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Nottingham 3 Internet-Draft June 30, 2005 4 Expires: January 1, 2006 6 Feed History: Enabling Stateful Syndication 7 draft-nottingham-atompub-feed-history-01 9 Status of this Memo 11 By submitting this Internet-Draft, each author represents that any 12 applicable patent or other IPR claims of which he or she is aware 13 have been or will be disclosed, and any of which he or she becomes 14 aware will be disclosed, in accordance with Section 6 of BCP 79. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on January 1, 2006. 34 Copyright Notice 36 Copyright (C) The Internet Society (2005). 38 Abstract 40 This document specifies mechanisms that allow feed publishers to give 41 hints about the nature of the feed's statefulness, and a means of 42 retrieving "missed" entries from a stateful feed. 44 1. Introduction 46 Syndication documents (e.g., those in formats such as Atom and RSS) 47 usually only contain the last several entries in a longer-lived 48 channel (or "feed") of information. Often, consuming software keeps 49 copies of all entries that have been previously seen, effectively 50 keeping a history of the feed's contents. 52 However, not all feeds benefit from this practice; in some, old 53 entries are not relevant to the current contents of the feed. For 54 example, it's not desireable to keep history in this manner with a 55 "top ten" feed; showing old entries would imply that the previous 56 number one is now number eleven, and so forth. 58 Feeds that encourage this practice have a different problem. If 59 consuming software does not poll often enough, some entries may be 60 missed, causing them to be silently omitted. For some applications, 61 this is a serious error on its own. Even in non-critical 62 applications, this phenomenon can cause publishers to make Feed 63 Documents contain more entries than reasonably necessary, just to 64 assure that consumers have an amply large window in which to 65 reconstruct the feed's state. 67 This document specifies mechanisms that allow feed publishers to give 68 hints as to the nature of the feed with regard to state, and a means 69 of retrieving "missed" entries from a stateful feed. Although it 70 refers to Atom normatively, the mechanisms described herein can be 71 used with similar syndication formats, such as the various flavours 72 of RSS. 74 2. Notational Conventions 76 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 77 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 78 document are to be interpreted as described in BCP 14, [RFC2119], as 79 scoped to those conformance targets. 81 In this specification, "subscription document" refers to an Atom Feed 82 Document or similar syndication format (e.g., RSS) that is intended 83 to be subscribed to; i.e., it contains the most recent entries 84 available in the feed. 86 In this specification, "archive document" refers to an Atom Feed 87 Document or similar syndication format (e.g., RSS) that is archived; 88 i.e., the set of entries inside it does not change over time. Note 89 that some entries in the archive document may also be present in the 90 subscription document; in other words, some (but not necessarily all) 91 "live" entries might already be archived. 93 In this specification, "head section" refers to the children of a 94 feed document's document-wide metadata container; e.g., the child 95 elements of the atom:feed element in an Atom Feed Document. 97 This specification uses XML Namespaces to uniquely identify XML 98 element names. It uses the following namespace prefix for the 99 indicated namespace URI; 101 "fh": [[TBD]] 103 This specification uses terms from the XML Infoset. However, this 104 specification uses a shorthand; the phrase "Information Item" is 105 omitted when naming Element Information Items. Therefore, when this 106 specification uses the term "element," it is referring to an Element 107 Information Item in Infoset terms. 109 3. The 'fh:stateful' Element 111 The fh:stateful element indicates whether the Feed is stateful, and 112 MAY occur in a subscription document's head section. Its content 113 MUST be either "true" or "false". Whitespace in its content MUST be 114 ignored by processors. 116 For example, 118 true 120 If the content of the fh:stateful element is "false", it indicates 121 that the subscription document is a complete representation of the 122 entire feed; previous entries SHOULD NOT be considered part of the 123 feed by consumers. 125 For example, a feed that represents a ranking that varies over time, 126 such as "Top Twenty Records" or "Most Popular Items" should be marked 127 with a fh:stateful element containing "false". 129 If the content of the fh:stateful element is "true", it indicates 130 that the subscription document is a potentially partial 131 representation of the entire feed; previous entries MUST be 132 considered part of the feed by consumers. 134 For example, a feed that represents a chronological list, such as 135 "ExampleCo Press Releases" or "Widget Project Updates" should be 136 marked with a fh:stateful element containing "true". 138 A subscription document whose fh:stateful element contains "true" 139 MUST contain a fh:prev element, unless there are no previous entries 140 in the feed. A subscription document whose fh:stateful element 141 contains "false" MUST NOT contain a fh:prev element. 143 4. The 'fh:prev' Element 145 The fh:prev element conveys the location of an archive of previous 146 entries in the feed, and MAY occur in a subscription document's head 147 section. It MUST occur in an archive document's head section, unless 148 there are no previous entries in the feed. 150 Its content MUST be a URI reference indicating the previous archive 151 document's location. 153 For example, 155 http://www.example.com/feed/archive/2005/05 157 5. State Reconstruction 159 When presented with a partial representation of a feed, a consumer 160 MAY reconstruct the entire feed in a local store by following these 161 steps, starting with the subscription document as the current 162 document: 164 1. Add all of the entries in the current document to the store. 165 2. Dereference the fh:prev URI, if present. If it is not present, 166 stop processing. 167 3. Using the dereferenced archive document as the current document, 168 start at step one (i.e., apply these steps recursively). 170 An implementation MAY stop when it encounters an fh:prev URI whose 171 entries have been successfully stored beforehand when following this 172 process. 174 Note that implementations MAY cache archive documents and/or use a 175 different method of reconstructing state, as long as the result is 176 the same as that achieved by following these steps. 178 User-Agents SHOULD warn when they do not have the complete state of a 179 feed (e.g., by alerting the user that an archive document is 180 unavailable, or inserting pseudo-entries that inform the user that 181 some entries may be missing). 183 Note that publishers are not required to make all archive documents 184 available. 186 6. Examples 188 Atom Subscription Document with History 190 191 193 Example Feed 194 195 2003-12-13T18:30:02Z 196 197 John Doe 198 199 urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6 200 true 201 http://example.org/2003/11/index.atom 203 204 Atom-Powered Robots Run Amok 205 206 urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a 207 2003-12-13T18:30:02Z 208 Some text in a new, fresh entry. 209 211 212 Atom Archive Document with History 214 215 217 Example Feed 218 219 2003-11-24T12:00:00Z 220 221 John Doe 222 223 urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6 224 http://example.org/2003/10/index.atom 226 227 Atom-Powered Robots Scheduled To Run Amok 228 229 urn:uuid:cd3272ef-b09c-42fd-806b-e25580e59b39 230 2003-11-24T12:00:00Z 231 Some text from an old, different entry. 232 234 235 RSS 2.0 Subscription Document with History 237 238 239 240 Liftoff News 241 http://liftoff.msfc.nasa.gov/ 242 Liftoff to Space Exploration. 243 en-us 244 Tue, 10 Jun 2003 04:00:00 GMT 245 Tue, 10 Jun 2003 09:41:01 GMT 246 http://blogs.law.harvard.edu/tech/rss 247 Weblog Editor 2.0 248 editor@example.com 249 webmaster@example.com 250 true 251 http://liftoff.msfc.nasa.gov/2003/05/feed.rss> 253 254 Star City 255 http://liftoff.msfc.nasa.gov/2003/06/news-starcity 256 How do Americans get ready to work with Russians 257 aboard the International Space Station? They take a crash course 258 in culture, language and protocol at Russia's Star 260 City. 261 Tue, 03 Jun 2003 09:39:21 GMT 262 http://liftoff.msfc.nasa.gov/2003/06/03.html#item573 263 264 265 266 RSS 2.0 Archive Document with History 268 269 270 271 Liftoff News 272 http://liftoff.msfc.nasa.gov/ 273 Liftoff to Space Exploration. 274 en-us 275 Tue, 30 May 2003 08:00:00 GMT 276 Tue, 30 May 2003 10:31:52 GMT 277 http://blogs.law.harvard.edu/tech/rss 278 Weblog Editor 2.0 279 editor@example.com 280 webmaster@example.com 281 true 282 http://liftoff.msfc.nasa.gov/2003/04/feed.rss> 284 285 Sky watchers in Europe, Asia, and parts of 286 Alaska and Canada will experience a partial eclipse of the Sun 287 on Saturday, May 31st. 288 Fri, 30 May 2003 11:06:42 GMT 289 http://liftoff.msfc.nasa.gov/2003/05/30.html#item572 290 291 292 The Engine That Does More 293 http://liftoff.msfc.nasa.gov/2003/05/news-VASIMR.asp 294 Before man travels to Mars, NASA hopes to 295 design new engines that will let us fly through the Solar 296 System more quickly. The proposed VASIMR engine would do 297 that. 298 Tue, 27 May 2003 08:37:32 GMT 299 http://liftoff.msfc.nasa.gov/2003/05/27.html#item571 300 301 302 304 7. Security Considerations 306 Feeds using the mechanisms described here could be crafted in such a 307 way as to cause a User-Agent to initiate excessive (or even an 308 unending sequence of) network requests, causing denial of service 309 (either to the User-Agent, the target server, and/or intervening 310 networks). This risk can be mitigated by requiring user intervention 311 after a certain number of requests, or by limiting requests either 312 according to a hard limit, or with heuristics. 314 User-Agents should be mindful of resource limits when storing feed 315 state; to reiterate, they are not required to always store or 316 reconstruct feed state when conforming to this specification; they 317 only need inform the user when state is partial. 319 8. Normative References 321 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 322 Requirement Levels", BCP 14, RFC 2119, March 1997. 324 Author's Address 326 Mark Nottingham 328 Email: mnot@pobox.com 329 URI: http://www.mnot.net/ 331 Intellectual Property Statement 333 The IETF takes no position regarding the validity or scope of any 334 Intellectual Property Rights or other rights that might be claimed to 335 pertain to the implementation or use of the technology described in 336 this document or the extent to which any license under such rights 337 might or might not be available; nor does it represent that it has 338 made any independent effort to identify any such rights. 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Please address the information to the IETF at 353 ietf-ipr@ietf.org. 355 Disclaimer of Validity 357 This document and the information contained herein are provided on an 358 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 359 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 360 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 361 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 362 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 363 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 365 Copyright Statement 367 Copyright (C) The Internet Society (2005). This document is subject 368 to the rights, licenses and restrictions contained in BCP 78, and 369 except as set forth therein, the authors retain all their rights. 371 Acknowledgment 373 Funding for the RFC Editor function is currently provided by the 374 Internet Society.