This post describes existing vocabularies for expressing licenses.
License for a Linked Data dataset can be specified within the data, or outside of it, for example in a HTML document linking the data. If the license is specified as RDF, Dublin Core license or other elements can be used, pointing to a well known license
The most widespread way of specifying a license is making a Dublin Core license (http://purl.org/dc/terms/license) to point to a pre-defined license. For example,the GoodRelations ontology is licensed as a CC-BY (Creative Commons Attribution license) simply using the license element (yes, ontologies themselves can be licensed):
<owl:Ontology rdf:about="http://purl.org/goodrelations/v1">
<dcterms:license rdf:resource="http://creativecommons.org/licenses/by/3.0/"/>
</owl:Ontology>
Please note that DublinCore also defines a class LicenseDocument: A legal document giving official permission to do something with a Resource.
The DublinCore rights (http://purl.org/dc/elements/1.1/rights), defined as Information about rights held in and over the resource has also been used. For example, the knowledge base at http://oai.rkbexplorer.com/ describes the licensing information under the rights element (see example.
Some other vocabularies have derived a license property from the DublinCore's. The most relevant is the Creative Commons':
http://creativecommons.org/ns#licenseSome other vocabularies have defined classes and properties. The following list only shows the relations that could be assumed to be like the DublinCore "license".
- Description of a Project (DOAP): http://usefulinc.com/ns/doap#license
- Ontology Metadata Vocabulary (OMV): http://omv.ontoware.org/2005/05/ontology/hasLicense
- Data Dictionary for Preservation of Metadata (PREMIS): http://multimedialab.elis.ugent.be/users/samcoppe/ontologies/Premis/premis.owl#licenseTerms
- Music Ontology (MO): http://purl.org/ontology/mo/license
- Vocabulary Of Attribution and Governance (VOAM): http://www.linkedmodel.org/schema/vaem#hasLicenseType
Somewhat more exotic is using the XHTML specification of license. For example, the Biological Taxonomy Vocabulary was licensed using the xhtml vocabulary.
<rdf:RDF
xmlns:cc="http://creativecommons.org/ns#"
xmlns:dcterms="http://purl.org/dc/terms/"
xmlns:xhv="http://www.w3.org/1999/xhtml/vocab#"
xmlns:owl="http://www.w3.org/2002/07/owl#">
<owl:Ontology rdf:about="http://purl.org/NET/biol/ns#">
<xhv:license>
<cc:License rdf:about="http://creativecommons.org/licenses/by-sa/2.0/uk/">
<dcterms:title>Creative Commons Attribution-Share Alike 2.0 (UK: England & Wales) Licence</dcterms:title>
</cc:License>
</xhv:license>
</owl:Ontology>
</rdf:RDF>
Some licenses have been specifically tailored for data:
- PDDL: Public Domain Dedication and License (PDDL) — “Public Domain for data/databasesBut it can happen that pre-defined licenses do not suit well for the purposes of the publishers. In that case, richer vocabularies allow expressing further details.
The Open Digital Rights Language (ODRL), whose first version was edited by Renato Ianella in 2001, is a well stablished XML format for expressing licenses with successful application in the industry. Ten years ago, OMA DRM adopted a ODRL profile, but still as of 2013 the eBook and news industries have also adhered to their standards. Since 2011, ODRL is formally a W3C Community Group.
ODRL version 2.0 (specified in 2012) supersedes the much spread version 1.1. It specifies a core model and extension vocabularies, and it can be serialized as XML and RDF as well.The ODRL initiative is still developing the OWL model
(ODRL Core Model) and a vocabulary.Creative Commons advocates for linking the license in the HTML document and using the microformat "license" element to declare that the linked URI is a license. For example:
<a rel="license" href="http://creativecommons.org/licenses/by-nc/3.0/">Creative Commons Attribution Non-Commercial 3.0 License</a>
But they also offered an almost full-fledged language, the Creative Commons Rights Expression Language (CC REL) to describe further describe the licensing information with RDF.
CC REL includes six classes (Work, License, Jurisdiction, Permission, Requirement, Prohibition). Permissions are limited to Reproduction, Distribution, Derivative Works and Sharing, and Requirements are: Notice, Attribution, Share Alike, Source Code, Copyleft and Lesser Copyleft. Prohibitions are only two: Commercial Use and High Income Nation Use. Works and Licenses are further described with some properties.
A resource can be directly linked to a license:
@prefixcc:<http://creativecommons.org/ns#>.But also the complete license can be described:
<http://myresource>
rdf:type <http://mytype>;
cc:license<http://XXX/licenses/bysa/3.0/>.
<http://creativecommons.org/licenses/bysa/3.0/>
cc:permits
cc:DerivativeWorks,
cc:Distribution,
cc:Reproduction;
cc:requires
cc:Attribution.
This language makes customizable licenses possible, at least to a certain level.
MPEG-21, which already defined a Rights Expression Language in XML, is currently ultimating their new part of the standard with a Media Contract Ontology (to appear soon). It largely relies on the Media Value Chain Ontology, which already provided elements for expressing permissions.