Cache-Control - HTTP 编辑

The Cache-Control HTTP header holds directives (instructions) for caching in both requests and responses. A given directive in a request does not mean the same directive should be in the response.

Syntax

Caching directives have the following rules to be valid:

• Case-insensitive, but lowercase is recommended.
• Multiple directives are comma-separated.
• Some directives have an optional argument, which can be either a token or a quoted-string. (See spec for definitions)

Cache request directives

Standard Cache-Control directives that can be used by the client in an HTTP request.

Cache-Control: max-age=<seconds>
Cache-Control: max-stale[=<seconds>]
Cache-Control: min-fresh=<seconds>
Cache-Control: no-cache
Cache-Control: no-store
Cache-Control: no-transform
Cache-Control: only-if-cached

Cache response directives

Standard Cache-Control directives that can be used by the server in an HTTP response.

Cache-Control: must-revalidate
Cache-Control: no-cache
Cache-Control: no-store
Cache-Control: no-transform
Cache-Control: public
Cache-Control: private
Cache-Control: proxy-revalidate
Cache-Control: max-age=<seconds>
Cache-Control: s-maxage=<seconds>

Extension Cache-Control directives

Extension Cache-Control directives are not part of the core HTTP caching standards document. Check the compatibility table for their support; user-agents that don't recognize them should ignore them.

Cache-Control: immutable
Cache-Control: stale-while-revalidate=<seconds>
Cache-Control: stale-if-error=<seconds>

Directives

Cacheability

Directives that define whether a response/request can be cached, where it may be cached, and whether it must be validated with the origin server before caching.

public

The response may be stored by any cache, even if the response is normally non-cacheable.

private

The response may be stored only by a browser's cache, even if the response is normally non-cacheable. If you mean to not store the response in any cache, use no-store instead. This directive is not effective in preventing caches from storing your response.

no-cache

The response may be stored by any cache, even if the response is normally non-cacheable. However, the stored response MUST always go through validation with the origin server first before using it, therefore, you cannot use no-cache in-conjunction with immutable. If you mean to not store the response in any cache, use no-store instead. This directive is not effective in preventing caches from storing your response.

no-store

The response may not be stored in any cache. Note that this will not prevent a valid pre-existing cached response being returned. Clients can set max-age=0 to also clear existing cache responses, as this forces the cache to revalidate with the server (no other directives have an effect when used with no-store).

Expiration

max-age=<seconds>

The maximum amount of time a resource is considered fresh. Unlike Expires, this directive is relative to the time of the request.

s-maxage=<seconds>

Overrides max-age or the Expires header, but only for shared caches (e.g., proxies). Ignored by private caches.

max-stale[=<seconds>]

Indicates the client will accept a stale response. An optional value in seconds indicates the upper limit of staleness the client will accept.

min-fresh=<seconds>

Indicates the client wants a response that will still be fresh for at least the specified number of seconds.

stale-while-revalidate=<seconds>

Indicates the client will accept a stale response, while asynchronously checking in the background for a fresh one. The seconds value indicates how long the client will accept a stale response. See "Keeping things fresh with stale-while-revalidate" for more information.

stale-if-error=<seconds>

Indicates the client will accept a stale response if the check for a fresh one fails. The seconds value indicates how long the client will accept the stale response after the initial expiration.

Revalidation and reloading

must-revalidate

Indicates that once a resource becomes stale, caches must not use their stale copy without successful validation on the origin server.

proxy-revalidate

Like must-revalidate, but only for shared caches (e.g., proxies). Ignored by private caches.

immutable

Indicates that the response body will not change over time. The resource, if unexpired, is unchanged on the server and therefore the client should not send a conditional revalidation for it (e.g. If-None-Match or If-Modified-Since) to check for updates, even when the user explicitly refreshes the page. Clients that aren't aware of this extension must ignore them as per the HTTP specification. In Firefox, immutable is only honored on https:// transactions. For more information, see also this blog post.

Other

no-transform

An intermediate cache or proxy cannot edit the response body, Content-Encoding, Content-Range, or Content-Type. It therefore forbids a proxy or browser feature, such as Google’s Web Light, from converting images to minimize data for a cache store or slow connection.

only-if-cached

Set by the client to indicate "do not use the network" for the response. The cache should either respond using a stored response, or respond with a 504 status code. Conditional headers such as If-None-Match should not be set. There is no effect if only-if-cached is set by a server as part of a response.

Examples

Preventing caching

To disable caching of a resource, you can send the following response header:

Good:

Cache-Control: no-store

The no-store directive will prevent a new resource being cached, but it will not prevent the cache from responding with a non-stale resource that was cached as the result of an earlier request. Setting max-age=0 as well forces the cache to revalidate (clears the cache).

Cache-Control: no-store, max-age=0

Bad:

Cache-Control: private,no-cache,no-store,max-age=0,must-revalidate,pre-check=0,post-check=0

Caching static assets

For the files in the application that will not change, you can usually add aggressive caching by sending the response header below. This includes static files that are served by the application such as images, CSS files and JavaScript files, for example. In addition, see also the Expires header.

Cache-Control: public, max-age=604800, immutable

Requiring revalidation

no-cache and max-age=0, must-revalidate indicates same meaning.
Clients can cache a resource but must revalidate each time before using it. This means HTTP request occurs each time though, it can skip downloading HTTP body if the content is valid.

Cache-Control: no-cache

Cache-Control: max-age=0, must-revalidate

Note: Following may serve stale resource if server is down or lose connectivity.

Cache-Control: max-age=0

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• HTTP Caching FAQ
• Caching Tutorial for Web Authors and Webmasters
• Guide: Cache-Control for civilians
• Age
• Expires
• Pragma