Network Working Group M. Nottingham
Internet-Draft February 26, 2006
Expires: August 30, 2006
Feed History: Enabling Incremental Syndication
draft-nottingham-atompub-feed-history-05
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This specification describes a 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4
4. Publishing Incremental Feeds . . . . . . . . . . . . . . . . . 4
5. Publishing Non-Incremental Feeds . . . . . . . . . . . . . . . 5
6. Reconstructing Feed State . . . . . . . . . . . . . . . . . . 5
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Introduction
Syndication documents (e.g., those in formats such as Atom [RFC4287]
and 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 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's state.
This specification describes a technique that allows feed publishers
to give hints as to the completeness of a feed document, and a means
of retrieving "missed" entries from a partial, or incremental, 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. Terminology
In this specification, "feed document" refers to an Atom Feed
Document, RSS document or similar syndication format 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.
"Subscription document" refers to a feed document that always
contains the most recent entries available in the feed (often, the
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 themselves change, however.
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"Incremental feed" refers to a set of associated feed documents
(namely, one subscription document and any number of archive
documents) that can be 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.
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 link relation registry [2]
for more information about specific values.
4. Publishing Incremental Feeds
Publishers who wish to make a feed available incrementally should:
1. Periodically remove the least recent entries from the
subscription document, moving them into archive documents.
2. In the subscription document's head section, add a "previous"
link relation, containing a URI reference to the most recent
archive documents.
3. In archive documents' head sections, add a "previous" link
relation, containing a URI reference to the next most recent
archive (if any).
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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 be
unable to serve (e.g., with HTTP status code 404) an archive
document.
Publishers may also duplicate entries between the subscription
document and the 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 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.
5. Publishing Non-Incremental Feeds
Some consumers may attempt to keep a history of feed 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 ranking that varies over time,
such as "Top Twenty Records" or "Most Popular Items" should not have
newer entries displayed alongside older ones.
The fh:complete element, when present in a feed's head section,
indicates that the subscription document is a complete representation
of the logical feed's entries.
For example,
<fh:complete/>
6. Reconstructing Feed State
When presented with a feed document, a consumer MAY reconstruct the
feed's state in a local store S by following these steps:
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1. If the feed document's head section contains a "current" link
relation value C, dereference C and use the resulting feed
document as D.
2. If the fh:complete element is present in D's head section:
1. Remove all entries present in local store S.
2. Add all of the document D's entries to local store S.
3. Stop processing.
3. Otherwise:
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 entries in document D as E.
4. If the document D has a "previous" 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 "previous" 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 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 have the complete feed,
or an error is encountered in the reconstruction process (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).
Consumers MAY cache archive documents and/or use a different method
of 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 "self" link relation, if present, SHOULD be
used as the URI of the feed document 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 Feed 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 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>
<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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Incremental Atom Feed: 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="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>
<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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Incremental RSS 2.0 Feed: 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="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>
<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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Incremental RSS 2.0 Feed: 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="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
[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.
[1] <http://blogs.law.harvard.edu/tech/rss>
[2] <http://www.iana.org/assignments/link-relations>
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, Aristotle Pagaltzis, Dave Pawson, Garrett
Rooney, Robert Sayre, James Snell, Henry Story.
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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