Abstract

For many information sources on the Web, it is useful to have some standardized way of subscribing to information updates. Syndication formats such as RSS and Atom can be used by these information sources to publish a feed of updated information items. While RSS and Atom are read-only formats, the Atom Publishing Protocol (AtomPub) build on top of Atom and provides a protocol for submitting new items to feeds.

Content Feeds

• Early Web content was static or updated very infrequently
  • there was not yet the requirement to reuse content in different contexts
• Frequently updated Web content quickly became a very common scenario
  • as commercial interests took over the Web, users should have a reason to re-visit a site
  • presenting a steady stream of new content creates the image of a live Web site
• There are two major use cases where HTML is not sufficient
  1. users want an efficient way to get the updated content from a site
  2. sites want to aggregate updated content from other sites and re-publish it
• Syndication Formats are designed to support these two use cases
  • container formats for updated items
  • a small amount of metadata about these items for automated processing

RSS History

• The Myth of RSS Compatibility provides a good overview
• RSS is a schoolbook example for why standards are a good thing
  • RSS 0.9 was created for the My Netscape portal in March 1999
  • RSS 0.91 (a simplification) was introduced in July 1999 (as an interim solution)
  • the AOL/Netscape merger removed the format from the company's portal
  • RSS was without an owner, and different parties claimed/denied ownership
  • RSS 1.0 was created by an informal developer group
  • RSS 0.92 (and 0.93 and 0.94) were published without acknowledging RSS 1.0
  • finally, RSS 2.0 was released as a follow-up to the RSS 0.9x versions
• Using RSS has become an exercise in managing a menagerie of versions

RSS 0.9

• RSS means RDF Site Summary (or Rich Site Summary?)
  • based on an RDF draft and not compatible with the final RDF specification
  • RDF was considered too cumbersome and unstable
  • 0.90 (proto-RDF) was quickly replaced by the non-RDF 0.91 version
• RSS 0.92+ versions were developed as unilateral specifications
  • starting with RSS 0.91, RSS means Rich Site Summary
  • it is no longer built on RDF, instead it simply uses XML
  • the 0.9x branch eventually was renamed to RSS 2.0

RSS 0.91 Example

<rss version="0.91">
 <channel>
  <title>XML.com</title>
  <link>http://www.xml.com/</link>
  <description>XML.com features a rich mix of information and services for the XML community.</description>
  <language>en-us</language>
  <item>
   <title>Normalizing XML, Part 2</title>
   <link>http://www.xml.com/pub/a/2002/12/04/normalizing.html</link>
   <description>In this second and final look at applying relational normalization techniques to W3C XML Schema data modeling, Will Provost discusses when not to normalize, the scope of uniqueness and the fourth and fifth normal forms.</description>
  </item>

RSS 1.0

• RSS means RDF Site Summary (this time for real)
  • based on the final RDF specification and thus incompatible with any RSS 0.9
  • developed when the Semantic Web and RDF were first heavily marketed (1999)
  • RDF was expected to become the format for metadata on the Web
• RSS 1.0 makes heavy use of XML Namespaces
• RSS 1.0 introduces features which were not present in 0.91
  • date information for published items (very relevant for news feeds)
  • individual authors for various items in a feed
• RSS 1.0 is the latest version of RDF-based RSS
  • the Semantic Web wave is not over yet, but RDF has lost its novelty appeal
  • for a more XML-oriented encoding, RSS 0.9 provides a better foundation

RSS 1.0 Example

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="http://purl.org/rss/1.0/" xmlns:dc="http://purl.org/dc/elements/1.1/">
 <channel rdf:about="http://www.xml.com/cs/xml/query/q/19">
  <title>XML.com</title>
  <link>http://www.xml.com/</link>
  <description>XML.com features a rich mix of information and services for the XML community.</description>
  <language>en-us</language>
  <items>
   <rdf:Seq>
    <rdf:li rdf:resource="http://www.xml.com/pub/a/2002/12/04/normalizing.html"/>
    <rdf:li rdf:resource="http://www.xml.com/pub/a/2002/12/04/som.html"/>
    <rdf:li rdf:resource="http://www.xml.com/pub/a/2002/12/04/svg.html"/>
   </rdf:Seq>
  </items>
 </channel>
 <item rdf:about="http://www.xml.com/pub/a/2002/12/04/normalizing.html">
  <title>Normalizing XML, Part 2</title>
  <link>http://www.xml.com/pub/a/2002/12/04/normalizing.html</link>
  <description>In this second and final look at applying relational normalization techniques to W3C XML Schema data modeling, Will Provost discusses when not to normalize, the scope of uniqueness and the fourth and fifth normal forms.</description>
  <dc:creator>Will Provost</dc:creator>
  <dc:date>2002-12-04</dc:date>
 </item>

RSS 2.0

• RSS now means Really Simple Syndication
  • RSS 2.0 is the continuation of the 0.91 branch (which dropped RDF)
  • together with RSS 1.0 it is the most popular version of RSS
  • migration from 0.91 to 2.0 is easily possible
• RSS 2.0 tries to avoid the use of XML Namespaces
• RSS 2.0 is increasingly used with extensions for vendor-specific information
  • the RSS core is minimal, so many applications need extensions
  • many extensions have overlapping functionality
  • most extensions have unclear semantics and unclear versioning policies

RSS 2.0 Example

<rss version="2.0" xmlns:dc="http://purl.org/dc/elements/1.1/">
 <channel>
  <title>XML.com</title>
  <link>http://www.xml.com/</link>
  <description>XML.com features a rich mix of information and services for the XML community.</description>
  <language>en-us</language>
  <item>
   <title>Normalizing XML, Part 2</title>
   <link>http://www.xml.com/pub/a/2002/12/04/normalizing.html</link>
   <description>In this second and final look at applying relational normalization techniques to W3C XML Schema data modeling, Will Provost discusses when not to normalize, the scope of uniqueness and the fourth and fifth normal forms.</description>
   <dc:creator>Will Provost</dc:creator>
   <dc:date>2002-12-04</dc:date>
  </item>

The Case for Content Management

• RSS is very rarely produced by hand
  • by definition, RSS contains redundant information for a specific purpose
• If a Content Management System (CMS) is used, RSS can be generated
  • basic metadata can be generated by the CMS (title, author, date)
  • better tagging of content results in better tagging of feeds
  • well-tagged feeds are better foundations for large-scale reuse of feed items
• Blogging is simply a specialized case of a CMS
  • Web-based interface for controlling everything
  • strictly time-ordered sequenced of published items
  • navigation features primarily based on the time-specific facets of the blog (maybe tags)
  • all blogging tools include feed support

Consuming RSS

• RSS feeds often have quality problems
  • surprisingly often feeds do not even deliver well-formed XML
  • the use of embedded markup in RSS is not well-defined
• Writing an RSS reader from scratch is not a good idea
• There are three major tasks which RSS readers must do
  1. accept non-XML RSS feeds and fix them to be XML
  2. look at the feed contents and bring them into a unified form
  3. produce a unified view of feeds regardless of the RSS version

RSS Technical Problems

• What to put into an item's description
  • the fundamental question is whether a description is text or HTML
  • if there is no well-defined way, then interpretation is client-specific

    <description>This is a <em>very important</em> blog post …

    <description>This is a &lt;em>very important&lt;/em> blog post …

    <description>This is a blog post about <em> in RSS feeds …

    <description>This is a blog post about &lt;em> in RSS feeds …

    <description>This is a blog post about &amp;lt;em> in RSS feeds …

• Underspecified and not very robust in various other areas
  • broken RSS is accepted by most readers (but fixing it can change the interpretation)
  • the interpretation of relative URIs is not mentioned in the specifications
  • some minimal semantics (classification) for items would be very useful

RSS Political Problems

• Multiple and incompatible RSS History are still in widespread use
  • RSS 1.0 and RSS 2.0 are incompatible by design (RDF vs. non-RDF)
  • none of the RSS versions is maintained by a universally accepted standards body
• None of the specifications is being updated or fixed
  • some of the lessons learned by RSS deployment are not used in a new version
  • it is unlikely that a new version will be produced which merges the RSS landscape
• Invent something new instead of trying to fix RSS
  • Atom started in 2003 (called Echo at first)
  • W3C or IETF would have been promising candidates for a new RSS
  • W3C is more formal, IETF is more developer-centered
  • IETF was chosen over W3C because the of Atom community's preferences

Atom History

• RSS's shortcomings were very apparent and could not be fixed
• In mid-2003, discussions started about an improved format
• It also became apparent that the format should have a protocol
• Atom 0.3 was released in December 2003 but had no formal home
• IETF was chosen as the new home with a working group in June 2004
• RFC 4287 was published in December 2005
• AtomPub has been published as RFC 5032 in October 2007

Atom vs. RSS

• Standardized by the IETF (well-defined process)
• Classification of entries (user-defined categories)
• More XML-like markup design (more nesting)
• Namespaces are used and supported as standard mechanism
• Atom feeds must be well-formed XML (there even is a schema)
• Interpretation of content is well-defined (various content types)
• Support for xml:lang and xml:base

Atom Example

<feed xmlns="http://www.w3.org/2005/Atom" xml:lang="en-us">
 <title>ongoing</title>
 <id>http://www.tbray.org/ongoing/</id>
 <link rel='self' href="http://www.tbray.org/ongoing/ongoing.atom"/>
 <updated>2007-04-11T12:55:09-07:00</updated>
 <author>
  <name>Tim Bray</name>
 </author>
 <subtitle>ongoing fragmented essay by Tim Bray</subtitle>
 <entry xml:base="When/200x/2007/04/02/">
  <title>Atom Publishing Protocol Interop!</title>
  <id>http://www.tbray.org/ongoing/When/200x/2007/04/02/APP-Interop</id>
  <published>2007-04-02T13:00:00-07:00</published>
  <updated>2007-04-10T14:24:00-07:00</updated>
  <category scheme="http://www.tbray.org/ongoing/What/" term="Technology/Atom"/>
  <category scheme="http://www.tbray.org/ongoing/What/" term="Technology"/>
  <category scheme="http://www.tbray.org/ongoing/What/" term="Atom"/>
  <content type="xhtml">
   <div xmlns="http://www.w3.org/1999/xhtml">
    <p>Mark your calendar: <a href="http://www.intertwingly.net/wiki/pie/April2007Interop">April 16-17 at Google</a>. <em>Everybody</em> is invited, provided they bring along an APP implementation, client or server. This was just announced a couple of days ago, and as I write this there are already <s>six</s> twelve client and <s>seven</s> fourteen server implementations signed up to be there and try to <a href="http://www.intertwingly.net/wiki/pie/InteropGrid">fill in the grid</a>. Let’s drop some names, in alphabetical order: AOL, Flock, Google, IBM, Lotus, Microsoft, Oracle, O’Reilly, Six Apart, Sun, WordPress. Um, have I mentioned that the APP is going to be huge?</p>
   </div>
  </content>
 </entry>
</feed>

Atom Content

• RSS had no safe way of finding out what an entry's content is
  • this led to different implementations using smart ways of what the RSS author really wanted
  • one of Atom's main goals was to improve this in a well-defined way
  • Atom allows escaped markup (the only way to include non-XML HTML in an XML format)
• Each content element should have a type (the default is text)
• Atom's content interpretation algorithm (use first applicable rule):
  1. if type is text, no child elements are allowed (plain text content)
  2. if type is html then RSS's method of escaped markup is used
  3. if type is xhtml then there must be an div containing XHTML markup
  4. if type is an XML MIME type then the content should be treated as this type
  5. if type starts with text/ then no child elements are allowed
  6. for all other values, the content must be an base64-encoded entity of the specified MIME type

Atom Content Examples

<content type="xhtml">
  <div xmlns="http://www.w3.org/1999/xhtml">
    One <strong>bold</strong> foot forward
  </div>
</content>

<content>The "atom:content" element either contains or links to the content of the entry. The content of atom:content is Language-Sensitive.</content>

<content type="html">The &lt;code>atom:content&lt;/code> element either contains or links to the content of the entry. The content of &lt;code>atom:content&lt;/code> is &lt;a href="http://www.ietf.org/rfc/rfc3066.txt">Language-Sensitive&lt;/a>.</content>

<content type="image/png">
iVBORw0KGgoA … TAAAAAElFTkSuQmCC
</content>

<content src="image.png" type="image/png"/>

Atom Categories

• Atom allows to assign categories to entries
  • each category element must have a term attribute for the category
  • an optional scheme identifies the categorization scheme (ontology, taxonomy, …)
  • an optional label attribute provides a human-readable label for the category
• AtomPub defines a document format for Category Documents
• Three different cases of categorization can be distinguished
  1. use a well-known scheme (such as Dublin Core)
  2. use a private but well-designed scheme (which has a URI and can be reused reliably)
  3. use tags without schemes, which then are little more than content labels
• Widely-known tags are not easy to handle
  • they are more than just privately assigned tags
  • there is no formal scheme for them, just an emerging consensus

Switching from RSS to Atom

• Generate both feeds but serve RSS with a HTTP redirect (301)
  • old subscribers with broken clients can still use the RSS feed
  • old subscribers with correct clients will use the Atom feed
• Atom exposes more information than RSS (category for tags)
  • the mapping of publishing info to the feed has to be changed/extended
  • for standard metadata use Atom's built-in metadata elements
  • for application-specific metadata consider reusing an existing metadata schema
• Atom can be used to publish snippets as well as full content
  • content allows any type of content to be used and may contain a complete entry
  • summary allows only text and should provide a condensed version of an entry
  • some Atom sources publish two feeds for summaries and content
• Generate good Atom and downgrade it to RSS 1.0 & 2.0

End-User Aggregation

• Users often have a small number of preferred Web sites
  • news can be tracked by subscribing to their feeds
  • this requires an RSS-aware client (Firefox is not a good RSS reader)
• AJAX-based RSS could be implemented as a client-side application
  • retrieve the feed, transform it to HTML, and render the result
  • but the security restriction of XMLHttpRequest gets into the way
  • there is a way around this as used in the Ajax RSS reader
• How can users find a feed?
  • feeds have URIs (they are Web resources) and can be referenced from within HTML

<link rel="alternate" type="application/rdf+xml" title="…" href="…" />
<link rel="alternate" type="application/rss+xml" title="…" href="…" />

<link rel="alternate" type="application/atom+xml" title="…" href="…" />

Aggregation Intermediaries

• RSS is a frequently used format between content providers
  • news agencies want to distribute news items as easy as possible
  • by using a single format, producers and consumers can more easily interoperate
• The RSS History caused publishers to look for something better
  • RSS had a head start and still is widely used
  • Atom is a much better format and will eventually replace RSS
• User-configured portals are the merger between both scenarios
  1. users select a number of feeds as their preferred information sources
  2. the feeds are presented on a single personalized Web page
  3. by sharing and re-publishing items, users are becoming aggregation intermediaries

Syndication Format Protocols

• LiveJournal (very simple text-based protocol)
  • not very good at handling structures (re-inventing for encoding structure)
• Blogger (now at Google after Google bought Pyra)
  • no support for titles or any other sort of entry metadata
  • protocol from the early days of blogging before tagging became popular
• MetaWeblog (an attempt to improve Blogger)
  • extends Blogger using a very bad design (RSS XML as XML-RPC structure encoded as XML)
• Atom Publishing Protocol (AtomPub) is an attempt to provide a clean alternative
  • use the same document structures for feeds and the protocol interacting with them
  • use a REST approach to provide a simple and Web-compatible protocol
  • add Service Documents and Category Documents for additional tasks

RESTified Syndication

• Atom is a format for retrieving a set of entries as a feed document
  • feeds often are time-based and are refreshed periodically or whenever needed
  • feeds can use any other strategy for deciding what to publish
• Read-only access to feeds should be complemented by full access
  • full access needs the CUD out of the CRUD set of operations
  • many Web-centric technologies try to build on the Web's REST model of interaction
• AtomPub builds on Atom and adds a REST-based protocol on top of it
  • POST for creating new entries (sending the request to the collection)
  • PUT for updating existing entries (overwriting the existing entry)
  • DELETE for deleting entries from a collection

Collections, Members, Entries, Media

• AtomPub's top-level concept is a collection
  • collections are used for managing and organizing members
  • Atom feed documents are the representation of collections
• Members of a collection can be entry and media resources
  • entry resources represent metadata and are represented as Atom entries
  • media resources can have any media type and are the data described by entries
  • a media link entry is an entry associated with a member

Protocol Summary

Service Documents

Service Documents represent server-defined groups of Collections, and are used to initialize the process of creating and editing resources.

• The real top-level construct of AtomPub is the workspace
  • collections on a server are organized into different workspaces
  • workspaces have no AtomPub semantics and no operations can be performed on them
• Service documents list constraints on the members of collections
  • accept specifies a comma-separated list of media ranges (with entry as special value)
  • categories defines the list of categories that can be applied to members (can be fixed)
  • APP servers are likely to reject operations not satisfying these constraints

Service Document Example

<service xmlns="http://purl.org/atom/app#" xmlns:atom="http://www.w3.org/2005/Atom">
 <workspace>
  <atom:title>Main Site</atom:title>
  <collection href="http://example.org/reilly/main">
   <atom:title>My Blog Entries</atom:title>
   <categories href="http://example.com/cats/forMain.cats"/>
  </collection>
  <collection href="http://example.org/reilly/pic">
   <atom:title>Pictures</atom:title>
   <accept>image/*</accept>
  </collection>
 </workspace>
 <workspace>
  <atom:title>Side Bar Blog</atom:title>
  <collection href="http://example.org/reilly/list">
   <atom:title>Remaindered Links</atom:title>
   <accept>entry</accept>
   <categories fixed="yes">
    <atom:category scheme="http://example.org/extra-cats/" term="joke"/>
    <atom:category scheme="http://example.org/extra-cats/" term="serious"/>
   </categories>
  </collection>
 </workspace>
</service>

Category Documents

• Categories are important for creating and reading entries
  • they may contain metadata using any classification scheme
• Service Documents contain a list of allowed categories
• APP defines a document format for standalone category documents
  • a useful interface between APP systems and other systems using classification schemes

Category Document Example

<app:categories xmlns:app="http://purl.org/atom/app#" xmlns="http://www.w3.org/2005/Atom" fixed="yes" scheme="http://example.com/cats/big3">
 <category term="animal"/>
 <category term="vegetable"/>
 <category term="mineral"/>
</app:categories>

Semantic Web Light

• Syndication creates representations for universal concepts
• Atom adds some concepts to RSS's model
• Syndication revolves around the idea of interacting with items
• Atom-based interaction is one way of implementing REST
• For more semantics, Atom is only the foundation