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1. Introduction

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Original RFC Text​

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.
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