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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This document specifies version 1.2 of the OPDS Catalog specification.
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.
https://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.atom:feed
from the Atom Syndication Format [RFC4287] that this specification defines.rel
attribute value of an atom:link element.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].
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.
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.
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="https://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>https://opds-spec.org</uri>
</author>
<entry>
<title>Popular Publications</title>
<link rel="https://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="https://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>
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="https://www.w3.org/2005/Atom"
xmlns:dc="https://purl.org/dc/terms/"
xmlns:opds="https://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>https://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>https://opds-spec.org/authors/1285</uri>
</author>
<dc:language>en</dc:language>
<dc:issued>1917</dc:issued>
<category scheme="https://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="https://opds-spec.org/image"
href="/covers/4561.lrg.png"
type="image/png"/>
<link rel="https://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="https://opds-spec.org/acquisition"
href="/content/free/4561.epub"
type="application/epub+zip"/>
<link rel="https://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>https://opds-spec.org/authors/21285</uri>
</author>
<author>
<name>Alice McGee</name>
<uri>https://opds-spec.org/authors/21284</uri>
</author>
<author>
<name>Harold McGee</name>
<uri>https://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="https://opds-spec.org/image"
href="/covers/11241.lrg.jpg"
type="image/jpeg"/>
<link rel="https://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>
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 https://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.
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 https://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.
atom:feed
ElementThe 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:
atom:link
element with a rel
attribute value of self
. This is the preferred URI for retrieving the atom:feed
representing this OPDS Catalog Feed Document.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="https://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:
atom:author
atom:contributor
atom:title
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="https://www.w3.org/2005/Atom"
template="https://example.com/search?q={searchTerms}&author={atom:author}&contributor={atom:contributor}&title={atom:title}" />
With such a template, an OPDS Client could for example support the following search queries:
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:
https://opds-spec.org/facet
: An Acquisition Feed with a subset or an alternate order of the Publications listed.Links using this relation must only appear in Acquisition Feeds.
Example
<link rel="https://opds-spec.org/facet"
href="/sci-fi"
title="Science-Fiction"
opds:facetGroup="Categories"
opds:activeFacet="true" />
<link rel="https://opds-spec.org/facet"
href="/romance"
title="Romance"
opds:facetGroup="Categories"
thr:count="600" />
<link rel="https://opds-spec.org/facet"
href="/thrillers"
title="Thrillers"
opds:facetGroup="Categories"
thr:count="1200" />
<link rel="https://opds-spec.org/facet"
href="/sci-fi/english"
title="English"
opds:facetGroup="Language"
opds:activeFacet="true" />
<link rel="https://opds-spec.org/facet"
href="/sci-fi/french"
title="French"
opds:facetGroup="Language"
thr:count="40" />
opds:facetGroup
AttributeFacets 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.
opds:activeFacet
AttributeA 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.
thr:count
AttributeThe 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.
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.
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:
atom:id
. One or more dc:identifier
elements should be used to identify the represented Publication, if appropriate metadata is available, and must not identify the OPDS Catalog Entry.atom:updated
element indicating when the OPDS Catalog Entry was last updated. A dc:issued
element should be used to indicate the first publication date of the Publication and must not represent any date related to the OPDS Catalog Entry.atom:title
representing the Publication’s title and must not use dc:title
.atom:author
to represent the Publication’s creators and should not use dc:creator
.atom:category
to represent the Publication’s category, keywords, key phrases, or classification codes and should not use dc:subject
.atom:rights
to represent rights held in and over the Publication and should not use dc:rights
.atom:summary
and/or atom:content
to describe the Publication should not use dc:description
or dc:abstract
.atom:contributor
to represent contributors to the Publication beside its creators.atom:published
to indicate when the OPDS Catalog Entry was first accessible.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.”
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:
atom:category
atom:contributor
atom:rights
opds:price
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>https://opds-spec.org/authors/1285</uri>
</author>
<dc:language>en</dc:language>
<dc:issued>1917</dc:issued>
<category scheme="https://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="https://opds-spec.org/image"
href="/covers/4561.lrg.png"
type="image/png"/>
<link rel="https://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="https://opds-spec.org/acquisition"
href="/content/free/4561.epub"
type="application/epub+zip"/>
<link rel="https://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>https://opds-spec.org/authors/1285</uri>
</author>
<dc:language>en</dc:language>
<dc:issued>1917</dc:issued>
<category scheme="https://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="https://opds-spec.org/image"
href="/covers/4561.lrg.png"
type="image/png"/>
<link rel="https://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="https://opds-spec.org/acquisition"
href="/content/free/4561.epub"
type="application/epub+zip"/>
<link rel="https://opds-spec.org/acquisition"
href="/content/free/4561.mobi"
type="application/x-mobipocket-ebook"/>
</entry>
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
}
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.
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 “https://opds-spec.org/acquisition”:
https://opds-spec.org/acquisition
: A generic relation that indicates that the complete representation of the content Resource may be retrieved.https://opds-spec.org/acquisition/open-access
: Indicates that the complete representation of the Resource can be retrieved without any requirement, which includes payment and registration.https://opds-spec.org/acquisition/borrow
: Indicates that the complete representation of the content Resource may be retrieved as part of a lending transaction.https://opds-spec.org/acquisition/buy
: Indicates that the complete representation of the content Resource may be retrieved as part of a purchase.https://opds-spec.org/acquisition/sample
or preview
: Indicates that a subset of the content Resource may be retrieved.https://opds-spec.org/acquisition/subscribe
: Indicates that the complete representation of the content Resource may be retrieved as part of a subscription.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 https://opds-spec.org/image
:
https://opds-spec.org/image
: a graphical Resource associated to the OPDS Catalog Entryhttps://opds-spec.org/image/thumbnail
: a reduced-size version of a graphical Resource associated to the OPS Catalog EntryImage resources related by https://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 https://opds-spec.org/image
thumbnail image Resources using the data
URL scheme from [RFC2397].
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.
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="https://opds-spec.org/acquisition"
type="video/mp4v-es"
href="https://specs.opds.io/content/free/4561.mp4"/>
If the Publication was available in multiple formats as unique Resources, they would simply be listed:
<link rel="https://opds-spec.org/acquisition/borrow"
href="https://specs.opds.io/content/borrow/4561.mobi"
type="application/x-mobipocket-ebook" />
<link rel="https://opds-spec.org/acquisition/borrow"
href="https://specs.opds.io/content/borrow/4561.epub"
type="application/epub+zip" />
If the Publication requires payment, at least one opds:price
element is required:
<link rel="https://opds-spec.org/acquisition/buy"
href="https://specs.opds.io/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="https://opds-spec.org/acquisition/buy"
href="https://specs.opds.io/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="https://opds-spec.org/acquisition/buy"
href="https://specs.opds.io/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>
atom:entry
ElementThe 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:
https://opds-spec.org/acquisition
.atom:link
ElementThe 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:
atom:link
elements with a rel attribute value set to https://opds-spec.org/acquisition/buy
must contain at least one opds:price
element.atom:link
elements with a rel attribute set to https://opds-spec.org/acquisition/borrow
, https://opds-spec.org/acquisition/subscribe
, or https://opds-spec.org/acquisition/sample
may contain one or more opds:price
elements.opds:price
ElementThe 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.
opds:indirectAcquisition
ElementIn 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:
start
: The OPDS Catalog Root.subsection
: An OPDS Feed not better described by a more specific relation.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.
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:
https://opds-spec.org/shelf
: A Resource that includes a user’s existing set of Acquired Content, which may be represented as an OPDS Catalog.https://opds-spec.org/subscriptions
: A Resource that includes a user’s set of subscriptions, which may be represented as an OPDS Catalog.An OPDS client may use these links to automatically sync content to the user’s local shelf or check for periodical content updates.
This specification defines two new relations, which begin with https://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.
https://opds-spec.org/sort/new
: An Acquisition Feed with newly released OPDS Catalog Entries. These Acquisition Feeds typically contain a subset of the OPDS Catalog Entries in an OPDS Catalog based on the publication date of the Publication.https://opds-spec.org/sort/popular
: An Acquisition Feed with popular OPDS Catalog Entries. These Acquisition Feeds typically contain a subset of the OPDS Catalog Entries in an OPDS Catalog based on a numerical ranking criteria.Acquisition Feeds using the https://opds-spec.org/sort/new
relation should be ordered with the most recent items first.
Acquisition Feeds using the https://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.
https://opds-spec.org/featured
: An Acquisition Feed with featured OPDS Catalog Entries. These Acquisition Feeds typically contain a subset of the OPDS Catalog Entries in an OPDS Catalog that have been selected for promotion by the OPDS Catalog provider. No order is implied.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.
https://opds-spec.org/recommended
: An Acquisition Feed with recommended OPDS Catalog Entries. These Acquisition Feeds typically contain a subset of the OPDS Catalog Entries in an OPDS Catalog that have been selected specifically for the user.Acquisition Feeds using the https://opds-spec.org/recommended
relation should be ordered with the most recommended items first.
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].
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.
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.
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
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
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].
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].
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
.
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.
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].
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.
The content and concepts within this specification are a product of the OPDS Community.
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