Internet-Draft HTTP Cache Groups April 2024
Nottingham Expires 13 October 2024 [Page]
Workgroup:
Network Working Group
Internet-Draft:
draft-ietf-httpbis-cache-groups-latest
Published:
Intended Status:
Standards Track
Expires:
Author:
M. Nottingham

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 opaque 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/.

Discussion of this document takes place on the HTTP Working Group mailing list (mailto:ietf-http-wg@w3.org), which is archived at https://lists.w3.org/Archives/Public/ietf-http-wg/. Working Group 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.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 13 October 2024.

Table of Contents

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's common for a set of closely-related resources to be deployed on a site, such as is the case for many JavaScript libraries and frameworks. These resources are typically deployed with long freshness lifetimes for caching. When that period passes, the cache will need to revalidate each stored response in a short period of time. Grouping these resources can be used to allow a cache to consider them all as being revalidated when any single response in the group is revalidated, removing the need to revalidate all of them individually and avoiding the associated overhead.

Likewise, when some resources change, it implies that other resources may have also changed. 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 revalidation and invalidation events, the relationships indicated by grouping can also be used by caches to optimise their operation; for example, it could be used to inform the operation of cache eviction algorithms.

Section 2 introduces a means of describing the relationships between a set of stored responses in HTTP caches by associating them with one or more opaque strings. It also describes how caches can use that information to apply revalidation and invalidation events to members of a group.

Section 3 introduces one new source of such events: a HTTP response header 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. They do not address this 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, Parameter.

2. The Cache-Groups Response Header Field

The Cache-Groups HTTP Response Header is a List of Strings [STRUCTURED-FIELDS]. Each member of the list is an opaque value that identifies a group that the response belongs to.

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

This specification defines one Parameter for Cache-Groups, "revalidate", that indicates that the resources associated with that group share revalidation; see Section 2.2.1.

The ordering of members of Cache-Groups is not significant.

2.1. Identifying Grouped Responses

Two responses stored in the same cache are considered to have 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.

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

  3. If being considered for revalidation (Section 2.2.1), they both have the "revalidate" Parameter.

2.2. Cache Behaviour

2.2.1. Revalidation

A cache that successfully revalidates a stored response MAY consider any stored responses that share a group (per Section 2.1) as also being revalidated at the same time.

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 revalidate such responses.

2.2.2. Invalidation

A cache that invalidates a stored response MAY invalidate any stored responses that share groups (per Section 2.1) with that response.

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 [STRUCTURED-FIELDS]. Each member of the list is an opaque value that identifies a group that the response invalidates, per Section 2.2.2.

For example, a POST request that has side effects on two cache groups 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", "kylie-minogue"

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.

4. IANA Considerations

IANA should perform the following tasks:

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

  • Comments:

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 which have side effects upon them -- either invalidating stored responses or extending their lifetime.

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, , <https://www.rfc-editor.org/rfc/rfc9110>.
[HTTP-CACHING]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Caching", STD 98, RFC 9111, DOI 10.17487/RFC9111, , <https://www.rfc-editor.org/rfc/rfc9111>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
[STRUCTURED-FIELDS]
Nottingham, M. and P. Kamp, "Structured Field Values for HTTP", Work in Progress, Internet-Draft, draft-ietf-httpbis-sfbis-05, , <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-sfbis-05>.

6.2. Informative References

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

Appendix A. Acknowledgements

Thanks to Stephen Ludin for his review and suggestions.

Author's Address

Mark Nottingham
Prahran
Australia