rfc9875.original   rfc9875.txt 
HTTP M. Nottingham Internet Engineering Task Force (IETF) M. Nottingham
Internet-Draft 16 May 2025 Request for Comments: 9875 Cloudflare
Intended status: Standards Track Category: Standards Track October 2025
Expires: 17 November 2025 ISSN: 2070-1721
HTTP Cache Groups HTTP Cache Groups
draft-ietf-httpbis-cache-groups-07
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.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-httpbis-cache-groups/.
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information can be found at https://httpwg.org/.
Source for this draft and an issue tracker can be found at
https://github.com/httpwg/http-extensions/labels/cache-groups.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 1.1. Notational Conventions
2. The Cache-Groups Response Header Field . . . . . . . . . . . 3 2. The Cache-Groups Response Header Field
2.1. Identifying Grouped Responses . . . . . . . . . . . . . . 4 2.1. Identifying Grouped Responses
2.2. Cache Behaviour . . . . . . . . . . . . . . . . . . . . . 4 2.2. Cache Behaviour
2.2.1. Invalidation . . . . . . . . . . . . . . . . . . . . 4 2.2.1. Invalidation
3. The Cache-Group-Invalidation Response Header Field . . . . . 4 3. The Cache-Group-Invalidation Response Header Field
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 4. IANA Considerations
4.1. HTTP Field Names . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6 6. References
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 6.1. Normative References
6.1. Normative References . . . . . . . . . . . . . . . . . . 6 6.2. Informative References
6.2. Informative References . . . . . . . . . . . . . . . . . 6 Acknowledgements
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 7 Author's Address
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction 1. Introduction
HTTP caching [HTTP-CACHING] operates at the granularity of a single HTTP caching [HTTP-CACHING] operates at the granularity of a single
resource; the freshness of one stored response does not affect that resource; the freshness of one stored response does not affect that
of others. This granularity can make caching more efficient -- for of others. This granularity can make caching more efficient -- for
example, when a page is composed of many assets that have different example, when a page is composed of many assets that have different
requirements for caching. requirements for caching.
However, there are also cases where the relationship between stored However, there are also cases where the relationship between stored
skipping to change at page 3, line 14 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: a 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.
These mechanisms operate within a single cache, across the stored These mechanisms operate within a single cache, across the stored
responses associated with a single origin server (see Section 2.1). responses associated with a single origin server (see Section 2.1).
They do not address the issues of synchronising state between They do not address the issues of synchronising state between
multiple caches (e.g., in a hierarchy or mesh), nor do they multiple caches (e.g., in a hierarchy or mesh), nor do they
facilitate association of stored responses from disparate origins. facilitate association of stored responses from disparate origins.
1.1. Notational Conventions 1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"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, 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.
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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 page 5, line 37 skipping to change at line 198
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.
Implementations MUST support at least 32 groups in a field value, Implementations MUST support at least 32 groups in a field value,
with up to at least 32 characters in each member. Note that generic with up to at least 32 characters in each member. Note that generic
limitations on HTTP field lengths may constrain the size of this limitations on HTTP field lengths may constrain the size of this
field value in practice. field value in practice.
4. IANA Considerations 4. IANA Considerations
IANA should perform the following tasks: IANA has added the following entries to the "Hypertext Transfer
Protocol (HTTP) Field Name Registry":
4.1. HTTP Field Names
Enter the following into the Hypertext Transfer Protocol (HTTP) Field
Name Registry:
* Field Name: Cache-Groups
* Status: permanent
* Reference: RFC nnnn
* Comments:
* Field Name: Cache-Group-Invalidation
* Status: permanent
* Reference: RFC nnnn Field Name: Cache-Groups
Status: permanent
Reference: RFC 9875
* Comments: Field Name: Cache-Group-Invalidation
Status: permanent
Reference: RFC 9875
5. Security Considerations 5. Security Considerations
This mechanism allows resources that share an origin to invalidate This mechanism allows resources that share an origin to invalidate
each other. Because of this, origins that represent multiple parties each other. Because of this, origins that represent multiple parties
(sometimes referred to as "shared hosting") might allow one party to (sometimes referred to as "shared hosting") might allow one party to
group its resources with those of others, or to send signals which group its resources with those of others or to send signals that have
have side effects upon them. side effects upon them.
Shared hosts that wish to mitigate these risks can control access to Shared hosts that wish to mitigate these risks can control access to
the header fields defined in this specification. the header fields defined in this specification.
6. References 6. References
6.1. Normative References 6.1. Normative References
[HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110, Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022, DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/rfc/rfc9110>. <https://www.rfc-editor.org/info/rfc9110>.
[HTTP-CACHING] [HTTP-CACHING]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Caching", STD 98, RFC 9111, Ed., "HTTP Caching", STD 98, RFC 9111,
DOI 10.17487/RFC9111, June 2022, DOI 10.17487/RFC9111, June 2022,
<https://www.rfc-editor.org/rfc/rfc9111>. <https://www.rfc-editor.org/info/rfc9111>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[STRUCTURED-FIELDS] [STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024, HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024,
<https://www.rfc-editor.org/rfc/rfc9651>. <https://www.rfc-editor.org/info/rfc9651>.
6.2. Informative References 6.2. Informative References
[TARGETED] Ludin, S., Nottingham, M., and Y. Wu, "Targeted HTTP Cache [TARGETED] Ludin, S., Nottingham, M., and Y. Wu, "Targeted HTTP Cache
Control", RFC 9213, DOI 10.17487/RFC9213, June 2022, Control", RFC 9213, DOI 10.17487/RFC9213, June 2022,
<https://www.rfc-editor.org/rfc/rfc9213>. <https://www.rfc-editor.org/info/rfc9213>.
Appendix A. 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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