HTTP Cache Groups
RFC 9875



Internet Engineering Task Force (IETF) M. Nottingham
Request for Comments: 9875 Cloudflare
Category: Standards Track October 2025
ISSN: 2070-1721

 HTTP Cache Groups

Abstract

 This specification introduces a means of describing the relationships
 between stored responses in HTTP caches, grouping them by associating
 a stored response with one or more strings.

Status of This Memo

 This is an Internet Standards Track document.

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

 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc9875.

Copyright Notice

 Copyright (c) 2025 IETF Trust and the persons identified as the
 document authors. All rights reserved.

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Table of Contents

 1. Introduction
 1.1. Notational Conventions
 2. The Cache-Groups Response Header Field
 2.1. Identifying Grouped Responses
 2.2. Cache Behaviour
 2.2.1. Invalidation
 3. The Cache-Group-Invalidation Response Header Field
 4. IANA Considerations
 5. Security Considerations
 6. References
 6.1. Normative References
 6.2. Informative References
 Acknowledgements
 Author's Address

1. Introduction

 HTTP caching [HTTP-CACHING] operates at the granularity of a single
 resource; the freshness of one stored response does not affect that
 of others. This granularity can make caching more efficient -- for
 example, when a page is composed of many assets that have different
 requirements for caching.

 However, there are also cases where the relationship between stored
 responses could be used to improve cache efficiency.

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

 In addition to sharing invalidation events, the relationships
 indicated by grouping can also be used by caches to optimise their
 operation (e.g., to inform the operation of cache eviction
 algorithms).

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

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

 These mechanisms operate within a single cache, across the stored
 responses associated with a single origin server (see Section 2.1).
 They do not address the issues of synchronising state between
 multiple caches (e.g., in a hierarchy or mesh), nor do they
 facilitate association of stored responses from disparate origins.

1.1. Notational Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

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

2. The Cache-Groups Response Header Field

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

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

 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.

2.1. Identifying Grouped Responses

 Two responses stored in the same cache are considered to belong to
 the same group when all of the following conditions are met:

 1. They both contain a Cache-Groups response header field that
 contains the same String (in any position in the List), when
 compared character-by-character (case sensitive).

 2. They both share the same URI origin (per Section 4.3.1 of
 [HTTP]).

2.2. Cache Behaviour

2.2.1. Invalidation

 A cache that invalidates a stored response MAY invalidate any stored
 responses that share groups (per Section 2.1) with that response.
 Note that further grouped invalidations are not triggered by a
 grouped invalidation; i.e., this mechanism does not cascade.

 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 invalidate such
 responses.

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.

4. IANA Considerations

 IANA has added the following entries to the "Hypertext Transfer
 Protocol (HTTP) Field Name Registry":

 Field Name: Cache-Groups
 Status: permanent
 Reference: RFC 9875

 Field Name: Cache-Group-Invalidation
 Status: permanent
 Reference: RFC 9875

5. Security Considerations

 This mechanism allows resources that share an origin to invalidate
 each other. Because of this, origins that represent multiple parties
 (sometimes referred to as "shared hosting") might allow one party to
 group its resources with those of others or to send signals that have
 side effects upon them.

 Shared hosts that wish to mitigate these risks can control access to
 the header fields defined in this specification.

6. References

6.1. Normative References

 [HTTP] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
 Ed., "HTTP Semantics", STD 97, RFC 9110,
 DOI 10.17487/RFC9110, June 2022,
 <https://www.rfc-editor.org/info/rfc9110>.

 [HTTP-CACHING]
 Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
 Ed., "HTTP Caching", STD 98, RFC 9111,
 DOI 10.17487/RFC9111, June 2022,
 <https://www.rfc-editor.org/info/rfc9111>.

 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
 Requirement Levels", BCP 14, RFC 2119,
 DOI 10.17487/RFC2119, March 1997,
 <https://www.rfc-editor.org/info/rfc2119>.

 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
 May 2017, <https://www.rfc-editor.org/info/rfc8174>.

 [STRUCTURED-FIELDS]
 Nottingham, M. and P. Kamp, "Structured Field Values for
 HTTP", RFC 9651, DOI 10.17487/RFC9651, September 2024,
 <https://www.rfc-editor.org/info/rfc9651>.

6.2. Informative References

 [TARGETED] Ludin, S., Nottingham, M., and Y. Wu, "Targeted HTTP Cache
 Control", RFC 9213, DOI 10.17487/RFC9213, June 2022,
 <https://www.rfc-editor.org/info/rfc9213>.

Acknowledgements

 Thanks to Stephen Ludin for his review and suggestions.

Author's Address

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