• The source code is available at https://github.com/zetaben/opds-validator
• Online validation is available at http://opds-validator.appspot.com/

This is a major step forward for the OPDS ecosystem since this validator not only includes the Relax NG validation that was previously released, but also check various requirements extracted from the 1.1 specification.

I highly recommend anyone with an OPDS catalog in production or in development to check their feeds with the validator to ensure their full compliance with the OPDS 1.1 specification.

OPDS Catalogs is a component of the Internet Archive’s BookServer Project, a framework supporting open standards for discovering, lending, and selling books and other digital content on the web.

The new OPDS 1.1 release significantly enhances the usability of OPDS catalogs by supporting faceted search and browsing for easier navigation, and indirect acquisition for better handling of payments, content bundling, and content protection schemes.  A list of the enhancements from v1.0 is available at the OPDS changelog.

FOR DISTRIBUTORS:

For aggregators, publishers, and distributors, OPDS v1.1 offers unparalleled opportunities to market digital content directly to consumers, without ceding control to intermediaries.

Hadrien Gardeur, CEO of Feedbooks, and one of the primary project leads for OPDS, says, “Feedbooks distribute over 3 million books every month, providing access to the catalogs of the major US, French and German publishers, through more than a dozen reading clients.  This is only possible through OPDS.  The new version of OPDS will greatly improve the discovery and purchase of ebooks thanks to features like faceted search and browsing and indirect acquisition.”

Aaron Miller of Firebrand Technologies says, “OPDS will allow us to create standardized feeds so that all of our titles can be embedded in other systems.  This includes our mobile partners, such as Bluefire Reader, as well as various web communities.  With the new detail that the spec provides about indirect acquisition, it is much more applicable to our NetGalley service, which relies on various forms of DRM for distributed files.”

OPDS is seeing rapid international adoption as well, for content distributors and digital libraries.

“Digital publishing is booming in Asia and we see rapidly growing adoption of OPDS, particularly in Japan and Taiwan.  We look forward helping OPDS gain even further traction with influential publishers, content aggregators, digital distributors as well as the industry associations throughout Asia,” say Daihei Shiohama and Chris Rowan of Inbeatween, a Japanese digital content distributor.

FOR READING SYSTEMS -

For reading systems, OPDS offers a unique opportunity to present content from multiple partners, as well as direct user selection, providing a way of escaping the restraints that siloed or locked content platforms impose.

Patrick Thompson of Inkstone Software says, “Faceted browsing allows you to fly through book catalogs to find just the right book you want to read.  It is as if catalog browsing has gone from 2D to 3D.  We look forward to providing OPDS 1.1 faceted browsing in our QuickReader and MegaReader applications, making it easier to discover great books, new and old.”

Tim Jones, Lead Web Engineer for Bluefire Reader states, "For me, the biggest advance in OPDS 1.1 is the support for search facets which allow bookstores to direct their customers towards books they might be interested in with new levels of clarity and exploration.  It allows users to explore all sorts of relevant results that they may not have known about before."

Tiffany Wong of Aldiko, a leading Android ereader app, writes: “We're very excited about the OPDS 1.1.  New features like facets and the new link relations will greatly enhance content discovery experience.”

FOR FURTHER INFORMATION -

References for the OPDS Catalogs specification and opportunities to participate in the development of the Catalog project and upcoming work are located at http://www.opds-spec.org.

• support for facets (Spec)
• support for indirect acquisition (Spec #1, Spec #2 & Spec #3)
• recommendations for advanced search (Spec)
• new rel value for recommendations (Spec)
• new media parameter to identify navigation/acquisition feeds (Spec)
• updated Relax NG Schema (Spec)

For a quick introduction to these new features, you can read the following posts:

• Faceted Search & Browsing
• Understanding Acquisition Links
• Creating a better OPDS Catalog Root

Every publication available in an OPDS catalog is required to have at least one acquisition link with:

• a rel attribute, which indicates the type of acquisition that a client can expect
• a type attribute, which indicates the format of the publication

The following rel values are available:

• http://opds-spec.org/acquisition/open-access for Open Access publications
• http://opds-spec.org/acquisition/buy for publications that you can buy
• http://opds-spec.org/acquisition/borrow for publications that you can borrow
• http://opds-spec.org/acquisition/subscribe for publications that you can subscribe to
• http://opds-spec.org/acquisition/sample to sample a publication
• http://opds-spec.org/acquisition when none of the other values are appropriate or you don't have additional information

In the spec we offer the following example:

<link rel="http://opds-spec.org/acquisition" 
      type="video/mp4v-es" 
      href="/content/free/4561.mp4"/>

If the same publication is available in multiple formats as unique resources, they would simply be listed using multiple acquisition links in the same entry:

<link rel="http://opds-spec.org/acquisition/borrow" 
      href="/content/borrow/4561.mobi"
      type="application/x-mobipocket-ebook"/>
 
<link rel="http://opds-spec.org/acquisition/borrow" 
      href="/content/borrow/4561.epub"
      type="application/epub+zip"/>

Price element

If the publication requires payment, at least one opds:price element is required, where the value is the price and the currencycode attribute indicates the currency:

<link rel="http://opds-spec.org/acquisition/buy"
      href="/product/song1.mp3"
      type="audio/mpeg">
  <opds:price currencycode="USD">1.99</opds:price> 
</link>

The price element can only be used with the following rel values:

• http://opds-spec.org/acquisition/buy
• http://opds-spec.org/acquisition/subscription
• http://opds-spec.org/acquisition/sample

Indirect Acquisition

In certain cases, the acquisition link doesn't point directly to the publication.

This could be the case for example with a bundle, where a zip file is distributed with the same publication available in different formats inside.

In this case, an opds:indirectAcquisition element is required, where the type attribute is the media type:

<link type="application/zip" rel="http://opds-spec.org/acquisition/open-access" href="/item/1111/buy/">
  <opds:indirectAcquisition type="application/epub+zip" />
  <opds:indirectAcquisition type="application/pdf" />
  <opds:indirectAcquisition type="application/x-mobipocket-ebook" />
</link>

This could also be the case of a DRM format, such as ACS4 for EPUB & PDF, where the client acquires the final publication through an ACSM file:

<link type="application/vnd.adobe.adept+xml" rel="http://opds-spec.org/acquisition/buy" href="/item/4141/buy">
  <opds:price currencycode="USD">4.29</opds:price>
  <opds:indirectAcquisition type="application/epub+zip" />
</link>

Aside from container formats, indirect acquisition is also widely used to handle a transaction in the browser. In this case, the atom:link points to a webpage, while the opds:indirectAcquisition element indicates the format that the client can expect to acquire at the end of the transaction:

<link type="text/html" rel="http://opds-spec.org/acquisition/buy" href="/item/4141/buy">
  <opds:price currencycode="USD">4.29</opds:price>
  <opds:indirectAcquisition type="application/vnd.adobe.adept+xml">
    <opds:indirectAcquisition type="application/epub+zip" />
  </opds:indirectAcquisition>
</link>

Filtering publications

OPDS clients can filter publications based on these acquisition links and opds:indirectAcquisition elements and decide to display only a subset or even none of the publications if they consider that they can't open them or deal with the workflow described.

We are soliciting feedback and comments on this version 1.1 draft.

Please submit all critiques or comments to the OPDS mailing list by Monday, 27 June 2011.

The current crop of OPDS clients on smartphones rely entirely on a navigation based on a series of links (Navigation Feeds) and publications  (Acquisition Feeds).

On a larger device, there's room for a little more creativity though and using link relations from the OPDS specification along with feed-level links, it is already possible to create an experience as rich as any other tablet application.

Let's take an example: the Android Market application on Android 3.x.

Catalog Root / Navigation Feed

A basic OPDS catalog or an OPDS client for smartphones would only display the menu for categories:

This is the absolute minimum required of an OPDS Navigation Feed:

<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
  <id>urn:uuid:2853dacf-ed79-42f5-8e8a-a7bb3d1ae6a2</id>
  <link rel="self"
        href="/opds-catalogs/root.xml"
        type="application/atom+xml;profile=opds-catalog;kind=navigation"/>
  <link rel="start"
        href="/opds-catalogs/root.xml"
        type="application/atom+xml;profile=opds-catalog;kind=navigation"/>
  <title>OPDS Catalog Root Example</title>
  <updated>2010-01-10T10:03:10Z</updated>
  <author>
    <name>Spec Writer</name>
    <uri>http://opds-spec.org</uri>
  </author>
 
  <entry>
    <title>Games</title>
    <link rel="subsection"
          href="/opds-catalogs/games.xml"
          type="application/atom+xml;profile=opds-catalog;kind=acquisition"/>
    <updated>2010-01-10T10:01:01Z</updated>
    <id>urn:uuid:d49e8018-a0e0-499e-9423-7c175fa0c56e</id>
    <content type="text">View all games.</content>
  </entry>
  <entry>
    <title>Books and Reference</title>
    <link rel="subsection"
          href="/opds-catalogs/books.xml"
          type="application/atom+xml;profile=opds-catalog;kind=acquisition"/>
    <updated>2010-01-10T10:02:00Z</updated>
    <id>urn:uuid:d49e8018-a0e0-499e-9423-7c175fa0c56c</id>
    <content type="text">View all books and reference applications.</content>
  </entry>
 
  [...]
 
</feed>

Featured items

The Android Market application dedicates a lot of space on the screen to featured applications. On the homepage, two elements display featured applications: a carousel and a "Featured apps" block.

Using the exact same navigation feed that we provided in the previous section, it is very easy to add the kind of information required for an OPDS client to also display featured publications in a more attractive way.

The OPDS specification defines a dedicated relationship value for such elements: http://opds-spec.org/featured

To reproduce this example we could add two links to our catalog root:

<link rel="http://opds-spec.org/featured"
  href="/opds-catalogs/featured_games.xml"
  title="Featured Games"
  type="application/atom+xml;profile=opds-catalog;kind=acquisition"/>
 
<link rel="http://opds-spec.org/featured"
  href="/opds-catalogs/featured_apps.xml"
  title="Featured Apps"
  type="application/atom+xml;profile=opds-catalog;kind=acquisition"/>

With this syntax, any OPDS client can now be aware that the catalog would like to feature a selection of games and a selection of applications.

Even on smartphones, some OPDS 1.0 compatible clients already support this syntax.

For example, Aldiko can display a carousel on top of the other navigation elements, and Bluefire Reader has a block dedicated to such items.

Popular Items

In a similar way, the Android Market also displays a selection of popular apps in a second block under the featured items.

In OPDS, we also have a relationship for popular items: http://opds-spec.org/sort/popular

<link rel="http://opds-spec.org/sort/popular"
  href="/opds-catalogs/top.xml"
  title="Top Free Apps"
  type="application/atom+xml;profile=opds-catalog;kind=acquisition"/>

Other rel values

Aside from relationships for featured items (http://opds-spec.org/featured) and popular items (http://opds-spec.org/sort/popular), the OPDS specification also defines rel values for new items (http://opds-spec.org/sort/new), customized recommendations (http://opds-spec.org/recommended),  items that you've already acquired (http://opds-spec.org/shelf) and your subscriptions (http://opds-spec.org/subscriptions).

It is very easy for a catalog to add a few links with the proper relationships and enable clients to create a great looking experience. One benefit of this approach is the ability to fallback to something basic on smaller screens, and expand the experience on tablets/laptops/desktops without relying on a different catalog.

Clients can also entirely design how they'd like to handle these elements: how to place them, how many items should be displayed etc.

Facets play a key role in how we navigate on websites with a large set of items to browse, for example:

• Google Search navigation is now mostly designed using facets
• All top ecommerce websites use facets
• Major libraries like the Internet Archive, Gallica or the Library of Congress

While navigation in OPDS 1.0 is limited to a series of navigation feeds (which act like the equivalent of the main menu of a website), with OPDS 1.1 any catalog and client can now offer a much richer navigation model using facets.

Let's see how we support this…

Each facet is a link

Facets are represented as atom:link element at the top of an acquisition feed:

<link rel="http://opds-spec.org/facet"
  href="/recent"
  title="Most recent" />
<link rel="http://opds-spec.org/facet" 
  href="/popular"
  title="Most popular" />

New attribute – opds:activeFacet

By default, we consider that a facet is inactive. If a facet is active, an attribute opds:activeFacet is added to the link and set to "true".

<link rel="http://opds-spec.org/facet"
  opds:activeFacet="true"
  href="/recent"
  title="Most recent" />
<link rel="http://opds-spec.org/facet" 
  href="/popular"
  title="Most popular" />

New attribute – opds:facetGroup

Facets can be grouped together using an opds:facetGroup attribute. In a group, only a single facet can be active at a time.

<link rel="http://opds-spec.org/facet"
  opds:facetGroup="Order" 
  opds:activeFacet="true"
  href="/recent"
  title="Most recent" />
<link rel="http://opds-spec.org/facet" 
  opds:facetGroup="Order"
  href="/popular"
  title="Most popular" />

Counting entries

A facet can also advertise the number of entries expected using an element from the Atom Threading extension: thr:count

<link rel="http://opds-spec.org/facet"
  opds:facetGroup="Order" 
  opds:activeFacet="true"
  href="/recent"
  title="Most recent" />
<link rel="http://opds-spec.org/facet" 
  thr:count="600"
  opds:facetGroup="Order"
  href="/popular"
  title="Most popular" />

Designing for facets

The so-called application stores (AppStore & Android Market), do a very poor job when it comes to discovery, and part of this problem is due to the lack of faceted browsing.

While facets are not as common on mobile applications as they are on websites (due to the limited screen space or the fact that designing mobile applications is still a new area of expertise) , a few design patterns already exist.

The following article at UXmatters is an excellent introduction to these patterns: http://www.uxmatters.com/mt/archives/2010/04/design-patterns-for-mobile-faceted-search-part-i.php

On larger devices such as tablets, specialized UI elements such as the iOS split view (Official guidelines) or Android fragments (Official documentation) are a perfect fit for facets.

On desktop and in web apps, the most common design for facets is to display them in a left-hand panel, but some websites also put them on top or on the right.

While there are no official recommendations in terms of UI from the spec or the OPDS group, the best way to treat facets is to display them where people would expect them the most (based on the platform you're targeting).

Implementations

Feedbooks has a live catalog with facets available at http://www.feedbooks.com/catalog

Bluefire Reader is the first OPDS client to support this feature (see screenshots below).

The Internet Archive and Inkstone Software should also be among the first to support faceted search & browsing.

If you'd like to participate, join the OPDS mailing list at http://groups.google.com/group/openpub

Calls are also scheduled and announced on the mailing list on a regular basis. Meeting notes are available on our wiki:

• 23 March 2011
• 27 April 2011
• 18 May 2011

On the Feedbooks site.

With the permission of COSLA, I have excerpted the use case ("Scenario Two") that was included in the full report, as an excellent and simple introduction to the promise of OPDS Catalogs in libraries.

COSLA2270 Book Server [Extract from Final Report]