Cache-Control

The Cache-Control general-header field is used to specify directives for caching mechanisms in both requests and responses. Caching directives are unidirectional, meaning that a given directive in a request is not implying that the same directive is to be given in the response.

Syntax

The directives are case-insensitive and have an optional argument, that can use both token and quoted-string syntax. Multiple directives are comma-separated.

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. Be sure to check the compatibility table for their support.

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

Directives

Cacheability

public

Indicates that the response may be cached by any cache.

private

Indicates that the response is intended for a single user and must not be stored by a shared cache. A private cache may store the response.

no-cache

Forces caches to submit the request to the origin server for validation before releasing a cached copy.

only-if-cached

Indicates to not retrieve new data. The client only wishes to obtain a cached response, and should not contact the origin-server to see if a newer copy exists.

Expiration

max-age=<seconds>

Specifies the maximum amount of time a resource will be considered fresh. Contrary to Expires, this directive is relative to the time of the request.

s-maxage=<seconds>

Overrides max-age or the Expires header, but it only applies to shared caches (e.g., proxies) and is ignored by a private cache.

max-stale[=<seconds>]

Indicates that the client is willing to accept a response that has exceeded its expiration time. Optionally, you can assign a value in seconds, indicating the time the response must not be expired by.

min-fresh=<seconds>

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

stale-while-revalidate=<seconds>

Indicates that the client is willing to accept a stale response while asynchronously checking in the background for a fresh one. The seconds value indicates for how long the client is willing to accept a stale response.

stale-if-error=<seconds>

...

Revalidation and reloading

must-revalidate

The cache must verify the status of the stale resources before using it and expired ones should not be used.

proxy-revalidate

Same as must-revalidate, but it only applies to shared caches (e.g., proxies) and is ignored by a private cache.

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-store

The cache should not store anything about the client request or server response.

no-transform

No transformations or conversions should be made to the resource. The Content-Encoding, Content-Range, Content-Type headers must not be modified by a proxy. A non- transparent proxy might, for example, convert between image formats in order to save cache space or to reduce the amount of traffic on a slow link. The no-transform directive disallows this.

Examples

Preventing caching

To turn off caching, you can send the following response header. In addition, see also the Expires and Pragma headers.

Cache-Control: no-cache, no-store, must-revalidate

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=31536000

Specifications

Browser compatibility

See also

• HTTP Caching FAQ
• Age
• Expires
• Pragma