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Cache-Control tells caches how and when a response may be stored or reused; an ETag is a validator that helps determine whether a stored representation still matches the current one. They control different parts of HTTP caching and are often used together: freshness policy can allow reuse without a request to the origin, while an ETag can support validation after a response becomes stale.
Cache-Control and ETag have different jobs
Cache-Control is policy: its directives guide caches along the request-and-response chain. The IETF’s RFC 9111, HTTP Caching describes the field as listing directives for caches. An ETag, defined in RFC 9110, HTTP Semantics, is an opaque validator for a selected representation. It can distinguish representations that change over time or vary according to content negotiation.
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| Question | Cache-Control | ETag |
|---|---|---|
| Main job | Direct cache behavior, including freshness and reuse. | Identify or validate a selected representation. |
| Core question | May the cache reuse this response under the applicable rules? | Does the stored representation still match the current one? |
| Illustrative example | Cache-Control: max-age=3600 |
ETag: "v7" |
| When it matters | Storage, freshness, and reuse decisions. | Conditional requests and validation. |
The example values are illustrative; HTTP standards do not prescribe them.
How freshness and validation work together
Freshness and validation are separate decisions. A fresh response may be reused under the applicable cache rules. When a stored response is stale, a cache may ask the origin to validate it rather than immediately downloading the representation again.
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A server returns a representation with
Cache-Control: max-age=60and an ETag such as"v7". These values are illustrative. -
While the response is fresh, a cache can reuse it under the applicable rules without contacting the origin.
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After it becomes stale, the cache can send a conditional request with
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If the representation has not changed, the origin can validate the stored copy. If it has changed, the origin sends a new representation and validator.
This is the standards-defined relationship, not a guarantee that every server or cache behaves identically in every deployment.
What Cache-Control directives determine
Freshness calculations depend on the response’s caching metadata. Under RFC 9111, a shared cache uses s-maxage when present; otherwise, max-age takes precedence over Expires. When explicit expiration information is absent, heuristic freshness may apply.
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s-maxage=Nsupplies a freshness lifetime for shared caches and takes precedence there when present. -
must-revalidateprohibits reuse of a stale response until it has been successfully validated. If the cache is disconnected and cannot validate, it must return an error rather than reuse that stale response.Rank #4
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no-cachedoes not mean “do not store.” It means a stored response must be validated before reuse.
Request and response directives are distinct and unidirectional: a directive in a request does not mean the same directive is automatically present in the response. For example, request max-age expresses a client preference about the age of a response, while response max-age defines when that response becomes stale.
What an ETag does—and does not—tell you
An ETag is not a freshness duration. It is an opaque validator: HTTP does not require its value to be a timestamp, a particular hash, or any other specific construction. A cache can use it in If-None-Match when validating a stored representation, but the tag itself does not say how long the representation may be reused.
That distinction also explains why an ETag does not replace Cache-Control. The cache’s freshness policy answers when reuse is allowed without validation; the validator helps the cache check whether its stored representation remains current when validation is needed.
Freshness is not the same as a browser refresh
Freshness governs cache behavior; it does not force a user agent to refresh what is displayed or reload a resource. A browser’s display and history behavior are not the same thing as whether a cached response is fresh under HTTP rules.
When stale-response extensions are involved
RFC 5861 defines extensions such as stale-while-revalidate and stale-if-error, which describe circumstances in which stale content may be served while revalidation occurs or when an error occurs. Support in deployed caches is implementation-dependent, so their presence in the specification does not establish universal support.
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