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Network Working Group                                      M. Nottingham
Internet-Draft                                         September 1, 2005                                         February 26, 2006
Expires: March 5, August 30, 2006


             Feed History: Enabling Incremental Syndication
                draft-nottingham-atompub-feed-history-04
                draft-nottingham-atompub-feed-history-05

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   This Internet-Draft will expire on March 5, August 30, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005). (2006).

Abstract

   This document specifies specification describes a mechanism technique for feed publishers to give
   hints about the completeness of a feed, and a means of retrieving
   "missed" entries from a incremental feed.









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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.   Notational Conventions  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.   The 'fh:incremental' Element .  Notational Conventions . . . . . . . . . . . . . . .   4
   4.   The 'fh:archive' Element . . . . .  4
   4.  Publishing Incremental Feeds . . . . . . . . . . . . .   5
   5.   The 'fh:prev' Element . . . .  4
   5.  Publishing Non-Incremental Feeds . . . . . . . . . . . . . . .  5
   6.  Reconstructing Feed Reconstruction  . State  . . . . . . . . . . . . . . . . . . .   6  5
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  Normative References . . . . . . . . . . . . . . . . . . . .  11
        Author's Address . 11
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 11
   Author's Address . . .  11
   A.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  11 12
   Intellectual Property and Copyright Statements . . . . . . .  12 . . . 13





































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1.  Introduction

   Syndication documents (e.g., those in formats such as Atom
   [AtomSyntax] [RFC4287]
   and RSS
   <http://purl.org/rss/1.0/spec><http://blogs.law.harvard.edu/tech/
   rss>) 2.0 [1]) usually only contain the last several entries in a
   larger channel (or "feed") of information.  Often, consuming software
   keeps copies of all entries that have been previously seen,
   effectively keeping a history of the feed's contents.

   However, not all feeds benefit from this practice; in some, previous
   entries are not relevant to the current contents of the feed.  For
   example, it's not desireable desirable to keep history in this manner with a
   "top ten" feed; showing old entries would imply that the previous
   number one is now number eleven, and so forth.

   Feeds that encourage this practice have a different problem.  If
   consuming software does not poll often enough, some entries may be
   missed, causing them to be silently omitted.  For some applications,
   this is a serious error on its own.  Even in non-critical
   applications, this phenomenon can cause publishers to make Feed
   Documents contain more entries than reasonably necessary, just to
   assure that consumers have an amply large window in which to
   reconstruct the feed. feed's state.

   This document specifies specification describes a mechanism technique that allows feed publishers
   to give hints as to the completeness of a feed, feed document, and a means
   of retrieving "missed" entries from a partial, or incremental, feed. feed
   by linking archives of the feed together.

   Although it refers to Atom normatively, the mechanism described
   herein can be used with similar syndication formats, such as the
   various flavours of RSS.


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 BCP 14, [RFC2119], as
   scoped to those conformance targets.  Terminology

   In this specification, "feed document" refers to an Atom Feed
   Document, RSS document or similar syndication format instance. instance
   document.  It may contain any number of entries (in RSS, items), and
   may or may not be a complete representation of the logical feed.

   Similarly, "subscription

   "Subscription document" refers to a feed document that is
   intended to be subscribed to; i.e., it always
   contains the most recent entries available in the feed.




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   feed document that should be subscribed to).

   "Archive document" refers to a feed document that is archived; i.e.,
   the set of entries inside it does not change over time.  Entries
   within an archive MAY change themselves.  Note that some entries in
   the archive document may also be present in the themselves change, however.




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   "Incremental feed" refers to a set of associated feed documents
   (namely, one subscription
   document; in other words, some (but not necessarily all) "live"
   entries might already document and any number of archive
   documents) that can be archived. combined to form a single, logical feed.

   Finally, "head section" refers to the children of a feed document's
   document-wide metadata container; e.g., the child elements of the
   atom:feed element in an Atom Feed Document.


3.  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 BCP 14, [RFC2119], as
   scoped to those conformance targets.

   This specification uses XML Namespaces [W3C.REC-xml-names-19990114]
   to uniquely identify XML element names.  It uses the following
   namespace prefix for the indicated namespace URI;

   "fh": "http://purl.org/syndication/history/1.0"

   This specification uses terms from the XML Infoset [W3C.REC-xml-
   infoset-20040204].  However, this specification uses a shorthand; the
   phrase "Information Item" is omitted when naming Element Information
   Items.  Therefore, when this specification uses the term "element,"
   it is referring to an Element Information Item in Infoset terms.

3.  The 'fh:incremental' Element

   The fh:incremental element indicates whether

   This specification also uses Atom link relations to identify
   different types of links; see the subscription
   document is a complete representation of the logical feed's entries, Atom specification [RFC4287] for
   information about their syntax, and SHOULD occur in a subscription document's head section.  It MUST
   NOT occur there the link relation registry [2]
   for more than once, or elsewhere in information about specific values.


4.  Publishing Incremental Feeds

   Publishers who wish to make a feed document.  Its
   content MUST be either "true" or "false".  Whitespace in its content
   MUST be ignored by consumers.

   For example,

     <fh:incremental>true</fh:incremental>

   Subscription feeds MAY omit available incrementally should:

   1.  Periodically remove the fh:incremental element if least recent entries from the fh:prev
   element is present; a consumer encountering a
       subscription document
   that contains a fh:prev element but does not contain a fh:incremental
   element MAY behave as if the fh:incremental is present and contains
   "true".

   If the content of the fh:incremental element is "false", it indicates
   that document, moving them into archive documents.
   2.  In the subscription document is document's head section, add a complete representation of the
   entire feed; previous entries SHOULD NOT be considered part of "previous"
       link relation, containing a URI reference to the
   feed by consumers.

   For example, most recent
       archive documents.
   3.  In archive documents' head sections, add a feed that represents "previous" link
       relation, containing a ranking that varies over time, URI reference to the next most recent
       archive (if any).





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   such as "Top Twenty Records"                 February 2006


   4.  In archive documents' head sections, add a "current" link
       relation, containing the subscription document's URI reference.

   Publishers are not required to make all archive documents available;
   they may refuse to serve (e.g., with HTTP status code 403), or "Most Popular Items" should be marked
   unable to serve (e.g., with a fh:incremental element containing "false".

   If HTTP status code 404) an archive
   document.

   Publishers may also duplicate entries between the content of subscription
   document and the fh:incremental element is "true", it indicates most current archive document; i.e., they may
   archive current entries.

   Note that because consumers are not required to re-fetch archived
   feeds that they've previously stored, changes to entries in those
   documents may not be apparent to all users.  Therefore, if a
   publisher requires a change to be visible to all users (e.g.,
   correcting factual errors), they should consider publishing the
   revised entry in the subscription document is a potentially partial
   representation of feed, in addition to (or instead
   of) the entire feed; previous entries MUST appropriate archive feed.  Conversely, unimportant changes
   (e.g., spelling corrections) might be
   considered part only effected in archive feeds.


5.  Publishing Non-Incremental Feeds

   Some consumers may attempt to keep a history of the feed by consumers. entries, even
   without explicit hints that enable it.  Because this is undesirable
   for some feeds, this section defines an explicit means of indicating
   that history should not be kept.

   For example, a feed that represents a chronological list, ranking that varies over time,
   such as
   "ExampleCo Press Releases" or "Widget Project Updates" "Top Twenty Records" or "Most Popular Items" should be
   marked with a fh:incremental element containing "true".

   A subscription document whose fh:incremental element contains "true"
   MUST contain a fh:prev element, unless there are no previous not have
   newer entries displayed alongside older ones.

   The fh:complete element, when present in the feed.  A subscription document whose fh:incremental element
   contains "false" MUST NOT contain a fh:prev element.

4.  The 'fh:archive' Element

   The fh:archive element is an empty element that feed's head section,
   indicates that the
   feed subscription document is an archive document and SHOULD occur in an archive
   document's head section.  It MUST NOT occur there more than once, or
   elsewhere in a feed document.

   For example,

       <fh:archive/>

   Consumers can use this element to distinguish between subscription
   documents and archive documents; e.g., to assure that an archive
   document isn't subscribed to by mistake.

5.  The 'fh:prev' Element

   The fh:prev element conveys the location of an archive complete representation
   of previous
   entries from the logical feed's entries.

   For example,

     <fh:complete/>


6.  Reconstructing Feed State

   When presented with a feed which sequentially precede those in the current document, and MAY occur in a subscription document's head section.
   It MUST occur in an archive document's head section, unless there are
   no previous entries in consumer MAY reconstruct the feed.  It MUST NOT occur there more than
   once, or elsewhere
   feed's state in a feed document.

   Its content MUST be a URI reference [RFC3986] indicating the previous
   archive document's location.  Whitespace surrounding its content MUST
   be ignored local store S by consumers.

   For example,

      <fh:prev>http://www.example.com/feed/archive/2005/05</fh:prev> following these steps:





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   Note that URI references may be relative,                 February 2006


   1.  If the feed document's head section contains a "current" link
       relation value C, dereference C and when they are used they
   must be absolutised, use the resulting feed
       document as described D.
   2.  If the fh:complete element is present in Section 5.1 D's head section:
       1.  Remove all entries present in local store S.
       2.  Add all of [RFC3986].

   Also, note that the fh:prev element, on its own, does not bestow any
   semantics on document D's entries to local store S.
       3.  Stop processing.
   3.  Otherwise:
       1.  Create an empty list L.
       2.  Consider the ordering URI of the last archive document successfully
           stored to local store S as A.
       3.  Consider the entries in a feed; document D as E.
       4.  If the only purpose of
   introducing ordering here document D has a "previous" link relation value P in
           its head section, and P is not A,
           1.  Append P to allow L.
           2.  Dereference P and use the resulting feed to be reconstructed. document as D.
       5.  Repeat the previous step until no new P is found.
       6.  Feed Reconstruction

   When presented with an incremental subscription document, a consumer
   MAY reconstruct  Add all of document D's entries to the entire feed in a local store by following these
   steps, starting S,
           replacing any entries with the subscription same identity.
       7.  Pop the last "previous" link relation from L, dereference its
           value and use the resulting feed document as D.
       8.  Repeat the current
   document:

   1. previous two steps until L is empty.
       9.  Add all of the entries in the current document E to the store.
   2.  Dereference local store S, replacing any entries
           with the fh:prev URI, if present.  If same identity.

   In the instructions above, the concept of an entry's identity is
   format-specific; e.g., in Atom, it is conveyed by the atom:id
   element; in RSS 2, it is indicated by the guid element.

   Consumers SHOULD warn users when they do not present,
       stop processing.
   3.  Using have the complete feed,
   or an error is encountered in the reconstruction process (e.g., by
   alerting the dereferenced user that an archive document as is unavailable, or
   displaying pseudo-entries that inform the current document,
       start at step one (i.e., apply these steps recursively).

   A consumer MAY stop when it encounters an fh:prev URI whose entries
   have been successfully stored beforehand when following this process.

   Note user that consumers some entries may
   be missing).

   Consumers MAY cache archive documents and/or use a different method
   of reconstructing reconstructing the feed, as long as the result is the same as that
   achieved by following these steps.

   In particular, consumers MAY use the "first" and "next" link
   relations, if available, to traverse the feed in a different manner.
   However, such consumers MUST NOT require their presense to
   reconstruct a feed's state.

   When comparing URIs to determine if an archive has been successfully
   stored, the value of the feed, as long "self" link relation, if present, SHOULD be
   used as the result is
   the same as that achieved by following these steps.

   Consumers SHOULD warn users when they do not have URI of the complete feed
   (e.g., by alerting the user that an archive document is unavailable,
   or inserting pseudo-entries that inform the user that some entries
   may be missing).

   Note that publishers are not required to make all archive documents
   available. it occurs in.




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   Note that URI references in link relation values may be relative, and
   when they are used they must be absolutised, as described in Section
   5.1 of [RFC3986].

   This specification does not bestow any semantics on the ordering of
   entries in a feed; the only purpose of introducing ordering between
   archive documents is to allow the feed to be reconstructed.


7.  Examples

   Non-Incremental Atom Subscription Feed Document

   <?xml version="1.0" encoding="utf-8"?>
   <feed xmlns="http://purl.org/atom/ns#draft-ietf-atompub-format-09"
     xmlns:history="http://purl.org/syndication/history/1.0"> xmlns="http://www.w3.org/2005/Atom"
    xmlns:fh="http://purl.org/syndication/history/1.0">
    <title>Example Feed</title>
    <link href="http://example.org/"/>
    <fh:complete/>
    <link rel="self" href="http://example.org/index.atom"/>
    <updated>2003-12-13T18:30:02Z</updated>
    <author>
      <name>John Doe</name>
    </author>
    <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>
     <history:incremental>true</history:incremental>
     <history:prev>http://example.org/2003/11/index.atom</history:prev>
    <entry>
      <title>Atom-Powered Robots Run Amok</title>
      <link href="http://example.org/2003/12/13/robots_here"/> href="http://example.org/2003/12/13/atom03"/>
      <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
      <updated>2003-12-13T18:30:02Z</updated>
      <summary>Some text in a new, fresh entry.</summary> text.</summary>
    </entry>
   </feed>


















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   Incremental Atom Archive Feed: Subscription Document

   <?xml version="1.0" encoding="utf-8"?>
   <feed xmlns="http://purl.org/atom/ns#draft-ietf-atompub-format-09"
     xmlns:history="http://purl.org/syndication/history/1.0"> xmlns="http://www.w3.org/2005/Atom">
    <title>Example Feed</title>
    <link href="http://example.org/"/>
    <link rel="self" href="http://example.org/index.atom"/>
    <link rel="previous" href="http://example.org/2003/11/index.atom"/>
    <updated>2003-12-13T18:30:02Z</updated>
    <author>
      <name>John Doe</name>
    </author>
    <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>
    <entry>
      <title>Atom-Powered Robots Run Amok</title>
      <link href="http://example.org/2003/12/13/atom03"/>
      <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
      <updated>2003-12-13T18:30:02Z</updated>
      <summary>Some text.</summary>
    </entry>
   </feed>

   Incremental Atom Feed: Archive Document

   <?xml version="1.0" encoding="utf-8"?>
   <feed xmlns="http://www.w3.org/2005/Atom">
    <title>Example Feed</title>
    <link rel="current" href="http://example.org/index.atom"/>
    <link rel="self" href="http://example.org/2003/11/index.atom"/>
    <link rel="previous" href="http://example.org/2003/10/index.atom"/>
    <updated>2003-11-24T12:00:00Z</updated>
    <author>
      <name>John Doe</name>
    </author>
    <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>
     <history:archive/>
     <history:prev>http://example.org/2003/10/index.atom</history:prev>
    <entry>
      <title>Atom-Powered Robots Scheduled To Run Amok</title>
      <link href="http://example.org/2003/11/24/robots_coming"/>
       <id>urn:uuid:cd3272ef-b09c-42fd-806b-e25580e59b39</id>
      <id>urn:uuid:cdef5c6d5-gff8-4ebb-assa-80dwe44efkjo</id>
      <updated>2003-11-24T12:00:00Z</updated>
      <summary>Some text from an old, different entry.</summary>
    </entry>
   </feed>








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   Incremental RSS 2.0 Feed: Subscription Document

   <?xml version="1.0"?>
   <rss version="2.0"
    xmlns:history="http://purl.org/syndication/history/1.0"> xmlns:atom="http://www.w3.org/2005/Atom">
    <channel>
     <title>Liftoff News</title>
     <link>http://liftoff.nasa.gov/</link>
     <description>Liftoff to Space Exploration.</description>
     <language>en-us</language>
     <pubDate>Tue, 10 Jun 2003 04:00:00 GMT</pubDate>
     <lastBuildDate>Tue, 10 Jun 2003 09:41:01 GMT</lastBuildDate>
     <docs>http://blogs.law.harvard.edu/tech/rss</docs>
     <generator>Weblog Editor 2.0</generator>
     <managingEditor>editor@example.com</managingEditor>
     <webMaster>webmaster@example.com</webMaster>
     <history:prev>http://liftoff.nasa.gov/2003/05/feed<history:prev>
     <atom:link rel="previous"
      href="http://liftoff.nasa.gov/2003/05/index.rss"/>

     <item>
      <title>Star City</title>
      <link>http://liftoff.nasa.gov/2003/06/news-starcity</link>
      <description>How do Americans get ready to work with Russians
      aboard the International Space Station? They take a crash course
      in culture, language and protocol at Russia's <a
      href="http://howe.iki.rssi.ru/GCTC/gctc_e.htm">Star
      City</a>.</description>
      href="http://howe.iki.rssi.ru/GCTC/gctc_e.htm"&gt;Star
      City</a&gt;.</description>
      <pubDate>Tue, 03 Jun 2003 09:39:21 GMT</pubDate>
      <guid>http://liftoff.nasa.gov/2003/06/03.html#item573</guid>
     </item>
    </channel>
   </rss>

   (note that the incremental element has been omitted in this instance,
   because the presense of the prev element indicates that the feed is
   incremental.)




















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   Incremental RSS 2.0 Feed: Archive Document

   <?xml version="1.0"?>
   <rss version="2.0"
    xmlns:history="http://purl.org/syndication/history/1.0"> xmlns:atom="http://www.w3.org/2005/Atom">
    <channel>
     <title>Liftoff News</title>
     <link>http://liftoff.nasa.gov/</link>
     <description>Liftoff to Space Exploration.</description>
     <language>en-us</language>
     <pubDate>Tue, 30 May 2003 08:00:00 GMT</pubDate>
     <lastBuildDate>Tue, 30 May 2003 10:31:52 GMT</lastBuildDate>
     <docs>http://blogs.law.harvard.edu/tech/rss</docs>
     <generator>Weblog Editor 2.0</generator>
     <managingEditor>editor@example.com</managingEditor>
     <webMaster>webmaster@example.com</webMaster>
     <history:archive/>
     <history:prev>http://liftoff.nasa.gov/2003/04/feed</history:prev>
     <atom:link rel="current" href="http://liftoff.nasa.gov/index.rss"/>
     <atom:link rel="previous"
      href="http://liftoff.nasa.gov/2003/04/index.rss"/>

     <item>
      <description>Sky watchers in Europe, Asia, and parts of
      Alaska and Canada will experience a partial eclipse of the Sun
      on Saturday, May 31st.</description>
      <pubDate>Fri, 30 May 2003 11:06:42 GMT</pubDate>
      <guid>http://liftoff.nasa.gov/2003/05/30.html#item572</guid>
     </item>
     <item>
      <title>The Engine That Does More</title>
      <link>http://liftoff.nasa.gov/2003/05/news-VASIMR.asp</link>
      <description>Before man travels to Mars, NASA hopes to
      design new engines that will let us fly through the Solar
      System more quickly.  The proposed VASIMR engine would do
      that.</description>
      <pubDate>Tue, 27 May 2003 08:37:32 GMT</pubDate>
      <guid>http://liftoff.nasa.gov/2003/05/27.html#item571</guid>
     </item>
    </channel>
   </rss>


8.  Security Considerations

   Feeds using the mechanisms described here could be crafted in such a
   way as to cause a consumer to initiate excessive (or even an unending
   sequence of) network requests, causing denial of service (either to
   the consumer, the target server, and/or intervening networks).
   Consumers can mitigate this risk by requiring user intervention after
   a certain number of requests, or by limiting requests either



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   according to a hard limit, or with heuristics.

   Consumers should be mindful of resource limits when storing feed
   documents; to reiterate, they are not required to always store or
   reconstruct the feed when conforming to this specification; they only
   need inform the user when the reconstructed feed is not complete.

9.  Normative References

   [AtomSyntax]
              Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", June 2005.

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4287]  Nottingham, M. and R. Sayre, "The Atom Syndication
              Format", RFC 4287, December 2005.

   [W3C.REC-xml-infoset-20040204]
              Cowan, J. and R. Tobin, "XML Information Set (Second
              Edition)", W3C REC REC-xml-infoset-20040204,
              February 2004.

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


Author's Address

   Mark Nottingham

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

   [1]  <http://blogs.law.harvard.edu/tech/rss>

   [2]  <http://www.iana.org/assignments/link-relations>


Appendix A.  Acknowledgements

   The author would like to thanks thank the following people for their
   contributions, comments and help: Danny Ayers, Thomas Broyer, Stefan
   Eissing, David Hall, Aristotle Pagaltzis, Dave Pawson, Garrett
   Rooney, Robert Sayre, James Snell, Henry Story.  [[This is who showed
   up in the e-mail archive; contact me if I missed you.]]

   Any errors herein remain the author's, not theirs.










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Author's Address

   Mark Nottingham

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













































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