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After freshnessexpires, caches serve the stale copy whilerevalidating in the background. The stale-while-revalidate directive extends theusability window of a stale response. The public directive is unnecessary whenmust-revalidate or s-maxage is already present.
Request directives
The s-maxage (shared max-age) directive overridesmax-age for shared caches such as CDNs and proxyservers. After the hour elapses, caches treat theresponse as stale and either revalidate or fetch anew copy depending on other directives present. The min-fresh directive requests a stored responsewhose remaining freshness lifetime is at least thespecified number of seconds. The HTTP Cache-Control header carries directivescontrolling how browsers, proxies, and CDNs store andserve cached responses.
proxy-revalidate
Caches are encouraged to treat the value as if it were 0 (this is noted in the Calculating Freshness Lifetime section of the HTTP specification). Usually, the revalidation is done through a conditional request. Cache that exists between the origin server and clients (e.g., Proxy, CDN).
private
No-cache allows caches to store a response but requires them to revalidate it before reuse. If you want caches to always check for content updates while reusing stored content, no-cache is the directive to use. This usually means the response can be reused for subsequent requests, depending on request directives.
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The public response directive indicates that the response can be stored in a shared cache. The private response directive indicates that the response can be stored only in a private cache (e.g., local caches in browsers). The no-store response directive indicates that any caches of any kind (private or shared) should not store this response. The proxy-revalidate response directive is the equivalent of must-revalidate, but specifically for shared caches only.
Directives
If /assets/ files are suitable for storing in a shared cache, you also need one of public, s-maxage or must-revalidate. Note that no-cache means “it can be stored but don’t reuse before validating” — so it’s not for preventing a response from being stored. If a cache has a stored response, even a stale one, it will be returned. For example, a request with the header above indicates that the browser will accept a stale response from the cache that has expired within the last hour. The must-understand response directive indicates that a cache should store the response only if it understands the requirements for caching based on status code. This means that the response is access-controlled for restricted users (who have accounts), and it’s fundamentally not shared-cacheable, even if it has https://monsterenergyhouse.com/en-in/ max-age.
Folders and files
The client indicates that an already-cached response should be returned. In the example above, the response is fresh for 7 days (604800s). Revalidation will make the cache be fresh again, so it appears to clients that it was always fresh during that period — effectively hiding the latency penalty of revalidation from them.
- The immutable directive guarantees the response bodywill not change during the freshness lifetime.
- Here, an error is considered any response with a status code of 500, 502, 503, or 504.
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Installation and Usage
In some cases, this is undesirable for the content provider. For example, some convert images to reduce transfer size. Some intermediaries transform content for various reasons. If a cache doesn’t support must-understand, it will be ignored. Must-understand should be coupled with no-store for fallback behavior. You can use the public directive to unlock that restriction.
If you don’t add a Cache-Control header because the response is not intended to be cached, that could cause an unexpected result. You can add a long max-age value and immutable because the content will never change. Note that the major browsers do not support requests with min-fresh.
- For example, a request with the header above indicates that the browser will accept a stale response from the cache that has expired within the last hour.
- The must-understand response directive indicates that a cache should store the response only if it understands the requirements for caching based on status code.
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- The response no-transform directive preventsintermediaries from altering the response body beforeforwarding, whether the intermediary caches thecontent or not.
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The no-store directive prevents any cache fromstoring the response. This directive is useful when an originserver is temporarily unreachable and a slightly staleresponse is acceptable. Unrecognized directives are ignored by caches.This allows new directives to be introducedwithout breaking older implementations. Origins, intermediaries, and clients all rely onCache-Control to agree on when a stored responseremains usable and when a fresh copy is needed. Adding no-cache to the response causes revalidation to the server, so you can serve a fresh response every time — or if the client already has a new one, just respond 304 Not Modified.
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Sharedcaches such as CDNs and proxy servers discard theresponse. The private directive restricts storage to privatecaches, typically the end user’s browser. Once stale, the cachecontacts the origin to revalidate before serving theresponse again.
Clients can use this header when the user requires the response to not only be fresh, but also requires that it won’t be updated for a period of time. When you use a cache-busting pattern for resources and apply them to a long max-age, you can also add immutable to avoid revalidation. When a user reloads the browser, the browser will send conditional requests for validating to the origin server. The immutable response directive indicates that the response will not be updated while it’s fresh. No-transform indicates that any intermediary (regardless of whether it implements a cache) shouldn’t transform the response contents. If a cache supports must-understand, it stores the response with an understanding of cache requirements based on its status code.
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proxy-revalidate
Cache storage is allowed to cache it heuristically — so if you have any requirements on caching, you should always indicate them explicitly, in the Cache-Control header. When you update the library or edit the picture, new content should have a new URL, and caches aren’t reused. Note that the major browsers do not support requests with max-stale. Browsers usually add no-cache to requests when users are force reloading a page. No-cache allows clients to request the most up-to-date response even if the cache has a fresh response. If no request happened during that period, the cache became stale and the next request will revalidate normally.
In such a case, you could address the caching needs by using a specific, numbered version of the library, and including the hash of the picture in its URL. Caches are encouraged to treat the value as if it were 0. Many browsers use this directive for reloading, as explained below. After the stale-if-error period passes, the client will receive any error generated. Afterwards, it becomes stale, but can be used for an extra 1 day (86400s) when an error is encountered. Here, an error is considered any response with a status code of 500, 502, 503, or 504.
Directives
Note that s-maxage or must-revalidate also unlock that restriction. If the sense of “don’t cache” that you want is actually “don’t store”, then no-store is the directive to use. It is a https://bh-marketinggroup.com/en-in/ criterion for whether a response is fresh or stale. However, the cached response is not always reused as-is.
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It stores a single response and reuses it with multiple users — so developers should avoid storing personalized contents to be cached in the shared cache. The command make install will install the shared library, manpages and the nocache, cachestats and cachedel commandsunder /usr/local. The stale-if-error directive allows caches to servea stale response when the origin returns an errorstatus (500, 502, 503, or 504) or isunreachable. The proxy-revalidate directive works identically tomust-revalidate, except the requirement applies onlyto shared caches. The response no-transform directive preventsintermediaries from altering the response body beforeforwarding, whether the intermediary caches thecontent or not. The max-age directive declares the number of secondsthe response remains fresh after generation.