Network Working Group M. Nottingham
Internet-Draft August 14, 2005
Expires: February 15, 2006
Feed History: Enabling Stateful Syndication
draft-nottingham-atompub-feed-history-03
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document specifies a mechanism that allows feed publishers to
give hints about the nature of the feed's statefulness, and a means
of retrieving "missed" entries from a stateful feed.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. The 'fh:stateful' Element . . . . . . . . . . . . . . . . . 4
4. The 'fh:archive' Element . . . . . . . . . . . . . . . . . . 5
5. The 'fh:prev' Element . . . . . . . . . . . . . . . . . . . 5
6. State Reconstruction . . . . . . . . . . . . . . . . . . . . 6
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . 10
9. Normative References . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . 12
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1. Introduction
Syndication documents (e.g., those in formats such as Atom
[AtomSyntax] and RSS
<http://purl.org/rss/1.0/spec><http://blogs.law.harvard.edu/tech/
rss>) 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 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 document specifies a mechanism that allows feed publishers to
give hints as to the nature of the feed with regard to state, and a
means of retrieving "missed" entries from a stateful feed. 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.
In this specification, "feed document" refers to an Atom Feed
Document, RSS document or similar syndication format instance. 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 document" refers to a feed document that is
intended to be subscribed to; i.e., it contains the most recent
entries available in the feed.
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"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 subscription
document; in other words, some (but not necessarily all) "live"
entries might already be archived.
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.
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:stateful' Element
The fh:stateful element indicates whether the feed is stateful, and
MAY occur in a subscription document's head section. It MUST NOT
occur there more than once, or elsewhere in a feed document. Its
content MUST be either "true" or "false". Whitespace in its content
MUST be ignored by consumers.
For example,
<fh:stateful>true</fh:stateful>
If the content of the fh:stateful element is "false", it indicates
that the subscription document is a complete representation of the
entire feed; previous entries SHOULD NOT be considered part of the
feed by consumers.
For example, a feed that represents a ranking that varies over time,
such as "Top Twenty Records" or "Most Popular Items" should be marked
with a fh:stateful element containing "false".
If the content of the fh:stateful element is "true", it indicates
that the subscription document is a potentially partial
representation of the entire feed; previous entries MUST be
considered part of the feed by consumers.
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For example, a feed that represents a chronological list, such as
"ExampleCo Press Releases" or "Widget Project Updates" should be
marked with a fh:stateful element containing "true".
A subscription document whose fh:stateful element contains "true"
MUST contain a fh:prev element, unless there are no previous entries
in the feed. A subscription document whose fh:stateful element
contains "false" MUST NOT contain a fh:prev element.
A consumer encountering a subscription document that contains a fh:
prev element but does not contain a fh:stateful element MAY behave as
if the fh:stateful is present and contains "true".
4. The 'fh:archive' Element
The fh:archive element is an empty element that indicates that the
feed 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 of previous
entries from the 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 the feed. It MUST NOT occur there more than
once, or elsewhere in a feed document.
Its content MUST be a URI reference indicating the previous archive
document's location.
For example,
<fh:prev>http://www.example.com/feed/archive/2005/05</fh:prev>
Note that the fh:prev element, on its own, does not bestow any
semantics on the ordering of entries in a feed; the only purpose of
introducing ordering here is to allow the state of the feed to be
reconstructed.
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6. State Reconstruction
When presented with a partial representation of a feed, a consumer
MAY reconstruct the entire feed in a local store by following these
steps, starting with the subscription document as the current
document:
1. Add all of the entries in the current document to the store.
2. Dereference the fh:prev URI, if present. If it is not present,
stop processing.
3. Using the dereferenced archive document as 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 that consumers MAY cache archive documents and/or use a
different method of reconstructing state, as long as the result is
the same as that achieved by following these steps.
Consumers SHOULD warn users when they do not have the complete state
of a 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.
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7. Examples
Atom 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">
<title>Example Feed</title>
<link href="http://example.org/"/>
<updated>2003-12-13T18:30:02Z</updated>
<author>
<name>John Doe</name>
</author>
<id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>
<history:stateful>true</history:stateful>
<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"/>
<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>
</entry>
</feed>
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Atom Archive 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">
<title>Example Feed</title>
<link href="http://example.org/"/>
<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>
<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 Subscription Document
<?xml version="1.0"?>
<rss version="2.0"
xmlns:history="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, 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>
<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>
(note that the stateful element has been omitted in this instance,
because the presense of the prev element indicates that the feed is
stateful.)
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RSS 2.0 Archive Document
<?xml version="1.0"?>
<rss version="2.0"
xmlns:history="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>
<history:archive/>
<history:prev>http://liftoff.nasa.gov/2003/04/feed</history:prev>
<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
state; to reiterate, they are not required to always store or
reconstruct feed state when conforming to this specification; they
only need inform the user when state is partial.
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.
[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/
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