Survey of Metadata Registries

Contents

• Characteristics of Metadata Registries
• Standards and APIs
• Registry Standards
• Schema Languages and Formats
• Access Standards and APIs
• Registries
• IEMSR
• SchemaWeb
• DCMI Registry
• Metadata Online Registry (METeOR)
• NSDL Metadata Registry
• XML.org Registry
• DoD Metadata Registry and Clearinghouse
• Other Registries
• Commercial Tools
• References

Characteristics of Metadata Registries

A metadata registry can be defined as a central location where metadata definitions are stored and maintained. Metadata registries are used by a variety of organisations to structure their metadata and enable interoperability, sharing and re-use of data. The registries act as a central source of authorative schemas or vocabularies for use within a domain.

Metadata registries differ in a number of key ways depending on their purpose and application domain.

1. schemas vs vocabularies (and the formats supported)
2. common model vs repository
3. breadth of domain
4. levels of access
5. human and software agents (and the APIs supported)

Metadata registries can be used to store either schemas or vocabularies. Schemas describe the structure of data or metadata. For example, XML Schemas describe the structure of XML documents and database schemas describe the table structure of relational databases. A central resource which contains specifications of schemas in formats such as SQL, UML, XML Schema or as application profiles, enables data creators or recorders within a domain to produce metadata which adheres to a common structure. This data is then more easily integrated.

Metadata registries that store domain vocabularies enable users to produce both structurally standard metadata and use common or standard terminology for metadata fields. Ontologies describe the semantics of a domain and can be used to connect or relate terms. These vocabularies, common terminologies or ontologies can be recorded in formats such as RDF Schema, OWL, DAML+OIL or SKOS.

If a metadata registry is used to store schemas or application profiles, then it is often desirable to relate these schemas through a common data model such as Dublin Core or IEEE LOM. This enables users to search for related terms and to produce metadata that can be integrated through this common model even though the schemas used may be different. Other metadata registries act more as repositories and store schemas or vocabularies without relating them to a common model.

Registries relating schema to a common data model are generally quite specific and restricted to a particular domain. Repositories, which do not use a common model, can be very broad and may store schema from a wide variety of domains. The interfaces to both these styles of registry need to provide users with convenient methods for searching, browsing and querying the stored schema or vocabularies.

Varying levels of access are also generally required by metadata registries. Some registries act as a service provider and do not support public registration of schema while others allow anybody to submit a schema for registration. Security issues can also be a concern particularly in sensitive or commercial applications.

Registries provide many different services and interfaces. Some of these interfaces are used by human agents to discover relevant schema or vocabularies. These interfaces may be built on advanced query languages such as RDFQL or SPARQL or use syndication/notification services based on RSS. Registries are also accessed by software agents to automatically define terms and discover relationships between metadata objects defined using schema in the registry. APIs (application programming interfaces) can be used for providing web services using standards such as SOAP, REST and WSDL.

This document describes a number of existing metadata registries, the approaches they use and standards they apply. Each registry is summarised according to its administrator, domain, type (schema or vocabulary), APIs used and availability.

Standards and APIs

Registry Standards

The ISO/IEC 11179 Metadata Registry (MDR) standard is a multipart standard that defines the structure and behaviour of an enterprise metadata registry. The standard describes a hierarchy of "concepts" with associated properties. Each concept and property has a precise "Data element definition". Many existing metadata registries or data models, chiefly from government organisations, have been based on ISO/IEC 11179. Examples include:

• Australian Institute of Health and Welfare - Metadata Online Registry (METeOR)
• Global Justice XML Data Model (JXDM)
• Environmental Data Registry (EDR)
• Cancer Data Standards Repository (caDSR)
• National Information Exchange Model (NIEM)

While the ISO/IEC 11179 standard is large and very comprehensive, there are still groups who are proposing extensions and modifications. One example is the Extended Metadata Registry which is proposed by a consortium of scientific organisations. Their aim is to extend ISO/IEC 11179 "to support more diverse types of metadata and enhanced capabilities for semantics specification and queries."

Schema Languages and Formats

XML: eXtensible Markup Language. A W3C standard, basic data storage format.

XML Schema. XML format for defining the structure (syntax) of XML documents.

RDF: Resource Description Framework. A W3C standard for describing triples - object-property-subject relationships.

RDF Schema. XML format for defining relationships (semantics) in RDF.

DAML+OIL: DArpa Markup Language + Ontology Inference Language. Used to define semantics in ontologies. Richer than RDF Schema.

OWL: Web Ontology Language. A W3C standard XML format for defining ontologies.

SKOS: Simple Knowledge Organization System. A set of three RDF vocabularies for describing "thesauri, classification schemes, subject heading lists, taxonomies, other types of controlled vocabulary, and perhaps also terminologies and glossaries, within the framework of the Semantic Web."

Access Standards and APIs

RDFQL: RDF Query Language. An earlier query language for RDF.

SPARQL: SPARQL Protocol And RDF Query Language. A W3C standard query language and protocol for querying and access RDF documents.

These are the RDF query languages most often referred to by metadata registries. Further information about RDF query languages can be found in [KAR99].

RSS: Really Simple Syndication. An XML format for describing news feeds. This is useful for providing simple, automatic updates as the contents of metadata registries change.

REST: Representational State Transfer. An architectural style proposed by Roy Fielding for use in web service based applications.

SOAP A W3C protocol for exchanging XML-based messages over a computer network, normally using HTTP. Used as the basis for web services.

Registries

The section briefly describes some existing metadata registries. Each registry is summarised according to its domain, type, the interfaces it provides and its availability.

Information Environment Metadata Schema Registry (IEMSR)

The IEMSR is preceded by a number of registry projects - DESIRE, SCHEMAS, CORES and MEG. It is based on software from the MEG registry project and aims to extend the work done there to include the IEEE Learning Objects Metadata (LOM) as a common model for application profiles. A document describing the predicted use cases [IEMSR-usage] is useful for understanding the scope of IEMSR.

SchemaWeb

SchemaWeb was announced on the w3c-rdf-interest list on the 19th of November 2003. It is a large and active repository containing a wide variety of schemas including standards such as OWL and RDF as well as ontologies for beer and music. A variety of interfaces are provided for both human and software agents.

Dublin Core Metadata Initiative (DCMI) Registry

Provided by the Dublin Core Metadata Initiative solely for use as a reference to the Dublin Core metadata model. It provides usage examples for elements in the models as well as definitions, descriptions, possible refinements and related schema. Multiple views of the underlying model are available in formats such as RDF/XML, N-triple and N3.

Metadata Online Registry (METeOR)

Developed to address the need in Australia for a formal approach for communicating national health data for statistical purposes. It was preceded by Knowledgebase and the National Health Data Dictionary.

NSDL Metadata Registry

Built on both the DCMI registry and CORES, the NSDL Metadata Registry intends to complement the existing NSDL Metadata Repository by providing the means to publish schemas and vocabularies. A useful list of terminology used by the project can be found in [NSDL-terms].

XML.org Registry

Appears to be currently inactive as the last submission was in 2001.

DoD Metadata Registry and Clearinghouse

Access to parts of the site containing content are restricted to users with an account.

Other Registries

• Distributed Interoperable Metadata Registry - proposes an architecture but no online implementation or demonstrator
• ROADS (Resource Organisation and Discovery in Subject-based services) Metadata Registry - circa 1997

Commercial Tools

• Oracle Enterprise Metadata Manager
• SAS Metadata Repository
• Data Foundations (consulting service)

References

[wikiMDR] Wikipedia - Metadata Registry

[KAR99] "RDF Query Languages: A state-of-the-art" Greg Karvounarakis.

[IEMSR-usage] Usage Scenarios for the IE Metadata Schema Registry Joint Information Systems Committee.

[NSDL-terms] Metadata Policy for NSDL