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With more and more Metadata standards or schemas used in various projects, the need of encoding, searching, and exchanging different Metadata sets in one interface becomes much more important. The Resource Description Framework (RDF) is being developed by the World Wide Web Consortium (W3C) to provide an infrastructure for the interoperability of Metadata in a wide range of applications.

Every Metadata must define the syntax, semantics, and structure of its elements for its applications. Using XML (eXtensible Markup Language) as its common syntax, RDF help users define the structure of Metadata elements used the application profile. Those Metadata elements can be from more than one Metadata schemas. Although RDF does not directly define the semantics of Metadata elements, with the Namespace feature of XML it specifies the Uniform Resource Identifier of schema and its vocabularies so that the semantics of elements of these schemas can be tracked and explained clearly without any ambiguity. Here the vocabularies are defined as the set of properties, or metadata elements for resource description.

One of the advantages of this framework is that people do not need to design and create a new Metadata schema for a project. He/she can combine several existing Metadata elements sets together, and choose any element that meet his specific needs from those sets by using XML namespace to indicate which element belongs which schema, and the address of DTD of that schema. This could let the common Metadata schemas reused, avoid the huge investment spent on creating new schema, as well as increase the Metadata interoperability.

Following is a small example that explains how XML namespace can combine different metadata schemas together:

 
<RDF:RDF 
  xmlns:RDF="http://www.w3.org/RDF/"
  xmlns:DC="http://purl.oclc.org/DC/"> 
   
     Jack Ma 
  
RDF:RDF>

The first line, which can be interpreted as the element RDF in the context of the RDF namespace, defines a simple wrapper that marks the boundaries of the document using RDF.

Next two lines are XML namespace declaration, which define the namespace prefix 'RDF' and 'DC', using the unique resource identifiers (here use two URLs) to indicate which schemas are used in the document.

The next is the RDF "Description" statement designed to group multiple descriptive statements (or elements) for one object (here is the document that specified by its URL http://uri-of-object1) into a "Description" element.

The DC:Creator declares that the "Creator" property element of the object within Description is defined using the property element of DC schema, and its value is "Jack Ma". The last two lines just close the declaration of Description and RDF.

If needed, Qualifiers can be used under element declaration to further define the property of the object. Other metadata schemas can be added into this sample with the similar namespace declaration. Here RDF works like a platform where different Metadata schemas can work together describing different types of property of the same object. Using RDF as a module for Metadata application, a central registry is normally required to support the declaration, and mapping of metadata schemas within the project. Each metadata schema declares the vocabularies of its elements set to provide the ability of unambiguously expressing semantics for encoding, exchanging, and machine processing of metadata consistently.

Eric Miller has a good article An Introduction to the Resource Description Framework for readers who want to learn more about the RDF.

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