Cache-Control(Http Header)

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 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>


Directives

Cacheability

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


Examples:

Preventing caching

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

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


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