Why cache-control HTTP header for requests?

Question

I recently went through this article. It says that not only response, the requests can also include cache-control options.

Though I understood, it can be utilized by server responses meaningfully, I don't clearly understand why they are needed in requests. Unfortunately I could find nothing on internet which got answers to my problem.

Any one has an idea?

Answer 1

HTTP/1.1 cache and the associated headers that control cache behavior are defined in the RFC 7234. Caching is an entirely optional feature of HTTP though.

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The Cache-Control header is used to specify directives for caches along the request/response chain. Such cache directives are unidirectional in that the presence of a directive in a request does not imply that the same directive is to be given in the response.

Cache directives are identified by a token, to be compared case-insensitively, and have an optional argument, that can use both token and quoted-string syntax. For the directives defined below that define arguments, recipients ought to accept both forms, even if one is documented to be preferred. For any directive not defined by this specification, a recipient MUST accept both forms.

Cache-Control   = 1#cache-directive

cache-directive = token [ "=" ( token / quoted-string ) ]

Find below the directives that can be used in the request for the Cache-Control header:

5.2.1.1. max-age

[...] The max-age request directive indicates that the client is unwilling to accept a response whose age is greater than the specified number of seconds. [...]

This directive uses the token form of the argument syntax: e.g., max-age=5 not max-age="5". [...]

5.2.1.2. max-stale

[...] The max-stale request directive indicates that the client is willing to accept a response that has exceeded its freshness lifetime. [...]

This directive uses the token form of the argument syntax: e.g., max-stale=10 not max-stale="10". [...]

5.2.1.3. min-fresh

[...] The min-fresh request directive indicates that the client is willing to accept a response whose freshness lifetime is no less than its current age plus the specified time in seconds. [...]

This directive uses the token form of the argument syntax: e.g., min-fresh=20 not min-fresh="20". [...]

5.2.1.4. no-cache

The no-cache request directive indicates that a cache MUST NOT use a stored response to satisfy the request without successful validation on the origin server.

5.2.1.5. no-store

The no-store request directive indicates that a cache MUST NOT store any part of either this request or any response to it. [...]

5.2.1.6. no-transform

The no-transform request directive indicates that an intermediary (whether or not it implements a cache) MUST NOT transform the payload [...].

5.2.1.7. only-if-cached

The only-if-cached request directive indicates that the client only wishes to obtain a stored response. [...]

In a similar way, RFC 7234 also defines the directives that can be used in the response for the Cache-Control header.