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The Cache-Control header coordinates cachingacross browsers, proxies, and CDNs through a set ofcomposable directives. A page with a longmax-age is re-fetched less often, whileno-cache signals frequent content changes andwarrants more frequent crawling. Google’s crawling infrastructure implementsheuristic HTTP caching.The max-age directive helps Googlebotdetermine recrawl frequency. The browser and any sharedcache store this response for one year withoutrevalidation. Once the background revalidation completes, the cachereplaces the stale entry with the fresh response.
Cache directives
If the origin is unreachable, thecache returns a 504 instead of serving stalecontent. The timerstarts from the moment the origin creates the response,so transit time and time spent in intermediate cachescount against the budget. The no-transform directive forbids intermediariesfrom modifying the response body, such as recompressingimages or converting media formats. To prevent a cache from storing a response https://leatherial.com/ atall, use no-store instead. Multiple directives are separated by commas in asingle header value or split across multipleCache-Control headers.
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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.
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Stale responses are served for up to 30seconds while the CDN revalidates in the background. The parameter sets the number of secondsbeyond the freshness lifetime during which the staleresponse remains usable as a fallback. Cachesskip conditional requests forimmutable resources, eliminating revalidation roundtrips for assets like versioned JavaScript bundles orfingerprinted images. The immutable directive guarantees the response bodywill not change during the freshness lifetime.
Response Directives
Imagine that clients/caches store a fresh response for a path, with no request flight to the server. The no-cache request directive asks caches to validate the response with the origin server before reuse. If you forget to add private to a response with personalized content, then that response can be stored in a shared cache and end up being reused for multiple users, which can cause personal information to leak. HTTP allows caches to reuse stale responses when they are disconnected from the origin server. The cache stores the response for five minutesand revalidates before reuse once stale. The public directive marks a response as eligiblefor storage in shared caches.
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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.
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 max-age.
proxy-revalidate
A request carrying no-cache requires the cache tovalidate the stored response with theorigin before serving the copy. The cache returns thestored copy only if the response will stay fresh forthe additional period. The max-stale directive signals acceptance of aresponse whose Age has exceeded the freshnesslifetime by up to the given number of seconds. Both requests and responses usethis header to coordinate Caching behavior acrossthe entire delivery chain.
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.
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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.
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The response no-cache directive requires caches torevalidate the stored response with theorigin before every reuse. The max-age directive tells caches to return a storedresponse only if the response is no older than thespecified number of seconds. The no-store request directive allows a client to request that caches refrain from storing the request and corresponding response — even if the origin server’s response could be stored. Indicates that caches can store this response and reuse it for subsequent requests while it’s fresh. Responses carrying anAuthorization header are not storedby shared caches unless the public directive ispresent.
Cache directives
- Many browsers use this directive for reloading, as explained below.
- This directive is useful when an originserver is temporarily unreachable and a slightly staleresponse is acceptable.
- Multiple directives are separated by commas in asingle header value or split across multipleCache-Control headers.
- The max-age directive declares the number of secondsthe response remains fresh after generation.
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).
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.
We fight for a safer, more transparent food system. Data provided by LabelINSIGHT® and backed by science-based food safety systems. Not all processed foods are bad but some are more concerning than others. EWG’s Food Scores helps you make informed choices about what’s in your food and how it’s produced and processed. https://thalassa-ile-oleron.com/ Also, you can use vmstat 1 to view cache statistics.
proxy-revalidate
- For example, some convert images to reduce transfer size.
- If the sense of “don’t cache” that you want is actually “don’t store”, then no-store is the directive to use.
- Not all processed foods are bad but some are more concerning than others.
- The parameter sets the number of secondsbeyond the freshness lifetime during which the staleresponse remains usable as a fallback.
The HTTP Cache-Control header holds directives (instructions) in both requests and responses that control caching in browsers and shared caches (e.g., Proxies, CDNs). Responses for requests with Authorization header fields must not be stored in a shared cache; however, the public directive will cause such responses to be stored in a shared cache. There are no cache directives for clearing already-stored responses from caches on intermediate servers. The must-revalidate response directive indicates that the response can be stored in caches and can be reused while fresh.
The stale-if-error response directive indicates that the cache can reuse a stale response when an upstream server generates an error, or when the error is generated locally. The stale-while-revalidate response directive indicates that the cache could reuse a stale response while it revalidates it to a cache. If a request doesn’t have an Authorization header, or you are already using s-maxage or must-revalidate in the response, then you don’t need to use public.
