Internet DRAFT - draft-nottingham-http-cache-channels


Network Working Group                                      M. Nottingham
Internet-Draft                                               Yahoo! Inc.
Intended status: Informational                          October 12, 2007
Expires: April 14, 2008

                          HTTP Cache Channels

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This specification defines "cache channels" to propagate out-of-band
   events from HTTP origin servers (or their delegates) to subscribing
   caches.  It also defines an event payload that gives finer-grained
   control over cache freshness.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  Cache Channels . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Channel Subscription . . . . . . . . . . . . . . . . . . .  5
     3.2.  Event Application  . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Atom Cache Channels  . . . . . . . . . . . . . . . . . . .  6
       3.3.1.  The 'cc:precision' Feed Extension  . . . . . . . . . .  7
       3.3.2.  The 'cc:lifetime' Feed Extension . . . . . . . . . . .  7
       3.3.3.  Example  . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Manging Freshness with Cache Channels  . . . . . . . . . . . .  8
     4.1.  The 'channel-maxage' Response Cache-Control Extension  . .  9
     4.2.  The 'stale' Cache Channel Event  . . . . . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  Normative References . . . . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 11
   Appendix B.  Operational Considerations  . . . . . . . . . . . . . 11
   Appendix C.  Implementation Notes  . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 13

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1.  Introduction

   This specification defines "cache channels" that propagate out-of-
   band events from HTTP [1] origin servers (or their delegates) to
   subscribing caches.  It also defines an event payload that gives
   finer-grained control over cache freshness.

   Typically, a cache will discover channels of interest by examining
   Cache-Control response headers for the "channel" extension; when
   present, it indicates that the response is associated with that
   channel.  Upon subscription, caches will receive all events in that

   To allow use of a variety of underlying protocols, the process of
   subscription and the means of propagating events in the channel are
   specific to the transport in use.  This specification does define one
   such channel transport, using the Atom Syndication Format [2].

   Likewise, channels may be used to convey a variety of events from
   origin servers to caches.  This specification defines one such
   payload, the 'stale' event, that affords finer-grained control over
   freshness than available in HTTP alone.

   Together, cache channels and the stale event enable an origin server
   to maintain control over the content of a set of caches while
   increasing their efficiency.  For example, "reverse proxies" are
   often used to accelerate HTTP servers by caching their content; cache
   channels and stale events can be used to more closely control their

   This use of cache channels is similar to an invalidation protocol,
   except that the protocol described here operates by extending cached
   responses' freshness lifetime, rather than invalidating them.  This
   preserves the semantics of the HTTP caching model and assures that
   the failure modes are safe.

   Additionally, the 'group' functionality of cache channels enables
   stale events to apply to several cached responses, thereby offering
   control over freshness of responses whose request-URIs may not be
   known.  For example, a HTTP search interface may have given several
   responses containing the same information; if they are grouped
   together, it is possible to force them to become stale without
   knowing their request-URIs.

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   document are to be interpreted as described in BCP 14 [3], as scoped
   to those conformance targets.

   This specification uses the augmented Backus-Naur Form of RFC2616
   [1], and includes the delta-seconds rule from that specification, and
   the absolute-URI rule from RFC3986 [4].

   Elements defined by this specification use the XML namespace [5] URI
   "".  In this specification,
   that URI is assumed to be bound to the prefix "cc".

3.  Cache Channels

   A cache channel is an out-of-band path from an origin server (or its
   delegate) to one or more interested caches.  It is identified by a
   URI [4].

   A channel contains events, each of which may be associated with one
   or more URIs.  The payload of each event is applied to its associated
   URIs when it is received.

   Typically, a cache channel will convey events applicable to a variety
   of URIs in an administrative domain; for example, it might carry all
   events that apply to a single origin server, or a group of origin

   From the perspective of a cache, a channel has several interesting

   o  unsubscribed: The channel is not being monitored.
   o  connected: The channel is being monitored, and events are able to
      be propagated.
   o  disconnected: The channel is being monitored, but events are not
      able to be propagated.

   Channels MUST make the events that they contain available for an
   advertised amount of time, known as the lifetime of the channel.
   This allows clients that have been disconnected to re-synchronise
   themselves with the contents of the channel upon becoming re-

   Channels SHOULD also advertise the desired maximum propagation delay
   for events, known as their precision.

   Two general processes facilitate the operation of cache channels;
   subscription and event application.

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3.1.  Channel Subscription

   Channels are advertised using the "channel" HTTP response cache-
   control extension.

   channel-extension = "channel" "=" <"> channel-URI <">
         channel-URI = absolute-URI

   A recipient of this cache-control extension can subscribe to the
   channel-URI when interested in receiving events associated with the

   The specific mechanism for subscription is determined by the channel-
   URI's scheme and other factors (e.g., the media type of the
   representation obtained by dereferencing that URI).

   If a subsequent response has a different channel-URI, or no channel-
   URI, and there are no other responses associated with the same
   channel-URI, a subscriber MAY unsubscribe from the channel.

   A response MUST NOT have more than one channel-extension.

   For example,

     Cache-Control: max-age=300, channel=""

3.2.  Event Application

   An event carries one or more event-URIs, which are used to determine
   what cached responses the event applies to.  This includes responses
   whose request-URI matches an event-URI, and those with a group-URI
   matching an event-URI.

   Responses can be associated with a group-URI using the "group"
   response Cache-Control extension;

    group-extension = "group" "=" <"> group-URI <">
          group-URI = absolute-URI

   A response MAY have any number of group-extensions.

   For example, a response carrying the following Cache-Control header;

    Cache-Control: max=age=600, channel="",

   will not only have those events which match the response's request-
   URI applied, but also events who match the URI

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   This mechanism allows application of events to arbitrary sets of
   responses using a synthetic identifier.

   For example, if "", ""
   and "" are all associated with the group
   identified by "urn:uuid:30A909D9-BC7A-4257-BE09-6F781AD6471F", an
   event with that group-URI will be applied to all three.

   By default, an event will apply to a matching response regardless of
   the headers used to select it (as indicated by the Vary response
   header).  However, a particular event type can be specified to
   override this behaviour.

   Note that an event MUST NOT be applied to responses whose channel-
   extension does not indicate that the response is associated with the
   channel that the event occurred in.

3.3.  Atom Cache Channels

   The Atom Syndication Format [2] -- when used with channel-specific
   extensions in a specific pattern over HTTP -- is one suitable cache
   channel transport.

   A channel is subscribed to by commencing regular polling of the
   channel-URI.  Polling MUST be attempted at least as often as the
   channel's precision, as indicted by the cc:precision feed extension.

   The feed's 'current' link relation value MUST contain the channel-
   URI, which MUST be a HTTP URI.

   The channel is considered disconnected when the last successful poll
   occurred more than channel-precision seconds ago, or a successful
   poll has not yet occurred.  A successful poll is defined as one that
   results in a fresh (according to the HTTP caching model) copy of a
   well-formed, valid feed document with a 'self' link relation whose
   value is character-for-character identical to the channel-URI.

   The feed SHOULD be archived [6] to allow the full state of the
   channel to be reconstructed, while minimising the amount of bandwidth
   that polling the channel consumes.  If a feed is archived, the
   channel is only considered connected when the entire contents of the
   logical feed.

   The cc:lifetime feed extension indicates the minimum span of time
   that events are available in the logical feed for.

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   Entries in the feed represent events, in reverse-chronological order.
   The atom:link element(s) with the "alternate" relation determines the
   URI(s) that an event is applicable to.

3.3.1.  The 'cc:precision' Feed Extension

   An Atom cache channel's precision is indicated by the cc:precision
   element.  This is an feed-level extension element whose content
   indicates the precision in seconds.

   For example;


   indicates that the precision of the channel it occurs in is one

3.3.2.  The 'cc:lifetime' Feed Extension

   An Atom cache channel's lifetime is indicated by the cc:lifetime
   element.  This is an feed-level extension element whose content
   indicates the lifetime in seconds.

   For example;


   indicates that the lifetime of the channel it occurs in is 30 days.

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3.3.3.  Example

  <feed xmlns=""
    <title>Invalidations for</title>
    <link rel="self"
    <link rel="prev-archive"
      <link href="urn:uuid:50D3565C-97A8-40E1-A5C8-CFA070166FEF"/>
      <link href="" type="image/gif"/>
      <link href="" type="image/png"/>

   This feed document contains two events, the most recent applying to
   the URI "urn:uuid:50D3565C-97A8-40E1-A5C8-CFA070166FEF" and the other
   to "" and
   "".  The previous archive document
   is located at "", to
   allow the client to reconstruct missed events if necessary.  The
   channel it represents has a precision of 60 seconds (thereby telling
   clients that they need to poll at least that often), and a lifetime
   of 30 days.

4.  Manging Freshness with Cache Channels

   One use of cache channels is the management of cached responses'

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   freshness lifetime.  This is achieved through use of the 'channel-
   maxage' response Cache-Control extension, which allows subscribed
   caches to extend the freshness of a response until an applicable
   'stale' cache channel event is received.

4.1.  The 'channel-maxage' Response Cache-Control Extension

   The channel-maxage response cache-control extension allows controlled
   extension of the freshness lifetime of a cached response.

     channel-maxage = "channel-maxage" [ "=" delta-seconds ]

   A response containing this extension MAY be considered fresh when all
   of the following conditions are true;

   o  The cache is connected to the channel indicated by the channel-
   o  The channel does not contain a 'stale' event applicable to the
      cached response.
   o  The response's current_age (as per HTTP [1], Section 13.2.3) is no
      more than delta-seconds, if specified.

   For example a response with this Cache-Control header;

     Cache-Control: channel="",
       channel-maxage=86400, max-age=30

   will be considered fresh for 30 seconds by a cache that is not
   subscribed to the channel "",
   but can be considered fresh for up to a day by one that is, as long
   as the channel is connected and does not contain an applicable
   'stale' event.

   Or, if the response contains;

     Cache-Control: channel="",
       channel-maxage, max-age=30

   then it will be considered fresh for up to the channels' lifetime,
   provided that the channel is connected and does not contain a 'stale'

4.2.  The 'stale' Cache Channel Event

   Cache channels can indicate that all cached responses associated with
   a URI are to be considered stale with the 'stale' event.  In Atom
   channels, this event is indicated by the cc:stale entry-level

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   When received, this event indicates that any cached response
   associated with the event's URI(s) MUST be considered stale (for
   purposes of extension by channel-maxage) within channel-precision

5.  Security Considerations

   Subscribing to a cache channel may incur more traffic than would
   otherwise be generated by typical operation of a cache.  Attackers
   might use this to cause an implementation to subscribe to many
   channels, reducing their capacity or even denying service.  As a
   result, caches that implement this protocol should have some
   mechanism of limiting or controlling the channels that will be
   subscribed to.

   The information in a cache channel may be considered sensitive by its
   publisher; in this case, they should require credentials to be
   presented by subscribers.

6.  Normative References

   [1]  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.

   [2]  Nottingham, M. and R. Sayre, "The Atom Syndication Format",
        RFC 4287, December 2005.

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

   [4]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
        Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
        January 2005.

   [5]  Bray, T., Hollander, D., and A. Layman, "Namespaces in XML", W3C
        REC REC-xml-names-19990114, January 1999.

   [6]  Nottingham, M., "Feed Paging and Archiving", RFC 5005,
        September 2007.

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Appendix A.  Acknowledgements

   Henrik Nordstrom has given invaluable advice and feedback on the
   design of this specification.  Thanks also to Jeff McCarrell, John
   Nienart, Evan Torrie, and Chris Westin for their suggestions.  The
   author takes all responsibility for errors and omissions.

Appendix B.  Operational Considerations

   There are a number of aspects to considered when using cache

   o  They are most efficient when a small number of channels (ideally,
      one) is used to convey events about a large number of associated
      resources (e.g., an entire Web site, or a number of related
   o  In particular, when using Atom channels care should be taken to
      assure that the additional requests necessary to poll the channel
      are offset by the load reduction achieved.  In doing so, the
      anticipated number of clients, channel-precision, change rate for
      cached responses and number of responses being monitored by the
      channel need to be considered.
   o  Feed documents from Atom-based channels should be cacheable, to
      allow clusters of caches and cache hierarchies to share them more
   o  When using channels to update freshness information, it is
      critical to assure that any new content is actually available
      before events are propagated; if the event is too early and stale
      cached content forces revalidation, it is possible that the
      updated content will not be loaded into cache.
   o  Responses that use the channel-maxage mechanism should also
      specify a max-age, both to allow channel-naive caches to store
      them in a limited fashion, and also to allow some types of channel
      implementations to initially store the response before

Appendix C.  Implementation Notes

   Handling of the 'stale' event in order to extend freshness can often
   be effected in an existing cache implementation with only small

   In particular, most caches can be easily modified to call a function
   (whether in-process or in a separate process) when a stale response
   is found, before the decision to validate it on the origin server is
   made.  Using the request-URI, the stored Cache-Control response

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   header and the age of the cached response as input, such a function
   should return either STALE, which indicates that the cached response
   is in fact stale, or FRESH, along with an indication of how much the
   freshness lifetime should be extended by.

   This function determines its response based upon its application of
   the following rules to the state that is collected about the channel
   in question;

   o  If there is no channel-maxage directive in the Cache-Control
      response header, STALE can be returned immediately.
   o  If the channel-URI is missing from the Cache-Control response
      header, the response is assumed to not be associated with any
      channel, and a STALE can immediately be returned.
   o  If the channel is unsubscribed, it should be scheduled for
      subscription, and STALE returned.
   o  If the channel is disconnected, return STALE.
   o  If there is a 'stale' event in the channel that applies to the
      request-URI or group-URI (if present), and cached response's age
      is greater than the age of the of that event, return STALE.
   o  If the cached response's age is greater than channel-maxage,
      return STALE.
   o  If the cached response's age is greater than the channel's
      lifetime, return STALE.
   o  Otherwise, return FRESH freshness=[channel-precision].

Author's Address

   Mark Nottingham
   Yahoo! Inc.


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