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 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 August 30, 2006. 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. Nottingham Expires August 30, 2006 [Page 1] Internet-Draft Feed History February 2006 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 Nottingham Expires August 30, 2006 [Page 2] Internet-Draft Feed History February 2006 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. Nottingham Expires August 30, 2006 [Page 3] Internet-Draft Feed History February 2006 "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). Nottingham Expires August 30, 2006 [Page 4] Internet-Draft Feed History February 2006 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, 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: Nottingham Expires August 30, 2006 [Page 5] Internet-Draft Feed History February 2006 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. Nottingham Expires August 30, 2006 [Page 6] Internet-Draft Feed History February 2006 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 Example Feed 2003-12-13T18:30:02Z John Doe urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6 Atom-Powered Robots Run Amok urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a 2003-12-13T18:30:02Z Some text. Nottingham Expires August 30, 2006 [Page 7] Internet-Draft Feed History February 2006 Incremental Atom Feed: Subscription Document Example Feed 2003-12-13T18:30:02Z John Doe urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6 Atom-Powered Robots Run Amok urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a 2003-12-13T18:30:02Z Some text. Incremental Atom Feed: Archive Document Example Feed 2003-11-24T12:00:00Z John Doe urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6 Atom-Powered Robots Scheduled To Run Amok urn:uuid:cdef5c6d5-gff8-4ebb-assa-80dwe44efkjo 2003-11-24T12:00:00Z Some text from an old, different entry. Nottingham Expires August 30, 2006 [Page 8] Internet-Draft Feed History February 2006 Incremental RSS 2.0 Feed: Subscription Document Liftoff News http://liftoff.nasa.gov/ Liftoff to Space Exploration. en-us Tue, 10 Jun 2003 04:00:00 GMT Tue, 10 Jun 2003 09:41:01 GMT http://blogs.law.harvard.edu/tech/rss Weblog Editor 2.0 editor@example.com webmaster@example.com Star City http://liftoff.nasa.gov/2003/06/news-starcity 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 Tue, 03 Jun 2003 09:39:21 GMT http://liftoff.nasa.gov/2003/06/03.html#item573 Nottingham Expires August 30, 2006 [Page 9] Internet-Draft Feed History February 2006 Incremental RSS 2.0 Feed: Archive Document Liftoff News http://liftoff.nasa.gov/ Liftoff to Space Exploration. en-us Tue, 30 May 2003 08:00:00 GMT Tue, 30 May 2003 10:31:52 GMT http://blogs.law.harvard.edu/tech/rss Weblog Editor 2.0 editor@example.com webmaster@example.com Sky watchers in Europe, Asia, and parts of Alaska and Canada will experience a partial eclipse of the Sun on Saturday, May 31st. Fri, 30 May 2003 11:06:42 GMT http://liftoff.nasa.gov/2003/05/30.html#item572 The Engine That Does More http://liftoff.nasa.gov/2003/05/news-VASIMR.asp 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. Tue, 27 May 2003 08:37:32 GMT http://liftoff.nasa.gov/2003/05/27.html#item571 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 Nottingham Expires August 30, 2006 [Page 10] Internet-Draft Feed History February 2006 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] [2] 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. Nottingham Expires August 30, 2006 [Page 11] Internet-Draft Feed History February 2006 Author's Address Mark Nottingham Email: mnot@pobox.com URI: http://www.mnot.net/ Nottingham Expires August 30, 2006 [Page 12] Internet-Draft Feed History February 2006 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. 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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 (2006). 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 August 30, 2006 [Page 13]