HTTP Cache Groups

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.¶

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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.¶

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.¶

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.¶

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.¶

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.¶

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.¶

Thanks to Stephen Ludin for his review and suggestions.¶