(19) 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 feed-aware client (Firefox is not a good feed reader)
• Ajax-based feeds 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 [http://www-128.ibm.com/developerworks/library/x-ajaxrss/]
  • Google Reader [http://www.google.com/reader/view/] accesses Google's own copy of the feed (FeedBurner [FeedBurner (1)])
• 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="…" />

(20) 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 [RSS History (1)] caused publishers to look for something better
  • RSS [RSS (1)] had a head start and still is widely used
  • Atom [Atom (1)] 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 [http://www.google.com/reader/view/]
  3. by sharing [http://www.google.com/reader/shared/16601496766743088901] and re-publishing [http://www.google.com/reader/public/atom/user/16601496766743088901/state/com.google/broadcast] items, users are becoming aggregation intermediaries

(26) Syndication Format Protocols

• LiveJournal (very simple text-based protocol)
  • not very good at handling structures (re-inventing for encoding structure)
• Blogger (now at Google [http://code.google.com/apis/blogger/overview.html] after Google bought Pyra [http://web.archive.org/web/20031008161432/http://weblog.siliconvalley.com/column/dangillmor/archives/000802.shtml])
  • no support for titles or any other sort of entry metadata
  • protocol from the early days of blogging before tagging became popular
• MetaWeblog [http://www.xmlrpc.com/metaWeblogApi] (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 [Service Documents (1)] and Category Documents [Category Documents (1)] for additional tasks

(27) 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

(28) Protocol Summary

(29) 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)
  • AtomPub servers are likely to reject operations not satisfying these constraints

(30) 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>

(31) Category Documents

• Categories are important for creating and reading entries
  • they may contain metadata using any classification scheme
• Service Documents [Service Documents (1)] contain a list of allowed categories
• AtomPub defines a document format for standalone category documents
  • a useful interface between AtomPub systems and other systems using classification schemes

(32) 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>

(34) 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