Network Working Group M. Nottingham
Internet-Draft June 24, 2005
Expires: December 26, 2005
Feed History: Enabling Stateful Syndication
draft-nottingham-atompub-feed-history-00
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on December 26, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document specifies mechanisms that allow feed publishers to give
hints about the nature of the feed's statefulness, and a means of
retrieving "missed" entries from a stateful feed.
1. Introduction
RFCxxxx describes a Feed Document as 'a representation of an Atom
feed, including metadata about the feed, and some or all of the
entries associated with it'. Because Feed Documents usually only
Nottingham Expires December 26, 2005 [Page 1]
Internet-Draft Feed History June 2005
contain the last several entries in a logical feed, consuming
software often keeps copies of all entries that have been previously
seen, effectively keeping a history of its contents.
However, not all feeds benefit from this practice; in some, old
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; it is not desireable to show old entries, because it
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 mechanisms that allow 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.
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. Note that these mechanisms MAY also be used in other XML-
based syndication formats, such as the various flavours of RSS.
In this specification, "head section" refers to the children of the
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 to uniquely identify XML
element names. It uses the following namespace prefix for the
indicated namespace URI;
"atom": [[TBD]]
"fh": [[TBD]]
Nottingham Expires December 26, 2005 [Page 2]
Internet-Draft Feed History June 2005
3. The Stateful Flag
The stateful flag is an XML element in a Feed Document's head section
whose content is either "0" or "1". Whitespace in its content MUST
be ignored by processors.
For example,
<fh:Stateful>1</fs:Stateful>
If the content of the stateful flag is "0", it indicates that the
Feed 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 stateful flag of "0".
If the content of the stateful flag is "1", it indicates that the
Feed Document is a potentially partial representation of the entire
feed; previous entries MUST be considered part of the feed by
consumers.
For example, a feed that represents a chronological list, such as
"ExampleCo Press Releases" or "Widget Project Updates" should be
marked with a stateful flag of "1".
4. The 'this' and 'prev' Link Relations
A Feed Document containing a stateful flag with the content "1"
SHOULD also contain an atom:link element with the relation "this",
and optionally a Link element with the relation "prev", in its head
section.
The value of the "this" link relation's href attribute MUST be a URI
indicating a permanent location that is unique to that Feed Document
instance; i.e., the content obtained by dereferencing that URI SHOULD
NOT change over time. This URI can be thought of as pointing to a
snapshot of the feed at a particular point in time.
The value of the "prev" link relation's href attribute MUST be a URI
indicating the location of the previous representation of the feed;
i.e., the last Feed Document's "this" URI.
Nottingham Expires December 26, 2005 [Page 3]
Internet-Draft Feed History June 2005
For example,
<atom:link rel="this" href="http://example.com/feed/2005/05/20"/>
<atom:link rel="prev" href="http://example.com/feed/2005/05/13"/>
5. 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 most recent Feed Document available:
1. Add all of the entries in the Feed Document to the store.
2. Dereference the 'prev' link in the Feed Document's head, if
present. If it is not present, stop processing.
3. Using the new Feed Document, start at step one (i.e., apply these
steps recursively).
Note that implementations MAY cache previous Feed 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.
User-Agents SHOULD warn when they do not have the complete state of a
feed (e.g., by alerting the user that a Feed 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 previous Feed
Documents available.
6. IANA Considerations
[[register the link relations]]
7. Security Considerations
Feed Documents using the mechainsms described here could be crafted
in such a way as to cause a User-Agent to initiate excessive (or even
an unending sequence of) network requests, causing denial of service
(either to the User-Agent, the target server, and/or intervening
networks). This risk can be mitigated by requiring user intervention
after a certain number of requests, or by limiting requests either
according to a hard limit, or with heuristics.
User-Agents 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.
Nottingham Expires December 26, 2005 [Page 4]
Internet-Draft Feed History June 2005
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Author's Address
Mark Nottingham
Email: mnot@pobox.com
URI: http://www.mnot.net/
Nottingham Expires December 26, 2005 [Page 5]
Internet-Draft Feed History June 2005
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Nottingham Expires December 26, 2005 [Page 6]
