(23) 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 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="…" />

(24) 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

(32) RESTful 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 RESTful 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

(33) 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
  • collections just exist; there is no way of interacting with them
• 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

(36) Collection Management

• Managing collections is outside the scope of AtomPub
  • AtomPub allows interactions with existing collections
  • Collection management is inaccessible or using a proprietary protocol
• Metacollections (collections of collections) can extend AtomPub
  • each member in a metacollection corresponds with a collection
  • accessing a metacollection means managing collections
  • AtomPub's existing features can be reused for collection management
• Ongoing discussion in the Atom community
  • currently there is no widely established method

(37) 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

(38) 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>

(39) 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
  • REST makes it important to make relevant resources accessible

(40) 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>

(42) Atom/AtomPub as Foundation

• Atom/AtomPub can be used as a foundation
  • apply the methodology of RESTful Web Service Design [RESTful Web Service Design (1)]
  • identify missing representations/links/states
  • add these to the Atom/AtomPub model
• Atom has a well-defined extension model (sort of)
  • foreign markup must be silently ignored by Atom processors
  • foreign markup is not allowed to change the semantics of Atom markup
  • every Atom extension must be well-behaving with plain Atom processors
• Atom can serve as a good example for extensible markup
  • if you're not using Atom, consider using Atom's extension model [http://tools.ietf.org/html/rfc4287#section-6]

(43) RESTful Web Service Design

• URIs are used for resource identification [What is REST?; Resource Identification (1)]
• HTTP's methods are used as the uniform interface [What is REST?; Uniform Interface (1)]
• Representations [What is REST?; Self-Describing Messages (1)] can use any syntax and structure
• Hypermedia [What is REST?; Hypermedia Driving Application State (1)] is based on links in representations
• Use transaction resources to create stateless interactions [What is REST?; Stateless Interactions (1)]

(44) RESTful Web Service Design Procedure

1. Figure out the data set
2. Split the data set into resources
3. Name the resources with URIs [What is REST?; Resource Identification (1)]
4. Expose a subset of the uniform interface [What is REST?; Uniform Interface (1)]
5. Design the representation(s) [What is REST?; Self-Describing Messages (1)] accepted from the client
6. Design the representation(s) [What is REST?; Self-Describing Messages (1)] served to the client
7. Design hypermedia integration [What is REST?; Hypermedia Driving Application State (1)] with other resources
8. Figure out the normal control flow [What is REST?; Stateless Interactions (1)]
9. Figure out error conditions

• (This is the ROA methodology)

(45) Watch Caching

• Decent HTTP libraries have built-in caching
  • caching can also be implemented by proxies or reverse proxies
• HTTP has various ways to control caching
  • Expires set an expiration data on a response
  • Cache-Control for various cache control directives
  • ETag for uniquely identifying a resource version
• Caching allows Conditional GETs
  • If-Modified-Since is conditional based on the Last-Modified date
  • If-None-Match is conditional based on the ETag label

(47) Toolboxes and Frameworks

• Toolboxes contain a number of low-level tools
  • tools for essential functionality (HTTP, URI, XML)
  • add-ons for frequent tasks (HTML tidying, XML validation, language tag parsing)
  • diagnostic and test tools (HTTP monitoring, protocol validators)
• Frameworks provide programming/structuring support
  • frameworks encourage and support a certain coding style and structure
  • REST focuses on exposing uniform interfaces and various media types
  • code should be structured by representation, not by function

(48) REST Frameworks

• JAX-RS
• RESTlet (covers any REST, not just RESTful HTTP)
• Ruby on Rails
• Django
• GData