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Network Working Group                                      M. Nottingham
Internet-Draft                                             June 24, 2005
Expires: December 26, 2005

              Feed History: Enabling Stateful Syndication

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   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

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   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

   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",
   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]]

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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,


   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

   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

   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

   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.

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   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.

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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/

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