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Network Working Group                                      M. Nottingham
Internet-Draft                                                  M. Kelly
Intended status: Informational                              May 27, 2011
Expires: November 28, 2011

                       Linked Cache Invalidation


   This memo defines Web link types that invalidate HTTP caches, along
   with an HTTP cache-control extension that allows caches that
   understand those link types to use responses containing them.
   Together, these mechanisms offer a way to avoid use of a response
   that has become stale due to another request that changes server-side
   state.  Collectively, this is referred to as Linked Cache
   Invalidation (LCI).

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 http://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 November 28, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . . . 5
   3.  The 'invalidates' link relation type  . . . . . . . . . . . . . 5
   4.  The 'inv-by' link relation type . . . . . . . . . . . . . . . . 6
   5.  The 'inv-maxage' response cache-control extension . . . . . . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 8
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   In normal operation, a HTTP [RFC2616] cache will invalidate a stored
   response if a state-changing request (e.g., POST, PUT or DELETE) is
   made for that URI.  HTTP also provides for such a state-changing
   request to invalidate related resources (using the Location and
   Content-Location headers in the response), but this is of limited
   utility, because those headers have defined semantics, and can only
   occur once each.

   Because of this, it is not practical to make a response that depends
   on the state of another resource cacheable.  For example, an update
   to a blog entry might change several different resources, such as the
   user's summary page, the blog's "front" page, the blog's Atom feed,
   and of course the blog entry itself.  If any of these resources is
   made cacheable, it will not reflect those changes, causing confusion
   if the user tries to verify that their changes have been correctly

   This memo introduces new Web link relation types [RFC5988] that allow
   more fine-grained relationships between resources to be defined, so
   that caches can invalidate all related resources when the state of
   one changes.  It also introduces a cache-control response extension,
   so that responses using the relations can be made cached by
   implementations that understand the relations.

1.1.  Example

   Taking the blog use case described above, imagine that we have the
   following related resources:

   o  http://example.com/blog/2011/05/04/hi {the blog entry}
   o  http://example.com/blog/2011/05/04/hi/comments {full comments for
      the entry}
   o  http://example.com/blog/ {the blog "home"}
   o  http://example.com/users/bob/ {the user page, listing his entries}

   When someone comments on Bob's blog entry, they might send a request
   like this:

   POST /cgi-bin/blog.cgi HTTP/1.1
   Host: example.com
   Content-Type: application/x-www-form-urlencoded
   Content-Length: 7890


   This request (if successful) should have the effect of invalidating

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   the related resources listed above.

   If the comment is successful, it's typical to redirect the client
   back to the original blog page, with a response like this:

   HTTP/1.1 302 Moved Temporarily
   Location: http://example.com/blog/2011/05/04/hi
   Content-Length: 0

   Which would invalidate the blog entry URI, as per HTTP's normal

   To invalidate the full comments page for the entry, the relationship
   can be described in that page's response headers:

   HTTP/1.1 200 OK
   Content-Type: text/html
   Content-Length: 5555
   Link: </blog/2011/05/04/hi>; rel="inv-by"
   Cache-Control: no-cache, inv-maxage=600


   This declares that whenever the entry page (the target of the link
   header) changes, this response (the full comments page) changes as
   well; it's invalidated by the link target.

   Note that the full comments page also carries a Cache-Control header
   that instructs "normal" caches not to reuse this response, but allows
   those caches that are aware of LCI to consider it fresh for ten

   To invalidate the blog home page and user page, it's impractical to
   list all of the resources that might change if a new entry is posted;
   not only are there many of them, but their URLs might not be known
   when the pages are cached.  To address this, the POST response itself
   can nominate resources to invalidate, using the 'invalidates'
   relation, making that response:

   HTTP/1.1 302 Moved Temporarily
   Location: http://example.com/blog/2011/05/04/hi
   Link: <http://example.com/blog/>; rel="invalidates",
         <http://example.com/users/bob/>; rel="invalidates"
   Content-Length: 0

   Depending on how important it is to see updates on the home page and

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   user page, those responses can either allow caching regardless of
   support for LCI, like this:

   Cache-Control: max-age=300

   ... or they can only allow caching by LCI-aware caches, like this:

   Cache-Control: no-cache, inv-maxage=300

   Together, these techniques can be used to invalidate a variety of
   related responses.

   It is important to note that the invalidations are only effective in
   the caches that the client's request stream travels through.
   Typically, this means that the client making the changes (e.g., the
   blog update above) will see the effects immediately, while other
   users whose requests travel through different caches will only see
   the changes once the content becomes stale (if it is cached).

   This makes Linked Cache Invalidation useful in a number of use cases,
   but not all; when it's important that changes be propagated quickly,
   the freshness lifetime of cached responses can be reduced, but there
   will still be lag.

   When multiple caches are close together, the HyperText Caching
   Protocol (HTCP) [RFC2756] can be used to propagate invalidation
   events between caches, reducing (but not eliminating) these effects.

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   This document uses the Augmented Backus-Naur Form (ABNF) notation of
   [RFC2616], and explicitly includes the following rules from it:

3.  The 'invalidates' link relation type

   The 'invalidates' link relation type allows a response that is an
   indication of a state change on the server to indicate one or more
   associated URIs whose states have also changed.

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   o  Relation name: invalidates
   o  Description: Indicates that when the link context changes, the
      link target also has changed.
   o  Reference: [this document]
   o  Notes:

4.  The 'inv-by' link relation type

   The 'inv-by' link relation type allows a response to nominate one or
   more other resources that affect the state of the resource it's
   associated with.  That is, when one of the nominated resources
   changes, it also changes the state of this response's resource.

   o  Relation name: inv-by
   o  Description: Indicates that when the link target changes, the
      link's context has also changed.
   o  Reference: [this document]
   o  Notes:

5.  The 'inv-maxage' response cache-control extension

   When present, the 'inv-maxage' cache-control extension indicates the
   number of seconds that caches who implement Linked Cache invalidation
   can consider responses fresh for.

   "inv-maxage" "=" delta-seconds

   HTTP caches MAY, if they fully implement this specification,
   disregard the HTTP response cache-control directives 'no-cache',
   'max-age' and 's-maxage' and use the value of inv-maxage as a
   replacement for max-age.

   HTTP caches using inv-maxage MUST invalidate all stored responses
   whose request-URIs (after normalisation) are indicated by the
   'invalidates' link relation type contained in a successful response
   to a state-changing request, provided that they are allowed.

   HTTP caches using inv-maxage MUST invalidate all stored responses
   containing the 'inv-by' relation that indicates the current request-
   URI (after normalisation) upon receipt of a successful response to a
   state-changing request.

   Here, a response is considered to "contain" a link relation if it is
   carried in the Link HTTP header [RFC5988].  I.e., it is not necessary
   to look at the response body.

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   "Invalidate" means that the cache will either remove all stored
   responses related to the effective request URI, or will mark these as
   "invalid" and in need of a mandatory validation before they can be
   returned in response to a subsequent request.

   A "successful" response is one with a 2xx or redirecting 3xx (e.g.,
   301, 302, 303, 307) status code.

   A "state-changing" request is one with an unsafe method (e.g., POST,
   PUT, DELETE, PATCH), or one that is not known to be safe.

   In this context, "normalisation" means, in the case of a relative
   request-URI, that it is absolutised using the value of the Host
   request header and the appropriate protocol scheme.

   Finally, an invalidation based upon "invalidates" is "allowed" if the
   host part of the request-URI (if absolute) or Host request header (if
   the request-URI is relative) matches the host part of the target URI.
   This prevents some types of denial-of-service attacks.

   Implementations SHOULD effect invalidations when they become aware of
   changes through other means; e.g., HTCP [RFC2756] CLR messages, or
   upon invalidations caused by other links (i.e., chained "cascades" of
   linked invalidations).

6.  Security Considerations

   Linked Cache Invalidation does not guarantee that invalidations will
   be effected; e.g., they can be lost due to network issues or cache
   downtime.  Furthermore, it does not guarantee that all caches that
   understand LCI will be made aware of invalidations that happen,
   because of how they originate.

   Therefore, care should be taken that LCI invalidations are not relied
   upon (e.g., to purge sensitive content).

   Furthermore, while some care is taken to avoid denial-of-service
   attacks through invalidation, cache efficiency may still be impaired
   under certain circumstances (e.g., arranging for one request to
   invalidate a large number of responses), leading to a reduction in
   service quality.

7.  IANA Considerations

   This document registers two entries in the Link Relation Type
   Registry; see Section 3 and Section 4.

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8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

8.2.  Informative References

   [RFC2756]  Vixie, P. and D. Wessels, "Hyper Text Caching Protocol
              (HTCP/0.0)", RFC 2756, January 2000.

Appendix A.  Acknowledgements

   Thanks to Michael Hausenblas for his input.

   The authors take all responsibility for errors and omissions.

Authors' Addresses

   Mark Nottingham

   Email: mnot@mnot.net
   URI:   http://www.mnot.net/

   Mike Kelly

   Email: mike@stateless.co
   URI:   http://stateless.co/

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