3. The Cache-Group-Invalidation Response Header Field

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Original RFC Text

3.  The Cache-Group-Invalidation Response Header Field

   The Cache-Group-Invalidation response header field is a List of
   Strings (Sections 3.1 and 3.3.3 of [STRUCTURED-FIELDS]).  Each member
   of the List is a value that identifies a group that the response
   invalidates, per Section 2.2.1.

   For example, following a POST request that has side effects on two
   cache groups, the corresponding response could indicate that stored
   responses associated with either or both of those groups should be
   invalidated with:

   HTTP/1.1 200 OK
   Content-Type: text/html
   Cache-Group-Invalidation: "eurovision-results", "australia"

   The Cache-Group-Invalidation header field MUST be ignored on
   responses to requests that have a safe method (e.g., GET; see
   Section 9.2.1 of [HTTP]).

   A cache that receives a Cache-Group-Invalidation header field on a
   response to an unsafe request MAY invalidate any stored responses
   that share groups (per Section 2.1) with any of the listed groups.

   Cache extensions can explicitly strengthen the requirement above.
   For example, a targeted cache control header field [TARGETED] might
   specify that caches processing it are required to respect the Cache-
   Group-Invalidation signal.

   The ordering of members is not significant.  Unrecognised Parameters
   are to be ignored.

   Implementations MUST support at least 32 groups in a field value,
   with up to at least 32 characters in each member.  Note that generic
   limitations on HTTP field lengths may constrain the size of this
   field value in practice.