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September 03, 2008

Proposed XML format for DC description sets

DCMI announced yesterday the availability for public comment of the document Expressing Dublin Core description sets using XML (DC-DS-XML). (Disclaimer: Andy and I are co-authors of the document!) This document, a DCMI Proposed Recommendation, describes an XML format called "DC-DS-XML" which supports the serialization of a "description set", the information structure defined by the DCMI Abstract Model. A note providing some further information on the background to the development of the specification, and its relationship to other specifications, was also published.

It's important to note that, in the terms of the document Interoperability levels for Dublin Core metadata which I mentioned a while ago, the DC-DS-XML format is intended to support what that document calls "level 3 interoperability", based on the creation/exchange of records structured as DC description sets. The DC-DS-XML format explicitly addresses a fairly minimal set of requirements, and does not seek to address the additional requirements of "level 4" in that document; in particular it does not concern itself with the implementation of the sorts of structural constraints which might be expressed in a Description Set Profile.

Also, the aim is not to promote DC-DS-XML as "the one and only" XML format for Dublin Core metadata - or even "the one and only" DCMI-owned XML format for Dublin Core metadata. The DCMI Architecture Forum continues to gather requirements for other formats, particularly requirements arising from the use of Description Set Profiles - i.e. from "level 4" in the Interoperability Levels document. The relationship between the checking of the structural constraints specified by a DSP and validation using XML schema technologies of various hues will be a factor to consider here. This is likely to be one of the topics for discussion at the f2f meeting of the DCMI Architecture Forum, to be held on Thursday 25 September at the DC-2008 conference in Berlin in a couple of weeks.

The DC-DS-XML format is based on a "TRiX-like" approach, by which I mean that it makes the structure of the description set explicit in the syntax in a similar fashion to the way the TRiX XML format makes explicit the structure of the RDF graph. Just as TRiX uses XML element names and XML attribute names corresponding to the names of the components of the RDF graph (<graph>, <triple>, <typedLiteral datatype="..."> etc), so DC-DS-XML uses XML element names and XML attribute names corresponding to the names of the components of the description set (<descriptionSet>, <description>, <statement>, <valueString sesURI="..."> etc). In DC-DS-XML, the various URIs in the description set model are represented as XML attribute values, and literals are represented as XML element content.

A GRDDL Namespace Transformation is provided, in the form of an XSLT stylesheet, following the mapping from a description set to an RDF graph described by the DCMI Recommendation Expressing DC metadata using RDF. This means that any instance of the DC-DS-XML format can be translated into an RDF/XML document, and a GRDDL-aware application can automatically extract an RDF graph corresponding to the description set encoded in a DC-DS-XML instance.

A W3C XML Schema for the DC-DS-XML format is provided. A (rather more drafty!) RELAX NG schema is also available.

Comments on the new document are welcome, and should be sent to the DC-Architecture Jiscmail mailing list.

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