Network Working Group M. Nottingham
Internet-Draft June 25, 2006
Expires: December 27, 2006
Extensions for Multi-Document Syndicated Feeds
draft-nottingham-atompub-feed-history-06
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This specification defines three types of syndicated feeds that
enable publication of entries across one or more feed documents.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Complete Feeds . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Paged Feeds . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Archived Feeds . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. Normative References . . . . . . . . . . . . . . . . . . . . . 14
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 15
Appendix B. Reconstructing Archived Feeds . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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1. Introduction
Syndicated feeds of information (using such formats as Atom [RFC4287]
or RSS 2.0) are often split up into multiple documents to save
bandwidth, allow "sliding window" access, or for other purposes.
This specification defines three types of feeds that allow the
reconstruction of their state from one or more feed documents;
"complete" feeds, "paged" feeds and "archived" feeds.
These types are complementary; 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 logical feed's 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.
See the feed type definitions below for examples of use cases for
each.
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.
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.
This specification also uses Atom link relations to identify
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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.
Although they refer to Atom normatively, the mechanisms described
herein can be used with similar syndication formats, such as the
various flavours of RSS.
3. Terminology
In this specification, "feed document" refers to an Atom Feed
Document, RSS document, or similar syndication 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.
"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.
A "logical feed" is the set of entries associated with a particular
feed (as contrasted with a feed document, which may contain a subset
of them).
4. Complete Feeds
A complete feed is a feed document that contains all of the entries
in the logical feed; any entry not actually in the feed document
SHOULD NOT be presented as part of that feed.
It is sometimes important to distinguish a complete feed, because
clients may attempt to keep a history of feed entries seen over time,
presenting the aggregate as the feed's contents. This is
undesireable in some situations.
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 them as
complete feeds, old entries are discarded when the feed 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.
For example,
<fh:complete/>
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4.1. Examples
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>NetTunes Queue</title>
<description>The CDs you'll receive next.</description>
<link href="http://example.org/"/>
<fh:complete/>
<link rel="self"
href="http://nettunes.example.org/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>A Rush of Blood to the Head</title>
<link href="http://nettunes.example.org/Coldplay/rush"/>
<id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
<updated>2003-12-13T18:30:02Z</updated>
<summary>More jangly guitars from Coldplay...</summary>
</entry>
</feed>
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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>NetTunes Queue</title>
<link>http://nettunes.example.org/</link>
<description>The CDs 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@nettunes.example.org</managingEditor>
<webMaster>webmaster@nettunes.example.org</webMaster>
<fh:complete/>
<item>
<title>A Rush of Blood to the Head</title>
<link>http://nettunes.example.org/Coldplay/rush</link>
<description>More jangly guitars from Coldplay...
</description>
<pubDate>Tue, 03 Jun 2003 09:39:21 GMT</pubDate>
<guid>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</guid>
</item>
</channel>
</rss>
5. Paged Feeds
A paged feed is a set of linked feed documents that contain the
entries in the 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
the client will be able to reconstruct the logical feed as the server
has published it. Some entries may be added to the feed as the pages
of the feed are acccessed, without the client becoming aware of them.
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
overwhelm the server, the network, or the client.
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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.
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].
5.1. Examples
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>
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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">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>
6. Archived Feeds
An archived feed is a set of feed documents that can be combined to
accurately reconstruct a logical feed.
Unlike paged feeds, archived feeds enable clients to do this without
losing any entries. This is achieved by publishing a single
subscription 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
(often, the feed document that should be subscribed to).
Archive documents are feed documents that contain less recent entries
in the feed. The set of entries contained in an archive document
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published at a particular URI MUST NOT change over time.
Likewise, the URI for a particular archive document MUST NOT change
over time, so that clients can recognise it and associate it with the
entries contained therein.
Typically, a logical feed will make a subscription feed available,
and link it to a set of archive documents (also linked together)
which contain progressively less recent entries.
Clients can then "subscribe" to the feed, polling the subscription
document for recent changes. If a client has missed some entries,
the archives can be used to synchronise its state by fetching the
archive documents it has not yet seen.
Note that because archive documents are considered stable, changes to
entries in them 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 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.
The following link relations are used to tie 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 archives available.
Archive documents SHOULD have "next-archive" and "current" link
relations.
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].
Archive document SHOULD also contain an fh:archive element in their
head sections, to indicate that they themselves are archives.
For example,
<fh:archive/>
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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.
Clients SHOULD warn users when they are not able to reconstruct the
complete, 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).
6.1. Examples
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>
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Atom-formatted 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="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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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">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 Archive 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, 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>
<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>
7. 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 link relations:
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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 ]
8. Security Considerations
Feeds using the mechanisms described here 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.
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.
9. 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.
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Appendix A. Acknowledgements
The author would like to thank the following people for their
contributions, comments and help: Danny Ayers, Thomas Broyer, Stefan
Eissing, David Hall, Bill de Hora, 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. Reconstructing Archived Feeds
One algorithm for reconstructing an archived feed into a complete,
logical feed (S), give the subscription document (D) follows.
1. Create an empty list L.
2. Consider the URI of the last archive document successfully stored
to local store S as A.
3. Consider the set of entries in document D as E.
4. If the document D has a "prev-archive" link relation value P in
its head section, and P is not A,
1. Append P to L.
2. Dereference P and use the resulting feed document as D.
5. Repeat the previous step until no new P is found.
6. Add all of document D's entries to the local store S, replacing
any entries with the same identity.
7. Pop the last "prev-archive" link relation from L, dereference its
value and use the resulting feed document as D.
8. Repeat the previous two steps until L is empty.
9. Add the entries E to the local store S, replacing any entries
with the same identity.
In these instructions, 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.
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Author's Address
Mark Nottingham
Email: mnot@pobox.com
URI: http://www.mnot.net/
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