Network Working Group M. Nottingham
Internet-Draft M. Kelly
Intended status: Informational May 27, 2011
Expires: November 28, 2011
Linked Cache Invalidation
draft-nottingham-linked-cache-inv-00
Abstract
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
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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.
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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
applied.
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
operation.
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
minutes.
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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:
delta-seconds.
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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