Diff: rfc9875v1.txt - rfc9875.txt

	rfc9875v1.txt	rfc9875.txt

	Internet Engineering Task Force (IETF) M. Nottingham	Internet Engineering Task Force (IETF) M. Nottingham

	Request for Comments: 9875 September 2025	Request for Comments: 9875 Cloudflare
	Category: Standards Track	Category: Standards Track October 2025
	ISSN: 2070-1721	ISSN: 2070-1721

	HTTP Cache Groups	HTTP Cache Groups

	Abstract	Abstract

	This specification introduces a means of describing the relationships	This specification introduces a means of describing the relationships

	between stored responses in HTTP caches, "grouping" them by	between stored responses in HTTP caches, grouping them by associating
	associating a stored response with one or more strings.	a stored response with one or more strings.

	Status of This Memo	Status of This Memo

	This is an Internet Standards Track document.	This is an Internet Standards Track document.

	This document is a product of the Internet Engineering Task Force	This document is a product of the Internet Engineering Task Force
	(IETF). It represents the consensus of the IETF community. It has	(IETF). It represents the consensus of the IETF community. It has
	received public review and has been approved for publication by the	received public review and has been approved for publication by the
	Internet Engineering Steering Group (IESG). Further information on	Internet Engineering Steering Group (IESG). Further information on
	Internet Standards is available in Section 2 of RFC 7841.	Internet Standards is available in Section 2 of RFC 7841.

	skipping to change at line 81 ¶	skipping to change at line 81 ¶

	For example, it is often necessary to invalidate a set of related	For example, it is often necessary to invalidate a set of related
	resources. This might be because a state-changing request has side	resources. This might be because a state-changing request has side
	effects on other resources, or it might be purely for administrative	effects on other resources, or it might be purely for administrative
	convenience (e.g., "invalidate this part of the site"). Grouping	convenience (e.g., "invalidate this part of the site"). Grouping
	responses together provides a dedicated way to express these	responses together provides a dedicated way to express these
	relationships, instead of relying on things like URL structure.	relationships, instead of relying on things like URL structure.

	In addition to sharing invalidation events, the relationships	In addition to sharing invalidation events, the relationships
	indicated by grouping can also be used by caches to optimise their	indicated by grouping can also be used by caches to optimise their

	operation; for example, it could be used to inform the operation of	operation (e.g., to inform the operation of cache eviction
	cache eviction algorithms.	algorithms).

	Section 2 introduces a means of describing the relationships between	Section 2 introduces a means of describing the relationships between
	stored responses in HTTP caches, by associating those responses with	stored responses in HTTP caches, by associating those responses with
	one or more groups that reflect those relationships. It also	one or more groups that reflect those relationships. It also
	describes how caches can use that information to apply invalidation	describes how caches can use that information to apply invalidation
	events to members of a group.	events to members of a group.

	Section 3 introduces one new source of such events: an HTTP response	Section 3 introduces one new source of such events: an HTTP response
	header field that allows a state-changing response to trigger a group	header field that allows a state-changing response to trigger a group
	invalidation.	invalidation.

	skipping to change at line 113 ¶	skipping to change at line 113 ¶
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in	"OPTIONAL" in this document are to be interpreted as described in
	BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all	BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
	capitals, as shown here.	capitals, as shown here.

	This specification uses the following terminology from	This specification uses the following terminology from
	[STRUCTURED-FIELDS]: List, String, and Parameter.	[STRUCTURED-FIELDS]: List, String, and Parameter.

	2. The Cache-Groups Response Header Field	2. The Cache-Groups Response Header Field


	The Cache-Groups HTTP Response Header is a List of Strings (Sections	The Cache-Groups response header field is a List of Strings (Sections
	3.1 and 3.3.1 of [STRUCTURED-FIELDS]). Each member of the list is a	3.1 and 3.3.3 of [STRUCTURED-FIELDS]). Each member of the List is a
	value that identifies a group that the response belongs to. These	value that identifies a group that the response belongs to. These

	strings are opaque -- while they might have some meaning to the	Strings are opaque -- while they might have some meaning to the
	server that creates them, the cache does not have any insight into	server that creates them, the cache does not have any insight into
	their structure or content (beyond uniquely identifying a group).	their structure or content (beyond uniquely identifying a group).

	HTTP/1.1 200 OK	HTTP/1.1 200 OK
	Content-Type: application/javascript	Content-Type: application/javascript
	Cache-Control: max-age=3600	Cache-Control: max-age=3600
	Cache-Groups: "scripts"	Cache-Groups: "scripts"

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

	skipping to change at line 152 ¶	skipping to change at line 152 ¶
	2. They both share the same URI origin (per Section 4.3.1 of	2. They both share the same URI origin (per Section 4.3.1 of
	[HTTP]).	[HTTP]).

	2.2. Cache Behaviour	2.2. Cache Behaviour

	2.2.1. Invalidation	2.2.1. Invalidation

	A cache that invalidates a stored response MAY invalidate any stored	A cache that invalidates a stored response MAY invalidate any stored
	responses that share groups (per Section 2.1) with that response.	responses that share groups (per Section 2.1) with that response.
	Note that further grouped invalidations are not triggered by a	Note that further grouped invalidations are not triggered by a

	grouped invalidation; i.e., this mechanism does not "cascade".	grouped invalidation; i.e., this mechanism does not cascade.

	Cache extensions can explicitly strengthen the requirement above.	Cache extensions can explicitly strengthen the requirement above.
	For example, a targeted cache control header field [TARGETED] might	For example, a targeted cache control header field [TARGETED] might
	specify that caches processing it are required to invalidate such	specify that caches processing it are required to invalidate such
	responses.	responses.

	3. The Cache-Group-Invalidation Response Header Field	3. The Cache-Group-Invalidation Response Header Field

	The Cache-Group-Invalidation response header field is a List of	The Cache-Group-Invalidation response header field is a List of

	Strings (Sections 3.1 and 3.3.1 of [STRUCTURED-FIELDS]). Each member	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	of the List is a value that identifies a group that the response
	invalidates, per Section 2.2.1.	invalidates, per Section 2.2.1.

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

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

	skipping to change at line 262 ¶	skipping to change at line 262 ¶
	Control", RFC 9213, DOI 10.17487/RFC9213, June 2022,	Control", RFC 9213, DOI 10.17487/RFC9213, June 2022,
	<https://www.rfc-editor.org/info/rfc9213>.	<https://www.rfc-editor.org/info/rfc9213>.

	Acknowledgements	Acknowledgements

	Thanks to Stephen Ludin for his review and suggestions.	Thanks to Stephen Ludin for his review and suggestions.

	Author's Address	Author's Address

	Mark Nottingham	Mark Nottingham

	Prahran	Cloudflare
		Melbourne
	Australia	Australia
	Email: mnot@mnot.net	Email: mnot@mnot.net
	URI: https://www.mnot.net/	URI: https://www.mnot.net/

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