cache behavior. | Consider separately when availability during origin errors matters and the data owner accepts the resulting age. |
no-cache and no-store are not synonyms. MDN explains that no-cache permits storage but requires validation before reuse; no-store directs caches not to store the response. See MDN’s Cache-Control reference.
Use validators to make revalidation efficient
A cache can validate a stored response with ETag and If-None-Match, or with Last-Modified and If-Modified-Since. If the representation has not changed, the origin can reply 304 Not Modified, allowing the cache to keep using its stored body rather than receive it again. If it has changed, the cache needs the updated representation. The MDN HTTP caching guide covers freshness, age, validators, and conditional requests.
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Keep representations and access boundaries correct
If request headers can change the representation—for example, the requested language—use Vary for the relevant headers so a cache does not serve one variant as another. Also check whether responses vary by identity or other request inputs before permitting shared caching. A cache policy that is fresh but mixes users’ or languages’ representations is not safe.
Review the policy before deploying it
- What is the maximum age a user can tolerate for this particular response?
- Which caches in the real request path are shared, and should their freshness differ from a private cache?
- Is the response safe to share, or does it vary by user, language, encoding, or another request input?
- How will the origin detect whether a stored representation changed?
- What should happen if validation or refresh fails, and is any stale fallback separately bounded?
- Do observed
Agevalues and the behavior of the actual browser, proxy, and CDN match the intended policy?
Avoid leaving freshness behavior implicit when correctness matters: HTTP can apply heuristic caching in some circumstances. Review the applicable semantics in RFC 9111 and test the deployed path. The appropriate values depend on the data and implementation; neither the HTTP standards nor the directives define one safe TTL for every frequently changing response.
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