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DRAFT UPDATE to Namespace Policy for the Dublin Core Metadata Initiative (DCMI)

Glossary:

This document uses the following terminology:

term

A property (i.e. element or element refinement), vocabulary encoding scheme, syntax encoding scheme or concept taken from a controlled vocabulary (concept space).

DCMI term

A term that is declared and maintained by the DCMI.

term URI

The URI that identifies a term.

term name

A human-readable token assigned to a term.

DCMI term URI

The URI for a term that is declared and managed by the DCMI.

vocabulary

A collection of terms (often as used in the context of an 'application profile').

DCMI namespace

A collection of DCMI term URIs where each term is assigned a URI that starts with the same 'base URI'. The 'base URI' is known as the DCMI namespace URI. (Note that a DCMI namespace is not the same as an 'XML namespace').

DCMI namespace URI

The URI that identifies a DCMI namespace.

DCMI recommendation

A human-readable document that may define one or more DCMI terms.

DCMI term declaration

A machine-processable representation of one or more DCMI terms, expressed in a schema language.

Note that the grouping of term URIs into a DCMI namespace is orthogonal to the grouping of terms into a vocabulary. Term URIs are grouped into DCMI namespaces in order to ease the assignment of URIs to terms and to streamline their use in particular encoding syntaxes. Terms are grouped into vocabularies in order to meet a functional need.

1. Introduction

All terms used in metadata descriptions that conform to the DCMI Abstract Model [DCAM] must be assigned a unique URI [RFC3986]. For convenience, the term URIs that are assigned and managed by the DCMI are grouped into collections known as DCMI namespaces. This document describes how term URIs are allocated by the DCMI and the policies associated with DCMI namespaces.

2. DCMI Namespace URIs

The DCMI namespace URI used for the collection of properties that make up the Dublin Core Metadata Element Set, Version 1.1 [DCMES] is:

http://purl.org/dc/elements/1.1/

The DCMI namespace URI used for the collection of all other DCMI properties and encoding schemes (i.e. other than those properties defined in the Dublin Core Metadata Element Set, Version 1.1 above) is:

http://purl.org/dc/terms/

The DCMI namespace URI used for the collection of DCMI terms in the DCMI Type Vocabulary [DCMI-TYPE] is:

http://purl.org/dc/dcmitype/

Therefore, the three currently approved DCMI namespace URIs are:

All DCMI namespace URIs will dereference to a machine-processable DCMI term declaration for all the terms with term URIs within that DCMI namespace.

Some example DCMI term URIs follow:

http://purl.org/dc/elements/1.1/title

is the DCMI term URI for the Title element in the Dublin Core Metadata Element Set, Version 1.1,

http://purl.org/dc/terms/extent

is the DCMI term URI for the Extent property, and

http://purl.org/dc/dcmitype/Image

is the DCMI term URI for the Image term in the DCMI Type Vocabulary. Each DCMI term can be so identified.

All future DCMI namespace URIs (additional DCMI controlled vocabularies for example) will conform to this pattern:

http://purl.org/dc/namespace_label/

3. Policy concerning classes of changes to DCMI terms

Changes to DCMI terms or term declarations will occur from time to time for a variety of reasons. Such changes have varying implications for DCMI term URIs and DCMI namespaces. The following classes of changes are identified along with examples and associated implications.

In all cases, any changes to DCMI terms or term declarations will result in an update to the versioning information carried in the DCMI recommendation and/or DCMI term declaration associated with that term.

A. Minor editorial errata

Errors of spelling, punctuation, or other clerical mistakes discovered in DCMI recommendations and/or DCMI term declarations will be corrected without a comment period, following notification to the DCMI Usage Board [DCMI-USAGE], as long as, in the judgment of the DCMI Directorate, there are no implications for negative impact on users or applications that rely on those DCMI term declarations.

Correction of minor editorial errata will result in no changes to DCMI term URIs.

B. Substantive editorial errata

Errors of substance discovered in DCMI recommendations and/or DCMI term declarations will trigger public notification of the correction to the DC-General mailing list [DC-GENERAL]. Errors that, in the judgment of the DCMI Directorate, compromise the immediate usefulness or accuracy of DCMI metadata systems will be corrected immediately (for example, an incorrect URL to a resource external to DCMI). Others will be corrected following a 14-day public comment period to assure that changes do not adversely effect systems or applications which rely on the DCMI namespace infrastructure.

Correction of substantive editorial errata will result in no changes in DCMI term URIs.

C. Semantic changes in DCMI terms

Changes of definitions within DCMI recommendations and/or DCMI term declarations will be reflected in the affected DCMI recommendation and/or DCMI term declaration. If, in the judgment of the DCMI Directorate, such changes of meaning are likely to have substantial impact on either machine processing of DCMI terms or the functional semantics of the terms, then these changes will be reflected in a change of URI for the DCMI term or terms in question. The URIs for any new DCMI namespacess resulting from such changes will conform to the DCMI namespace URI pattern defined above.

D. Addition of DCMI term declarations to existing DCMI namespaces

New DCMI term URIs will occasionally be added to existing DCMI namespaces. Addition of DCMI term URIs to existing DCMI namespaces will not trigger changes in DCMI namespace URIs.

4. Persistence Policy

The DCMI recognizes that people and applications depend on the persistence of formal documents and machine processable schemas that have been made publicly available. In particular, the stability of DCMI term URIs and DCMI namespace URIs is critical to interoperability over time. Thus, the wide promulgation of this set of URIs dictates that they be maintained to support legacy applications that have adopted them.

6. Justification

Two significant issues were raised during the development of this policy. Firstly, that DCMI namespace URIs should indicate the category of DCMI terms associated with that namespace. For example, it was proposed that different DCMI namespaces might be used to partition DCMI properties from DCMI encoding schemes, or to indicate that a particular term was originally defined by a particular community or within a particular domain. Secondly, that all DCMI namespace URIs should carry versioning information (for example a date stamp) that would be updated as terms within the namespace change.

On the first issue it was considered that the category of DCMI terms was not necessarily persistent. For example, terms defined initially by the education community might subsequently become useful to other communities. Associating particular URIs with particular categories of terms was not felt to be helpful to the long-term stability of DCMI namespaces or the URIs of DCMI terms within those namespaces.

On the second issue it was again considered that embedding versioning information within the DCMI namespace URI was unlikely to be helpful to the long-term stability of DCMI namespace URIs or DCMI term URIs within those DCMI namespaces. Rather, it was felt that versioning information should be carried within the DCMI recommendations and/or DCMI term declarations associated with DCMI namespaces and terms.

Finally it should be noted that, although the 15 elements currently within the http://purl.org/dc/elements/1.1/ DCMI namespace could have been assigned new URIs within the http://purl.org/dc/terms/ DCMI namespace, it was considered that the widespread existing usage of the former DCMI namespace URI mitigated against any change. Furthermore, the existing use of the purl.org domain for that DCMI namespace URI prompted its use for all DCMI namespace URIs.

References

[DCAM]
DCMI Abstract Model, DCMI Recommendation, March 2005
http://dublincore.org/documents/abstract-model/

[RFC3986]
IETF (Internet Engineering Task Force) RFC 3986: Uniform Resource Identifier (URI): Generic Syntax, eds T. Berners-Lee, R. Fielding, L. Masinter. January 2005
http://www.ietf.org/rfc/rfc3986.txt

[DCMES]
Dublin Core Metadata Element Set, Version 1.1: Reference Description
http://dublincore.org/documents/1999/07/02/dces/

[DCMI-TYPE]
DCMI Type Vocabulary, DCMI Recommendation, July 2000
http://dublincore.org/documents/dcmi-type-vocabulary/

[DCMI-USAGE]
DCMI Usage Board
http://www.dublincore.org/usage/

[DC-GENERAL]
DC-General mailing list
http://www.jiscmail.ac.uk/lists/dc-general.html