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KMR Group, NADA, KTH (Royal Institute of Technology), Sweden
|Status of Document:||This is a DCMI Recommended Resource|
|Description of Document:||This document serves as a guide to implementers to the changes introduced with the 2008-01-14 DCMI Recommendation "Expressing Dublin Core metadata using the Resource Description Framework (RDF)" (and subsequent revisions) with respect to legacy DCMI specifications.|
In January 2008, DCMI released "Expressing Dublin Core metadata in the Resource Description Framework (RDF)" as a DCMI Recommendation [DC-RDF]. This Recommendation replaces two legacy DCMI documents:
This document provides a guide to the changes introduced with the 2008 Recommendation.
Since 1997, the "Dublin Core data model" has evolved alongside W3C's Resource Description Framework (RDF). This process has resulted in the DCMI Abstract Model [ABSTRACT-MODEL], which was published in March 2005 as a DCMI Recommendation. The DCMI Abstract Model (DCAM) now provides a reference model on the basis of which particular Dublin Core expressions can be defined.
Since the publication of the DCAM, the Architecture Working Group has been preparing a new expression of Dublin Core in RDF. In March 2006, Mikael Nilsson (Royal Institute of Technology, Sweden) finalized an early draft of the 2008 Recommendation for a DCMI Public Comment period in June 2006. On the basis of feedback received in the Public Comment period and on the basis of an updated version of the DCMI Abstract Model, an updated version of the new RDF expression was prepared for Public Comment in April 2007 and finalized as as a DCMI Recommendation in January 2008.
The new specification represents a significant step in the evolution of the Dublin Core RDF expressions compared to the two earlier specifications. Historically, Dublin Core metadata expressed in RDF has suffered from a number of problems, including:
conflicting recommendations in the above documents regarding the use of literal strings in Dublin Core metadata;
implementation complexity due to the use of idiosyncrasic constructs;
difficulties in providing formal ranges and domains for Dublin Core properties.
The legacy RDF expression specifictions, which predate the DCAM, contain constructs that are incompatible with concepts in the DCAM.
The January 2008 specification addresses these problems in ways described below.
The most significant change introduced by the January 2008 Recommendation is the addition of support for domains and ranges of properties in general, and of DCMI-defined properties in particular. DCMI metadata terms have hitherto been defined exclusively in natural language. The RDF expression of the DCMI term set (e.g., http://dublincore.org/2003/03/24/dces) served essentially to convey these English-language definitions in a form ingestable by RDF applications. As part of the process of clarifying the RDF expression for Dublin Core metadata, it became evident that DCMI would benefit from supplementing these English-language definitions with machine-understandable declarations of domains and ranges. Such additional, machine-understandable precision is necessary as Dublin Core is deployed in the context of inference engines and ontology-based applications. In January 2008, the DCMI Usage Board is finalized the assignment of formal domains and ranges which make explicit the meanings intended in natural-language definitions [DOMAIN-RANGE]; see also the document "DCMI Metadata Terms" [DCMI-TERMS], which has been updated to reflect these changes, and the decision document "Revisions to DCMI Metadata Terms" [DCTERMS-CHANGES], which contains a detailed summary of all changes to the DCMI metadata terms introduced in the January 2008 specifications.
For most DCMI metadata terms, the process of clarifying domains and ranges machine-understandably is straightforward and unambiguous. However, one problem with regard to legacy metadata usage is serious enough to bear closer scrutiny.
Dublin Core community has long distinguished between Simple
and Qualified Dublin Core -- a distinction reflected in the
difference between the specifications "Expressing Simple
Dublin Core in RDF/XML" [DCMES-XML]
and the "Expressing Qualified Dublin Core in RDF/XML"
[DCQ-RDF-XML]. The two legacy specifications
differ with regard to
whether properties such as
values that are non-literal resources (e.g., a Person or
a Date, seen as entities), or literals representing the
resources (i.e., a value string). In "Expressing Simple
Dublin Core in RDF/XML", a
dc:creator is a name:
In "Expressing Qualified Dublin Core in RDF/XML",
in contrast, a
dc:creator is an entity, as in:
<http://www.example.com> dc:creator <http://www.example.org/person32>
<http://www.example.com> dc:creator _:xxx .
_:xxx rdf:type foaf:Person .
_:xxx rdf:value "John Smith" .
The new RDF encoding specification supports both of these constructs but bases the choice of one form over the other on the range of a property. A property with a "literal" range will follow the former pattern, while a property with a "non-literal" range will follow the latter.
In accordance with this approach, the DCMI Usage
Board has assigned appropriate ranges to the DCMI
properties. A range of "Agent" has been given to
dcterms:contributor, where "Agent" is defined
as "A resource that acts or has the power to act". Similarly, appropriate ranges have been specified
for the other DCMI terms. The range "Literal" applies
only to metadata terms which are typically associated with
a single value string, such as
Using this approach,
dcterms:creator refers to an entity which can
(e.g., assigned an identifier in an authority file) and described in its own right
(e.g., with a name, an affiliation, and a birth date). The
English-language definitions of these terms bear out this
dcterms:creator is "An entity primarily responsible for making the resource", examples being "a
person, an organization, or a service". However, the legacy usage
comment associated with this definition reflects the
ambiguity: "Typically, the name of a Creator should be used
to indicate the entity".
In most cases, the appropriate range of a term has become
reasonably obvious through a decade of implementation practice.
In some cases, such as
usage has been ambiguous, so the assignment of any specific
range would make one or another part of the legacy metadata
appear invalid in the context of machine processing. Declaring
"Agent" as the range of
dc:creator would mean that inferencing
applications would expect to treat the value of the
property as a non-literal entity. Where legacy metadata
represents names as literal values for
would need to treat these as "special cases" in order to merge
them with metadata in which those names were associated with
the expected non-literal entity constructs. The legacy specifications
did not properly address these
ambiguities, with the result that an unknown amount of Dublin
Core-based RDF data is inconsistent with the intended semantics
of the Dublin Core properties.
The clarification of these
ambiguities through the assignment of domains and ranges is
currently considered to be a desirable step towards ensuring
the long-term viability of Dublin Core in RDF. However, one important
compromise has been reached:
domains and ranges will only be asserted for properties in the
http://purl.org/dc/terms/ namespace, including copies of the properties
in the http://purl.org/dc/elements/1.1/ namespace. Thus,
will still have an unspecified range and can be used with both literal
and non-literal values, while
dcterms:creator will have a (non-literal)
range of Agent.
The declaration of domains and ranges for DCMI properties has important implications for the interpretation of legacy Dublin Core metadata in RDF. However, the interpretation of Dublin Core metadata in other formats, such as HTML [DCQ-HTML] and XML [DC-XML-GUIDELINES, [DC-XML], is not negatively affected by these developments. The rules for interpreting metadata in these syntaxes in terms of the DCAM are simpler than for RDF, as these other syntaxes are not bound by the semantics of RDF.
The declaration of domains and ranges helps clarify the formal semantics of DCMI properties. Metadata creators need to use syntactic constructs to ensure that RDF-consuming applications correctly interpret any value strings. The generation of Dublin Core metadata in RDF becomes slightly more complex for anyone producing metadata by hand. However, these measures eliminate the current ambiguity, enabling metadata that is mappable more consistently to the DCAM. Support by tools is improved by the machine-processable restrictions. In order to process legacy metadata, metadata consumers might need to "special-case" any metadata containing value strings associated directly with the affected Dublin Core properties (i.e., without intervening non-literal nodes).
The January 2008 Recommendation differs from the legacy RDF specifications in its handling of value strings.
The DCAM specifies that each value can be represented
in a DCAM statement by multiple value strings. The
new RDF expression supports this construct, using the
. This allows value strings in different
languages or using different syntax encoding schemes to be
used as representations of a single value.
Value strings are now expressed using
expressing value strings is no longer supported, as its definition does
not clearly fit this purpose. Of
course, the use of those properties is not forbidden, but
these properties are not considered to have any special
interpretation in terms of the DCAM.
RDF datatypes can now be used with value strings, corresponding to the DCAM concept of Syntax Encoding Schemes.
For value strings occurring as the object of a
property, this is a simple
The new specification also allows the use of datatyped or plain literals as direct values of properties when the value is a literal.
The January 2008 Recommendation deprecates several constructs described in the May 2002 specification [DCQ-RDF-XML].
The RDF Container constructs
rdf:Seq are no longer provided
as an alternative for constructing ordered and unordered
sets. They have no correspondence in the DCAM, and except in
the case when the range of a property includes one of these
classes, they should no longer be used.
The recursive use of
rdf:value for structured
values has been deprecated. It has no correspondance in
the DCAM and does not lend itself very well to automated
processing. The use of this construct is therefore no longer
The use of "poor-man's language qualification" in the 2002 specification does not fit the DCAM and does not take into account the language tagging of plain literals in RDF. It is no longer supported.
The dumb-down algorithm is independent of any particular expression of Dublin Core metadata (such as Dublin Core metadata in RDF) and is therefore out of scope for this specification. References to dumb-down have therefore been removed.
The use of reification is now considered to fall outside the scope of the specification and is therefore no longer part of the January 2008 Recommendation. As it does not interfere with the metadata itself, however, reification can still be used in accordance with RDF specifications.
The RDF schemas for DCMI properties and classes are part of the definitions of these terms and do not belong specifically to the RDF expression of Dublin Core metadata. They have been removed from the draft specification itself and can be accessed at http://dublincore.org/schemas/rdfs/. Human readable documentation matching the RDF Schemas is available at [DCMI-TERMS].
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