HTTP Cache Groups

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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 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; 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 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.¶

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: "scripts"

The ordering of members is not significant. Unrecognised Parameters MUST be ignored.¶

Implementations MUST support at least 128 groups in a field value, with up to at least 128 characters in each member. Note that generic limitations on HTTP field lengths may constrain the size of this field value in practice.¶

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 2.2.1.¶

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.¶

The ordering of members is not significant. Unrecognised Parameters MUST be ignored.¶

Implementations MUST support at least 128 groups in a field value, with up to at least 128 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 should perform the following tasks:¶

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.¶

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

Appendix A. Acknowledgements

Thanks to Stephen Ludin for his review and suggestions.¶

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