


Network Working Group                                      M. Nottingham
Internet-Draft                                            April 22, 2007
Intended status: Standards Track
Expires: October 24, 2007


                       Feed Paging and Archiving
                draft-nottingham-atompub-feed-history-09

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   This Internet-Draft will expire on October 24, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This specification defines three types of syndicated Web feeds that
   enable publication of entries across one or more feed documents.









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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Notational Conventions . . . . . . . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Complete Feeds . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Paged Feeds  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Archived Feeds . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Publishing Archived Feeds  . . . . . . . . . . . . . . . .  9
     4.2.  Consuming Archived Feeds . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 11
   Appendix B.  Use in RSS 2.0  . . . . . . . . . . . . . . . . . . . 11


































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1.  Introduction

   Syndicated Web feeds (using such formats as Atom [RFC4287]) are often
   split up into multiple documents to save bandwidth, allow "sliding
   window" access, or for other purposes.

   This specification formalizes two types of feeds that can span one or
   more feed documents; "paged" feeds and "archived" feeds.
   Additionally, it defines "complete" feeds to cover the case when a
   single feed document explicitly represents all of the feed's entries.

   Each has different properties and trade-offs:

   o  Complete feeds contain the entire set of entries in one document,
      and can be useful when it isn't desirable to "remember"
      previously-seen entries.
   o  Paged feeds split the entries among multiple temporary documents.
      This can be useful when entries in the feed are not long-lived or
      stable, and the client needs to access an arbitrary portion of
      them, usually in close succession.
   o  Archived feeds split them among multiple permanent documents, and
      can be useful when entries are long-lived and it is important for
      clients to see every one.

   The semantics of a feed that combines these types is undefined by
   this specification.

   Although they refer to Atom normatively, the mechanisms described
   herein can be used with similar syndication formats; see Appendix B
   for one such use.

1.1.  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].

   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"

1.2.  Terminology

   In this specification, "feed document" refers to an Atom Feed
   Document or similar syndication instance document.  It may contain
   any number of entries, and may or may not be a complete



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   representation of the logical feed.

   A "logical feed" is the complete set of entries associated with a
   feed (as contrasted with a feed document, which may contain a subset
   of them).

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

   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.

   This specification also uses Atom link relations to identify
   different types of links; see the Atom specification [RFC4287] for
   information about their syntax, and the IANA link relation registry
   for more information about specific values.

   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].

2.  Complete Feeds

   A complete feed is a feed document that contains all of the entries
   of a logical feed; any entry not actually in the feed document SHOULD
   NOT be considered to be part of that feed.

   For example, a feed that represents a ranking that varies over time
   (such as "Top Twenty Records" or "Most Popular Items") should not
   have newer entries displayed alongside older ones.  By marking this
   type feeds as complete, old entries are discarded when it is
   refreshed.

   The fh:complete element, when present in a feed's head section,
   indicates that the feed document it occurs in is a complete
   representation of the logical feed's entries.  It is an empty
   element; this specification does not define any content for it.









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   Example: Atom-formatted Complete Feed

   <?xml version="1.0" encoding="utf-8"?>
   <feed xmlns="http://www.w3.org/2005/Atom"
    xmlns:fh="http://purl.org/syndication/history/1.0">
    <title>NetMovies Queue</title>
    <description>The DVDs you'll receive next.</description>
    <link href="http://example.org/"/>
    <fh:complete/>
    <link rel="self"
     href="http://netmovies.example.org/jdoe/queue/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>Casablanca</title>
      <link href="http://netmovies.example.org/movies/Casablanca"/>
      <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
      <updated>2003-12-13T18:30:02Z</updated>
      <summary>Here's looking at you, kid...</summary>
    </entry>
   </feed>

   This specification does not address duplicate entries in complete
   feeds.

3.  Paged Feeds

   A paged feed is a set of linked feed documents that together contain
   the entries of a logical feed, without any guarantees about the
   stability of the documents' contents.

   Paged feeds are lossy; that is, it is not possible to guarantee that
   clients will be able to reconstruct the contents of the logical feed
   at a particular time.  Entries may be added or changed as the pages
   of the feed are accessed, without the client becoming aware of them.

   Therefore, clients SHOULD NOT present paged feeds as coherent or
   complete, or make assumptions to that effect.

   Paged feeds can be useful when the number of entries is very large,
   infinite, or indeterminate.  Clients can "page" through the feed,
   only accessing a subset of the feed's entries as necessary.

   For example, a search engine might make query results available as a
   paged feed, so that queries with very large result sets do not



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   overwhelm the server, the network, or the client.

   The feed documents in a paged feed are tied together with the
   following link relations:

   o  "first" - A URI that refers to the furthest preceding document in
      a series of documents.
   o  "last" - A URI that refers to the furthest following document in a
      series of documents.
   o  "previous" - A URI that refers to the immediately preceding
      document in a series of documents.
   o  "next" - A URI that refers to the immediately following document
      in a series of documents.

   Paged feed documents MUST have at least one of these link relations
   present, and should contain as many as practical and applicable.

   Example: Atom-formatted Paged Feed

   <?xml version="1.0" encoding="utf-8"?>
   <feed 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="next" href="http://example.org/index.atom?page=2"/>
    <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>

   This specification does not address duplicate entries in paged feeds.

4.  Archived Feeds

   An archived feed is a set of feed documents that can be combined to
   accurately reconstruct the entries of a logical feed.

   Unlike paged feeds, archived feeds enable clients to do this without
   losing entries.  This is achieved by publishing a single subscription



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   document and (potentially) many archive documents.

   A subscription document is a feed document that always contains the
   most recently added or changed entries available in the logical feed.

   Archive documents are feed documents that contain less recent entries
   in the feed.  The set of entries contained in an archive document
   published at a particular URI SHOULD NOT change over time.  Likewise,
   the URI for a particular archive document SHOULD NOT change over
   time.

   The following link relations are used to tie subscription and
   archived feeds together:

   o  "prev-archive" - A URI that refers to the immediately preceding
      archive document.
   o  "next-archive" - A URI that refers to the immediately following
      archive document.
   o  "current" - A URI that, when dereferenced, returns a feed document
      containing the most recent entries in the feed.

   Subscription documents and archive documents MUST have a "prev-
   archive" link relation, unless there are no preceding archives
   available.  Additionally, archive documents SHOULD have "next-
   archive" and "current" link relations.

   Archive document SHOULD also contain an fh:archive element in their
   head sections to indicate that they are archives. fh:archive is an
   empty element; this specification does not define any content for it.






















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   Example: Atom-formatted Subscription Document

   <?xml version="1.0" encoding="utf-8"?>
   <feed 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="prev-archive"
     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>

   Example: Atom-formatted Archive Document

   <?xml version="1.0" encoding="utf-8"?>
   <feed xmlns="http://www.w3.org/2005/Atom"
    xmlns:fh="http://purl.org/syndication/history/1.0">
    <title>Example Feed</title>
    <link rel="current" href="http://example.org/index.atom"/>
    <link rel="self" href="http://example.org/2003/11/index.atom"/>
    <fh:archive/>
    <link rel="prev-archive"
     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>
    <entry>
      <title>Atom-Powered Robots Scheduled To Run Amok</title>
      <link href="http://example.org/2003/11/24/robots_coming"/>
      <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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4.1.  Publishing Archived Feeds

   The requirement that archive documents be stable allows clients to
   safely assume that if they have retrieved one in the past, it will
   not meaningfully change in the future.  As a result, if an archive
   document's contents are changed, some clients may not become aware of
   it.

   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 feed, in addition to
   (or instead of) the appropriate archive feed.  Conversely,
   unimportant changes (e.g., spelling corrections) might be only
   effected in archive feeds.

   Publishers SHOULD construct their feed documents in such a way as to
   make duplicate removal unambiguous (see Section 4.2).

   Publishers are not required to make all archive documents available;
   they may refuse to serve (e.g., with HTTP status code 403 or 410), or
   be unable to serve (e.g., with HTTP status code 404) an archive
   document.

4.2.  Consuming Archived Feeds

   Typically, clients will "subscribe" to an archived feed by polling
   the subscription document for recent changes.  If URI contained in
   the prev-archive link relation has not been processed in the past,
   the client can "catch up" with any missed entries by dereferencing it
   and adding the contained entries to the logical feed.  This process
   should be repeated recursively until the client encounters a prev-
   archive link relation that has been processed, the end of the archive
   is indicated by a missing prev-archive link relation, or an error is
   encountered.

   If duplicate entries are found, clients SHOULD consider only the most
   recently updated entry to be part of the logical feed.  If duplicate
   entries have the same update time-stamp, or none is available, the
   entry sourced from the most recently updated feed document SHOULD
   replace all other duplicates of that entry.

   In Atom-formatted archived feeds, two entries are duplicates if they
   have the same atom:id element.  The update time of an entry is
   determined by its atom:updated element, and likewise the update time
   of a feed document is determined by its feed-level atom:updated
   element.

   Clients SHOULD warn users when they are not able to reconstruct the



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   entire logical feed (e.g., by alerting the user that an archive
   document is unavailable, or displaying pseudo-entries that inform the
   user that some entries may be missing).

5.  IANA Considerations

   The "previous", "next" and "current" link relations have been
   previously registered, and no IANA action regarding them is required.

   This specification defines the following new relations, to be added
   to the Link Relations registry:

      o  Attribute Value: prev-archive
      o  Description: A URI that refers to the immediately
         preceding archive document.
      o  Expected display characteristics: none
      o  Security considerations: See [ this document ]

      o  Attribute Value: next-archive
      o  Description: A URI that refers to the immediately
         following archive document.
      o  Expected display characteristics: none
      o  Security considerations: See [ this document ]

6.  Security Considerations

   Feeds using this mechanism have the same authentication and channel
   security concerns as explained in Atom [RFC4287].

   Feeds using these mechanisms could be crafted in such a way as to
   cause a client to initiate excessive (or even an unending sequence
   of) network requests, causing denial of service (either to the
   client, the target server, and/or intervening networks).  Clients can
   mitigate this risk by requiring user intervention after a certain
   number of requests, or by limiting requests either according to a
   hard limit, or with heuristics.  Servers can mitigate this risk by
   denying requests that they consider abusive (e.g., by closing the
   connection, or generating an error).

   Clients 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.

7.  References






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7.1.  Normative References

   [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.

7.2.  Informative References

   [RSS2]                          Winer, D., "RSS 2.0 Specification",
                                   2005, <http://blogs.law.harvard.edu/
                                   tech/rss>.

Appendix A.  Acknowledgements

   The author would like to thank the following people for their
   contributions, comments and help: Danny Ayers, Thomas Broyer, Lisa
   Dusseault, Stefan Eissing, David Hall, Bill de Hora, Vidya Narayanan,
   Aristotle Pagaltzis, John Panzer, Dave Pawson, Garrett Rooney, Robert
   Sayre, James Snell, Henry Story.

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

Appendix B.  Use in RSS 2.0

   As previously noted, while this specification's extensions are
   described in terms of the Atom feed format, they are also useful in
   similar formats.  This informative appendix demonstrates how they can
   be used in an RSS 2.0-formatted [RSS2] feed.




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   In RSS 2.0-formatted feeds, two entries are duplicates if they have
   the same guid element.  The update time of an entry is not defined by
   RSS 2.0, but the feed-level update time can be determined by the
   pubDate element.

   RSS 2.0-formatted Complete Feed

   <?xml version="1.0"?>
   <rss version="2.0"
    xmlns:fh="http://purl.org/syndication/history/1.0">
    <channel>
     <title>NetMovies Queue</title>
     <link>http://netmovies.example.org/</link>
     <description>The DVDs you'll receive next.</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@netmovies.example.org</managingEditor>
     <webMaster>webmaster@netmovies.example.org</webMaster>
     <fh:complete/>
     <item>
      <title>Casablanca</title>
      <link>http://netmovies.example.org/movies/Casablanca</link>
      <description>Here's looking at you, kid...
      </description>
      <pubDate>Tue, 03 Jun 2003 09:39:21 GMT</pubDate>
      <guid>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</guid>
     </item>
    </channel>
   </rss>



















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   RSS 2.0-formatted Paged Feed

   <?xml version="1.0"?>
   <rss version="2.0"
    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>
     <atom:link rel="next"
      href="http://liftof.nasa.gov/index.rss?page=2"/>
     <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"&gt;Star
      City</a>.</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>




















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   RSS 2.0-formatted Subscription Document

   <?xml version="1.0"?>
   <rss version="2.0" 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>
     <atom:link rel="prev-archive"
      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"&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>




















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   RSS 2.0-formatted Archive Document

   <?xml version="1.0"?>
   <rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom"
    xmlns:fh="http://purl.org/syndication/history/1.0">
    <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>
     <fh:archive/>
     <atom:link rel="current"
      href="http://liftoff.nasa.gov/index.rss"/>
     <atom:link rel="prev-archive"
      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>










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Author's Address

   Mark Nottingham

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













































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