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OPDS Catalog 1.2

The Open Publication Distribution System (OPDS) Catalog format is a syndication format for electronic publications based on Atom and HTTP. OPDS Catalogs enable the aggregation, distribution, discovery, and acquisition of electronic publications.

OPDS Catalogs use existing or emergent open standards and conventions, with a priority on simplicity.

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Status of this Document

This document specifies version 1.2 of the OPDS Catalog specification.

Table of Contents

1. Overview

1.1. Introduction

The Open Publication Distribution System (OPDS) Catalog format is a syndication format for electronic publications based on Atom [RFC4287] and HTTP [RFC2616]. OPDS Catalogs enable available electronic publications to be:

An OPDS Catalog is a set of one or more Atom Feeds, which are themselves listings of Atom Entries. The Atom Feeds that make up all OPDS Catalogs can be divided into two groups:

Each Atom Entry in an Acquisition Feed includes basic metadata about the publication, the publication’s format, and how the publication can be acquired.

These included Atom Entries may be minimal subsets of the complete metadata, with a link to a more extensive, standalone representation URI (see the Section Listing Acquisition Feeds for more).

OPDS Catalogs may be aggregated and combined into larger OPDS Catalogs.

1.2. Terminology

Acquisition Feed
An Atom Feed whose Atom Entries are exclusively OPDS Catalog Entries.
Acquisition Link
An atom:link element with a relation that begins with http://opds-spec.org/acquisition and refers to the Resource which holds the content of the described Publication or the Resource through which it may be acquired for any OPDS Catalog Entry. Check the following sections for more detail: Acquisition Relations and Acquiring Publications.
Complete Catalog Entry
An OPDS Catalog Entry that includes all known metadata about the described Publication and is referenced by a Partial Catalog Entry.
Facet
A subset or an alternate order for an Acquisition Feed. In an OPDS Catalog, a Facet is represented as an atom:link in an Acquisition Feed.
IRI
An Internationalized Resource Identifier as defined in [RFC3987].
Navigation Feed
An Atom Feed whose Atom Entries are not OPDS Catalog Entries but instead links to other Navigation Feeds, Acquisition Feeds, or other Resources to establish a hierarchical, browsable presentation of the OPDS Catalog.
OPDS Catalog
All of the Atom Feeds (Acquisition and Navigation) and Entries (Partial and Complete) following this specification published together to describe a consolidated group of available Publications.
OPDS Catalog Entry
An Atom Entry that provides a representation of an available Publication and includes an Acquisition Link. They are serialized as OPDS Catalog Entry Documents.
OPDS Catalog Entry Document
The formal name for the XML documents that are a refinement of an "atom:entry" from the Atom Syndication Format [RFC4287] that this specification defines.
OPDS Catalog Feed Document
The formal name for XML documents that are a refinement of an atom:feed from the Atom Syndication Format [RFC4287] that this specification defines.
OPDS Catalog Root
The Atom Feed that represents either the complete OPDS Catalog (a single Acquisition Feed) or the Atom Feed that starts a set of Navigation Feeds.
Partial Catalog Entry
An OPDS Catalog Entry that includes the minimal required metadata about the described Publication but no other metadata and links to the Complete Catalog Entry.
Publication
An electronic document, typically available as a single file. OPDS Catalogs are agnostic about the particular format of Publications.
relation (or "relation of")
Refers to the rel attribute value of an atom:link element.
Representation
An entity included with a request or response as defined in [RFC2616].
Resource
A network-accessible data object or service identified by an IRI, as defined in [RFC2616]. See [REC-webarch] for further discussion on Resources.
URI
A Uniform Resource Identifier as defined in [RFC3986]. In this specification, the phrase "the URI of a document" is shorthand for "a URI which, when dereferenced, is expected to produce that document as a representation".

1.3. Conformance Statements

The key words must, must not, required, shall, shall not, should, should not, recommended, may, and optional in this document are to be interpreted as described in [RFC2119].

2. OPDS Catalog Feed Documents

OPDS Catalog Feed Documents are Atom [RFC4287] Feeds. They serve two purposes:

While Navigation Feeds do provide a suggested hierarchy from the OPDS Catalog publisher, OPDS Catalog Feed Documents may contain other relations that allow for other methods of consumption and display.

Every OPDS Catalog Feed Document must either be an Acquisition Feed or a Navigation Feed. An Acquisition Feed can be identified by the presence of Acquisition Links in each Atom Entry.

2.1. OPDS Catalog Root

The OPDS Catalog Root is the top-level OPDS Catalog Feed Document. It is either a single Acquisition Feed in the simple case or the start of a set of Navigation Feeds. Every OPDS Catalog must have one and only one OPDS Catalog Root.

External links to the OPDS Catalog Resource should use the IRI of the OPDS Catalog Root.

Each OPDS Catalog Feed Document should contain an atom:link element with a link relation of start, which references the OPDS Catalog Root Resource.

2.2. Navigation Feeds

A Navigation Feed is an OPDS Catalog Feed Document whose Atom Entries serve to create a suggested hierarchy for presentation and browsing. A Navigation Feed must not contain OPDS Catalog Entries but instead contains Atom Entries that link to other Navigation or Acquisition Feeds or other Resources.

Links to Navigation Feeds must use the type attribute application/atom+xml;profile=opds-catalog;kind=navigation.

OPDS Catalog providers should choose the best relation for each Navigation Feed based on the relations in the section OPDS Catalog Relations. The relation subsection should be used if no other relation is more appropriate.

Example

An OPDS Catalog Root that is the top of a set of Navigation Feeds references three Acquisition Feeds using atom:link:

<?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>Popular Publications</title>
    <link rel="http://opds-spec.org/sort/popular"
          href="/opds-catalogs/popular.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">Popular publications from this catalog based on downloads.</content>
  </entry>
  <entry>
    <title>New Publications</title>
    <link rel="http://opds-spec.org/sort/new"
          href="/opds-catalogs/new.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">Recent publications from this catalog.</content>
  </entry>
  <entry>
    <title>Unpopular Publications</title>
    <link rel="subsection"
          href="/opds-catalogs/unpopular.xml"
          type="application/atom+xml;profile=opds-catalog;kind=acquisition"/>
    <updated>2010-01-10T10:01:00Z</updated>
    <id>urn:uuid:d49e8018-a0e0-499e-9423-7c175fa0c56d</id>
    <content type="text">Publications that could use some love.</content>
  </entry>
</feed>

2.3. Acquisition Feeds

An Acquisition Feed is an OPDS Catalog Feed Document that collects OPDS Catalog Entries into a single, ordered set. The simplest complete OPDS Catalog would be a single Acquisition Feed listing all of the available OPDS Catalog Entries from that provider. In more complex OPDS Catalogs, Acquisition Feeds are used to present and organize sets of related OPDS Catalog Entries for browsing and discovery by clients and aggregators.

Links to Acquisition Feeds must use the type attribute application/atom+xml;profile=opds-catalog;kind=acquisition.

Example

An Acquisition Feed listing OPDS Catalog Entries from the “Unpopular Publications” Entry provided in the Navigation Feed Example:

<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns="http://www.w3.org/2005/Atom"
      xmlns:dc="http://purl.org/dc/terms/"
      xmlns:opds="http://opds-spec.org/2010/catalog">
  <id>urn:uuid:433a5d6a-0b8c-4933-af65-4ca4f02763eb</id>

  <link rel="related"
        href="/opds-catalogs/vampire.farming.xml"
        type="application/atom+xml;profile=opds-catalog;kind=acquisition"/>
  <link rel="self"    
        href="/opds-catalogs/unpopular.xml"
        type="application/atom+xml;profile=opds-catalog;kind=acquisition"/>
  <link rel="start"  
        href="/opds-catalogs/root.xml"
        type="application/atom+xml;profile=opds-catalog;kind=navigation"/>
  <link rel="up"      
        href="/opds-catalogs/root.xml"
        type="application/atom+xml;profile=opds-catalog;kind=navigation"/>

  <title>Unpopular Publications</title>
  <updated>2010-01-10T10:01:11Z</updated>
  <author>
    <name>Spec Writer</name>
    <uri>http://opds-spec.org</uri>
  </author>

  <entry>
    <title>Bob, Son of Bob</title>
    <id>urn:uuid:6409a00b-7bf2-405e-826c-3fdff0fd0734</id>
    <updated>2010-01-10T10:01:11Z</updated>
    <author>
      <name>Bob the Recursive</name>
      <uri>http://opds-spec.org/authors/1285</uri>
    </author>
    <dc:language>en</dc:language>
    <dc:issued>1917</dc:issued>
    <category scheme="http://www.bisg.org/standards/bisac_subject/index.html"
              term="FIC020000"
              label="FICTION / Men's Adventure"/>
    <summary>The story of the son of the Bob and the gallant part he played in
      the lives of a man and a woman.</summary>
    <link rel="http://opds-spec.org/image"    
          href="/covers/4561.lrg.png"
          type="image/png"/>
    <link rel="http://opds-spec.org/image/thumbnail"
          href="/covers/4561.thmb.gif"
          type="image/gif"/>

    <link rel="alternate"
          href="/opds-catalogs/entries/4571.complete.xml"
          type="application/atom+xml;type=entry;profile=opds-catalog"
          title="Complete Catalog Entry for Bob, Son of Bob"/>

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

 <entry>
    <title>Modern Online Philately</title>
    <id>urn:uuid:7b595b0c-e15c-4755-bf9a-b7019f5c1dab</id>
    <author>
      <name>Stampy McGee</name>
      <uri>http://opds-spec.org/authors/21285</uri>
    </author>
    <author>
      <name>Alice McGee</name>
      <uri>http://opds-spec.org/authors/21284</uri>
    </author>
    <author>
      <name>Harold McGee</name>
      <uri>http://opds-spec.org/authors/21283</uri>
    </author>
    <updated>2010-01-10T10:01:10Z</updated>
    <rights>Copyright (c) 2009, Stampy McGee</rights>
    <dc:identifier>urn:isbn:978029536341X</dc:identifier>
    <dc:publisher>StampMeOnline, Inc.</dc:publisher>
    <dc:language>en</dc:language>
    <dc:issued>2009-10-01</dc:issued>
    <content type="text">The definitive reference for the web-curious
      philatelist.</content>
    <link rel="http://opds-spec.org/image"    
          href="/covers/11241.lrg.jpg"
          type="image/jpeg"/>

    <link rel="http://opds-spec.org/acquisition/buy"
          href="/content/buy/11241.epub"
          type="application/epub+zip">
      <opds:price currencycode="USD">18.99</opds:price>
      <opds:price currencycode="GBP">11.99</opds:price>
    </link>
 </entry>
</feed>

2.4. Listing Acquisition Feeds

OPDS Catalog Feed Documents, especially Acquisition Feeds, may contain large numbers of Atom Entries.

A client such as a web spider or web browser might be overwhelmed if the response to a GET contained every Atom Entry in an Acquisition Feed — in turn the server might also waste bandwidth and processing time on generating a response that cannot be handled. For this reason, servers MAY respond to Acquisition Feed GET requests with a paginated response: an OPDS Catalog Feed Document containing a partial list of the Acquisition Feed’s member Atom Entries and a link to the next partial Acquisition Feed, if it exists, as defined in Section 3 of [RFC5005].

OPDS Catalog providers should use Partial Catalog Entries in all Acquisition Feeds except Complete Acquisition Feeds, which are intended for crawling and are referenced using the http://opds-spec.org/crawlable relation.

Clients must not assume that an OPDS Catalog Entry returned in the Acquisition Feed is a full representation of an OPDS Catalog Entry Resource, as described in the Section Partial and Complete Entries.

2.5. Complete Acquisition Feeds

An OPDS Catalog provider may provide a single, consolidated Acquisition Feed that includes the complete representation of every unique OPDS Catalog Entry Document in an OPDS Catalog in an atom:feed to facilitate crawling and aggregation. Complete Acquisition Feeds should not be paginated unless they are extremely large.

This representation is called a Complete Acquisition Feed and each OPDS Catalog Entry must be ordered by atom:updated, with the most recently updated Atom Entries coming first in the document order.

If available, each OPDS Catalog Feed Document in the OPDS Catalog should contain an atom:link element with a relation of http://opds-spec.org/crawlable that references the Complete Acquisition Feed Resource.

A Complete Acquisition Feed must include a fh:complete element from [RFC5005] unless pagination is required. See Section 2 of [RFC5005] for the specification of the fh:complete element.

OPDS Catalog providers should use a compressed Content-Encoding when transmitting Complete Acquisition Feeds over HTTP. See Section 14.11 of [RFC2616] for more on compression.

OPDS Catalog providers must include Complete Catalog Entries when serializing a Complete Acquisition Feed.

2.6. Element Definitions

The atom:feed Element

The atom:feed element is the document (i.e., top-level) element of an OPDS Catalog Feed Document, acting as a container for metadata, hierarchy, and Publications associated with the OPDS Catalog. Its element children consist of metadata elements followed by zero or more atom:entry child elements.

The following child elements are refined by this specification:

An OPDS Catalog may provide a search facility through an [OpenSearch] description document.

Links to [OpenSearch] description documents must use the search relation value and the application/opensearchdescription+xml media type as defined in the “Autodiscovery” section of the [OpenSearch] specification.

<link rel="search"
      href="search.xml"
      type="application/opensearchdescription+xml"/>

In an [OpenSearch] description document, the search interface must use the media type associated to OPDS Catalogs:

<Url type="application/atom+xml;profile=opds-catalog;kind=acquisition"
     template="http://example.com/search?q={searchTerms}" />

An OPDS Catalog may also provide more advanced possibilities for its search endpoint, using one or more fully qualified parameters from the Atom namespace such as:

OPDS Catalog Feed Documents may include elements from the [OpenSearch] namespace such as opensearch:totalResults or opensearch:itemsPerPage in [OpenSearch] responses.

Open Search Description Document Example

In order to provide a search endpoint that supports both basic (keyword based) and advanced search, an OPDS Catalog could provide the following template in its [OpenSearch] Description document:

<Url type="application/atom+xml;profile=opds-catalog"
     xmlns:atom="http://www.w3.org/2005/Atom"
     template="http://example.com/search?q={searchTerms}&amp;author={atom:author}&amp;contributor={atom:contributor}&amp;title={atom:title}" />

With such a template, an OPDS Client could for example support the following search queries:

4. Facets

An Acquisition Feed may offer multiple links to reorder the Publications listed in the feed or limit them to a subset. This specification defines one new relation to identify such links as Facets:

Links using this relation must only appear in Acquisition Feeds.

Example

<link rel="http://opds-spec.org/facet" 
      href="/sci-fi" 
      title="Science-Fiction" 
      opds:facetGroup="Categories" 
      opds:activeFacet="true" />
 
<link rel="http://opds-spec.org/facet" 
      href="/romance" 
      title="Romance"  
      opds:facetGroup="Categories" 
      thr:count="600" />
 
<link rel="http://opds-spec.org/facet" 
      href="/thrillers" 
      title="Thrillers"  
      opds:facetGroup="Categories" 
      thr:count="1200" />
 
<link rel="http://opds-spec.org/facet" 
      href="/sci-fi/english" 
      title="English" 
      opds:facetGroup="Language" 
      opds:activeFacet="true" />
 
<link rel="http://opds-spec.org/facet" 
      href="/sci-fi/french" 
      title="French"  
      opds:facetGroup="Language" 
      thr:count="40" />

The opds:facetGroup Attribute

Facets may be grouped together by the OPDS Catalog provider using an opds:facetGroup attribute. The value of this attribute is the name of the group.

A Facet must not appear in more than a single group.

The opds:activeFacet Attribute

A Facet is considered active, if the attribute associated to the Facet is already being used to filter Publications in the current Acquisition Feed.

The OPDS Catalog provider should indicate that a Facet is active using the opds:activeFacet attribute set to true.

If the Facet is not active, the opds:activeFacet attribute should not appear in the link.

In a group of Facets, an OPDS Catalog provider must not mark more than one Facet as active.

The thr:count Attribute

The OPDS Catalog provider may provide an additional hint about the number of items expected in the Acquisition Feed, if an OPDS client follows a link.

The thr:count attribute, extracted from [RFC4685] may be added to a Facet in order to provide this information.

5. OPDS Catalog Entry Documents

OPDS Catalog Entry Documents are Atom [RFC4287] Entry documents that provide a complete representation of the metadata and data associated with an available Publication.

Each OPDS Catalog Entry Document must include at least one Acquisition Link.

5.1. Metadata

5.1.1. Relationship Between Atom and Dublin Core Metadata

OPDS Catalog providers are encouraged to include metadata from the Dublin Core Metadata Initiative (DCMI) Metadata Terms [DCTERMS] whenever appropriate. However, these elements are not required. Some of the elements defined in [DCTERMS] overlap in meaning with similar elements defined by Atom in [RFC4287]. In general, OPDS Catalog Entries prefer the Atom elements over the [DCTERMS] elements and the following rules should be used by OPDS Catalog providers when choosing elements:

All metadata elements required by Atom are required in OPDS Catalog Entries and Feeds.

Following Atom [RFC4287] Section 4.2.6, the content of an atom:id identifying an OPDS Catalog Entry must not change when the OPDS Catalog Entry is “relocated, migrated, syndicated, republished, exported, or imported” and “must be created in a way that assures uniqueness.”

5.1.2. Partial and Complete Catalog Entries

An OPDS Catalog Entry may be represented as either a Partial or Complete Catalog Entry. Partial Catalog Entries include only critical metadata elements to reduce the size of OPDS Catalog documents for bandwidth- and resource-limited clients and link to their alternate representation as a Complete Catalog Entries, which include all metadata elements.

A Partial Catalog Entry must include an alternate link relation referencing the Complete Catalog Entry Resource and that atom:link must use the type attribute application/atom+xml;type=entry;profile=opds-catalog.

Any OPDS Catalog Entry without a link to a Complete Catalog Entry must include all metadata elements.

Partial Catalog Entries should include the following metadata elements, if available:

OPDS Catalog providers are strongly encouraged to limit metadata elements not required by Atom, OPDS, or listed above from Partial Catalog Entries to help facilitate the efficient transmission and consumption of OPDS Catalog documents. Implementers are encouraged to include metadata elements from other vocabularies in Complete Catalog Entries.

Example

An example of a Partial Catalog Entry (as would appear in an Acquisition Feed) for a Publication available in two formats:

<entry>
  <title>Bob, Son of Bob</title>
  <id>urn:uuid:6409a00b-7bf2-405e-826c-3fdff0fd0734</id>
  <updated>2010-01-10T10:01:11Z</updated>
  <author>
    <name>Bob the Recursive</name>
    <uri>http://opds-spec.org/authors/1285</uri>
  </author>
  <dc:language>en</dc:language>
  <dc:issued>1917</dc:issued>
  <category scheme="http://www.bisg.org/standards/bisac_subject/index.html"
            term="FIC020000"
            label="FICTION / Men's Adventure"/>
  <summary type="text">The story of the son of the Bob and the gallant part he played in
    the lives of a man and a woman.</summary>
  <link rel="http://opds-spec.org/image"     
        href="/covers/4561.lrg.png"
        type="image/png"/> 
  <link rel="http://opds-spec.org/image/thumbnail" 
        href="/covers/4561.thmb.gif"
        type="image/gif"/>
 
  <link rel="alternate"
        href="/opds-catalogs/entries/4571.complete.xml"
        type="application/atom+xml;type=entry;profile=opds-catalog" 
        title="Complete Catalog Entry for Bob, Son of Bob"/>
 
  <link rel="http://opds-spec.org/acquisition" 
        href="/content/free/4561.epub"
        type="application/epub+zip"/>
  <link rel="http://opds-spec.org/acquisition" 
        href="/content/free/4561.mobi"
        type="application/x-mobipocket-ebook"/>
</entry>

The Complete Catalog Entry for the same Publication:

<entry>
  <title>Bob, Son of Bob</title>
  <id>urn:uuid:6409a00b-7bf2-405e-826c-3fdff0fd0734</id>
  <updated>2010-01-10T10:01:11Z</updated>
 
  <author>
    <name>Bob the Recursive</name>
    <uri>http://opds-spec.org/authors/1285</uri>
  </author>
  <dc:language>en</dc:language>
  <dc:issued>1917</dc:issued>
  <category scheme="http://www.bisg.org/standards/bisac_subject/index.html"
            term="FIC020000"
            label="FICTION / Men's Adventure"/>
 
  <summary type="text">The story of the son of the Bob and the gallant part he played in
    the lives of a man and a woman.</summary>
  <content type="text">The story of the son of the Bob and the gallant part
    he played in the lives of a man and a woman. Bob begins his humble life
    under the wandering eye of his senile mother, but quickly learns how to
    escape into the wilder world. Follow Bob as he uncovers his father's past
    and uses those lessons to improve the lives of others.</content>
 
  <link rel="http://opds-spec.org/image"     
        href="/covers/4561.lrg.png"
        type="image/png"/> 
  <link rel="http://opds-spec.org/image/thumbnail" 
        href="/covers/4561.thmb.gif"
        type="image/gif"/>
 
  <link rel="self"
        href="/opds-catalogs/entries/4571.complete.xml"
        type="application/atom+xml;type=entry;profile=opds-catalog"/>
 
  <link rel="http://opds-spec.org/acquisition" 
        href="/content/free/4561.epub"
        type="application/epub+zip"/>
  <link rel="http://opds-spec.org/acquisition" 
        href="/content/free/4561.mobi"
        type="application/x-mobipocket-ebook"/>
</entry>

5.1.3. Summary and Content

OPDS Catalog Entries use atom:content and atom:summary elements to describe a Publication. This is a distinction from Atom [RFC4287], which uses atom:content for the “content of the entry.” Instead, OPDS Catalog Entries reference an electronic document with the Publication’s content using Acquisition Links. OPDS Catalog Entries should include either atom:summary or atom:content elements or both to provide a description, summary, abstract, or excerpt of the Publication.

An atom:summary element in an OPDS Catalog Entry must use a type attribute of text and the content must not contain child elements. The content of an atom:summary element should not duplicate the content of atom:title or atom:content. Such text is intended to be presented to humans in a readable fashion. The restriction of the content of atom:summary and the type attribute is a restriction of Atom [RFC4287].

An atom:content element in an OPDS Catalog Entry either contains or links to a description, summary, abstract, or excerpt of the Publication. The behavior for atomOutOfLineContent and the src attribute are defined in Section 4.1.3 of [RFC4287].

If an OPDS Catalog Entry includes both atom:content and atom:summary, the atom:content should not be included in the Partial Catalog Entry. Both elements should be included in the Complete Catalog Entry.

atomSummary = 
  element atom:summary {
  atomCommonAttributes,
  attribute type { "text" }?,
    text
  }

5.2. Catalog Entry Relations

OPDS Catalog Entry Documents should include links to related Resources. This specification defines new relations for linking from OPDS Catalog Entry Documents. They are defined in the Sections Acquisition Relations and Artwork Relations.

OPDS Catalog providers are encouraged to use relations from [RFC5988] inside OPDS Catalog Entry Documents where appropriate.

5.2.1. Acquisition Relations

Acquisition relations describe the extent of the content referred to by an Acquisition Link or the manner in which that Resource may be acquired. This specification defines six Acquisition Relations, which all begin with “http://opds-spec.org/acquisition”:

5.2.2. Artwork Relations

OPDS Catalog Entries may include atom:link elements to images that provide a visual representation of the Publication. For many Publications, these images will be the Publication’s artwork. This specification defines two Artwork Relations, which begin with http://opds-spec.org/image:

Image resources related by http://opds-spec.org/image/thumbnail should be suitable for presentation at a small size.

The atom:link elements with these relations should include at least one link with a type attribute of image/gif, image/jpeg, or image/png and the image Resources must be in GIF, JPEG, or PNG format.

Additional atom:link elements may also include resources using a vector-based format.

While either image Resource is optional and may be included independently, OPDS Catalog providers are encouraged to provide both these relations and Resources whenever possible.

Some OPDS Catalog providers MAY choose to provide http://opds-spec.org/image thumbnail image Resources using the data URL scheme from [RFC2397].

5.2.3. Grouping Relations

An OPDS Catalog Entry may include one or more atom:link elements to Navigation Feeds and Acquisition Feeds which contain it, either directly or indirectly. This specification uses the IANA standard link relation collection for this purpose.

An OPDS client may choose to group together all entries in an OPDS Feed whose collection link points to a given target.

5.3. Acquiring Publications

The goal of OPDS Catalogs is to make Publications both discoverable and straightforward to acquire on a range of devices and platforms. To support that goal, this specification strives to provide a framework for describing how a Publication may be acquired while not attempting to constrain this very complex topic. Commonly-used acquisition scenarios may be specified in an update to this specification.

All Acquisition Links must include a type attribute that advises clients on the media type of the representation that is expected to be returned when the value of the href attribute is dereferenced. As with Atom, the value of the type attribute must conform to the syntax of a MIME media type [MIMEREG].

Publications in a format using Digital Rights Management should use a different value for the type attribute of the Acquisition Link than the same format without Digital Rights Management.

OPDS Catalog clients may only support a subset of all possible Publication media types. These clients will need to filter both the type attribute of Acquisition Links.

OPDS Catalogs may only provide certain Publications through an Indirect Acquisition, either through a container or a different Acquisition workflow. In such cases, it is up to the clients to filter these publications based on both the opds:indirectAcquisition and atom:link type attributes.

Examples

The simplest case is a Publication available in one format:

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

If the Publication was available in multiple formats as unique Resources, they would simply be listed:

<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" />

If the Publication requires payment, at least one opds:price element is required:

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

If the same Publication requires a payment through an HTML page, then an opds:indirectAcquisition element is required to describe the content type of the final Publication Representation:

<link rel="http://opds-spec.org/acquisition/buy"
      href="/product/1"
      type="text/html">
  <opds:price currencycode="USD">1.99</opds:price>
  <opds:indirectAcquisition type="audio/mpeg" />
</link>

Multiple opds:indirectAcquisition elements can also be used as child elements of an Acquisition Link or another opds:indirectAcquisition when this is necessary (a bundle would be a good example):

<link type="text/html" 
      rel="http://opds-spec.org/acquisition/buy" 
      href="/item/1111/buy/">
  <opds:price currencycode="EUR">10.99</opds:price>
  <opds:indirectAcquisition type="application/zip">
    <opds:indirectAcquisition type="application/epub+zip" />
    <opds:indirectAcquisition type="application/pdf" />
    <opds:indirectAcquisition type="application/x-mobipocket-ebook" />
  </opds:indirectAcquisition>
</link>

5.4. Element Definitions

The atom:entry Element

The atom:entry element is the document (i.e., top-level) element of an OPDS Catalog Entry Document, acting as a container for metadata and data associated with an available Publication.

The following child elements are refined by this specification:

The atom:link element defines a reference from an Atom Entry or Atom Feed to a Resource.

The following child elements are defined by this specification:

The opds:price Element

The opds:price element represents the acquisition price in a particular currency of an individual Publication in a particular format from a particular provider. This element can appear as a child of the atom:link element (in OPDS Catalog Entry Documents).

The content of opds:price is text describing a currency value. A currency sign must not be included.

In the opds:price element, the value of the currencycode attribute must be an active code from [ISO4217] representing a currency. It defines the currency used for the content of the opds:price element.

The opds:indirectAcquisition Element

In some cases, the OPDS Catalog provider may require the client to acquire an intermediate Resource before acquiring the final Publication. This can be the case for example with containers (archive formats, multimedia containers for various formats) or when a payment is required (need to go through a series of HTML pages to handle the transaction).

The opds:indirectAcquisition element represents these secondary media types [MIMEREG] that the client can expect to acquire if they follow the Acquisition Link.

These opds:indirectAcquisition elements may be arbitrarily nested to represent several levels of indirection.

OPDS Catalog Feed Documents should include links to other available Acquisition and Navigation Feeds and other related Resources to encourage independent navigation.

These relations may be used by clients to provide additional or alternative navigation, both in a Navigation Feed or an Acquisition Feed.

The following relations are derived from [RFC5988], with some clarification:

When creating an OPDS Catalog with Navigation and Acquisition Feeds, OPDS Catalog providers are encouraged to use the relations defined in this specification and [RFC5988]. If no appropriate relation is found, the Feeds should use a descriptive atom:title element and the atom:link elements should use a descriptive title attribute.

6.1. Relations for Previously Acquired Content

An OPDS catalog may track content that was previously acquired by the user. This specification defines two new relations to identify Acquisition Feeds listing such resources:

An OPDS client may use these links to automatically sync content to the user’s local shelf or check for periodical content updates.

6.2. Sorting Relations

This specification defines two new relations, which begin with http://opds-spec.org/sort, to describe how an Acquisition Feed is sorted. These relations SHOULD be used when creating Navigation Feeds and the OPDS Catalog Root, if applicable.

Acquisition Feeds using the http://opds-spec.org/sort/new relation should be ordered with the most recent items first.

Acquisition Feeds using the http://opds-spec.org/sort/popular relation should be ordered with the most popular items first.

This specification also defines a relation to describe an Acquisition Feed of featured items. This relation should be used when creating Navigation Feeds and the OPDS Catalog Root, if applicable.

6.4. Recommendations

This specification also defines a relation to describe an Acquisition Feed of recommended items. Since recommendations can be customized for the user, such feed may require some kind of prior authentication. This relation should be used when creating Navigation Feeds and the OPDS Catalog Root, if applicable.

Acquisition Feeds using the http://opds-spec.org/recommended relation should be ordered with the most recommended items first.

7. Other Considerations

7. Discovering OPDS Catalogs

OPDS Catalogs may be referenced in HTML/XHTML pages, HTTP headers, or using other techniques. These links may reference both OPDS Catalog Entries or Feeds.

Links to OPDS Catalog Entry Document Resources must use a type attribute of application/atom+xml;type=entry;profile=opds-catalog.

Links to OPDS Catalog Feed Document Resources must use a type attribute of application/atom+xml;profile=opds-catalog.

The most common mechanism for encouraging the auto-discovery of OPDS Catalogs is to link from an HTML document to the OPDS Catalog Root Resource, using the auto-discovery pattern popularized by the syndicated feed community [AUTODISCOVERY].

Multiple links to OPDS Catalog Resources may be expressed in a single HTML document.

An example of two links inside an HTML page about the same Publication:

<link rel="related"   
      href="/opds-catalogs/root" 
      type="application/atom+xml;profile=opds-catalog"  
      title="Example OPDS Catalog" /> 
 
<link rel="alternate" 
      href="/entry/1"
      type="application/atom+xml;type=entry;profile=opds-catalog" 
      title="Example OPDS Entry" />

Auto-discovery links may also be expressed using HTTP headers as defined in [RFC5988].

7. Aggregating OPDS Catalogs

OPDS Catalogs may be aggregated using the same techniques as Atom Feeds.

Aggregators should use the atom:source element from Section 4.2.11 of [RFC4287] to include information about the original OPDS Catalog.

7.1 Media Type Considerations

7.1.1. The Atom Format Type Parameter

The Atom Publishing Protocol [RFC5023] defines the Atom Format Type Parameter. Publishers of OPDS Catalogs should use the type parameter to help clients distinguish between relations to OPDS Catalog Entries and OPDS Catalog Feeds.

7.1.2. The OPDS Catalog Profile Parameter

Relations to OPDS Catalog Feed Document and OPDS Catalog Entry Document Resources must use a profile parameter following Section 4.3 of [RFC4288] with the value opds-catalog. This profile parameter provides clients with an advisory hint that the Resource should be a component of an OPDS Catalog.

The complete media type for a relation to an OPDS Catalog Entry Document Resource must be:

application/atom+xml;type=entry;profile=opds-catalog

7.1.3. The OPDS Kind Parameter

In addition to the new “profile” parameter, this specification also introduces a new kind parameter following Section 4.3 of [RFC4288] with the value acquisition or navigation. This kind parameter provides clients with an advisory hint whether the Resource should be an Acquisition Feed or a Navigation Feed.

This value is intended to make it easier for OPDS clients to display basic contextual information about the feed without requiring that those clients dereference, parse, and analyze linked resources. That said, the client must not assume this parameter to provide completely accurate information about the nature of the feed.

The complete media type for a relation to an Acquisition Feed must be:

application/atom+xml;profile=opds-catalog;kind=acquisition

The complete media type for a relation to a Navigation Feed must be:

application/atom+xml;profile=opds-catalog;kind=navigation

7.2. Security Considerations

OPDS Catalogs are Atom documents delivered over HTTP and thus subject to the security considerations found in Section 15 of [RFC2616] and Section 5 of [RFC4287].

7.2.1. Securing a Catalog

OPDS Catalogs are delivered over HTTP. Authentication requirements for HTTP are covered in Section 11 of [RFC2616].

The type of authentication required for any OPDS Catalog is a decision to be made by the OPDS Catalog provider. OPDS Catalog clients are likely to face authentication schemes that vary across OPDS Catalogs. At a minimum, client and server implementations must be capable of being configured to use HTTP Basic Authentication [RFC2617] in conjunction with a connection made with TLS 1.3 [RFC8446] or a subsequent standards-track version of TLS. It is recommended that OPDS Catalog clients be implemented in such a way that new authentication schemes can be deployed.

Because this protocol uses HTTP response status codes as the primary means of reporting the result of a request, OPDS Catalog providers are advised to respond to unauthorized or unauthenticated requests using an appropriate 4xx HTTP response code (e.g., 401 “Unauthorized” or 403 “Forbidden”) in accordance with [RFC2617].

7.2.2 Linked Resources

OPDS Catalogs can contain XML External Entities as defined in Section 4.2.2 of [REC-xml]. OPDS Catalog implementations are not required to load external entities. External entities are subject to the same security concerns as any network operation and can alter the semantics of an OPDS Catalog Feed Document or OPDS Catalog Entry Document. The same issues exist for Resources linked to by elements such as atom:link and atom:content.

7.2.3 URIs and IRIs

OPDS Catalog implementations handle URIs and IRIs. See Section 7 of [RFC3986] and Section 8 of [RFC3987] for security considerations related to their handling and use.

7.2.4 Code Injection and Cross Site Scripting

OPDS Catalogs can contain a broad range of content types including code that might be executable in some contexts. Malicious publishers could attempt to attack servers or other clients by injecting code into OPDS Catalog Feed Documents or OPDS Catalog Entry Documents or Media Resources.

Server implementations are strongly encouraged to verify that external content is safe prior to aggregating, processing, or publishing it. In the case of HTML, experience indicates that verification based on a white list of acceptable content is more effective than a black list of forbidden content.

Additional information about XHTML and HTML content safety can be found in Section 8.1 of [RFC4287].

7.3. Bandwidth and Processing Considerations

Many OPDS Catalog clients operate in mobile environments, which may impose strict limitations on bandwidth and processing resources. OPDS Catalog publishers are strongly encouraged to publish their OPDS Catalogs using compression and caching techniques and the partial feeds described in the Section Listing Acquisition Feeds. Implementers are encouraged to investigate and use alternative mechanisms regarded as equivalently good or better at the time of deployment. See [CACHING] for more on caching techniques.

8. References

8.1. Normative References

8.2. Informative References

Appendix A. Contributors

The content and concepts within this specification are a product of the OPDS Community.

Appendix B. RELAX NG Compact Schema

The RELAX NG schema explicitly excludes elements in the OPDS Catalog namespace that are not defined in this revision of the specification. Requirements for Atom Protocol processors encountering such markup are given in Sections 6.2 and 6.3 of [RFC4287].

The schema is maintained in version control at: https://github.com/opds-community/specs/blob/master/schema/1.2/opds.rnc