One part proposes facilities for associating datatypes with XML element types and attributes; this will allow XML software to do a better job of managing dates, numbers, and other special forms of information. The other part of the specification proposes methods for describing the structure and constraining the contents of XML documents. These drafts, which builds on earlier work submitted to W3C by several vendor and user organizations, are the first step in the development of a powerful, vendor-neutral format for defining the rules that govern particular kinds of XML data.
Current members of the W3C XML Schema Working Group are key industry players in Web publishing, XML processing, and database management system. In alphabetical order, they are: Adobe Systems, Agranat Systems, Arbortext, University of Bristol, Calico Commerce, Commerce One, University of Edinburgh, Extensibility, Graphic Communications Association, Health Level Seven, HP, IBM, University of Illinois at Chicago, Inso, Lawrence Berkeley National Laboratory, Lotus Development, Louisiana State University, Merrill Lynch, Microsoft, Microstar, Object Design, Omnimark Technologies, Oracle, SAP, SoftQuad, Software AG, Sybase, Vignette, Wall Data, and Xerox.
Following W3C's practice, the XML Schema Working Group provides a public mailing list for comments on the working drafts, in addition to the feedback channels defined by the W3C Process. The address for comments is given in the drafts themselves.
Extensible Markup Language (XML) was originally designed for encoding human-readable documents, but quickly attracted attention from groups interested in electronic commerce, interchange of data from relational and object-oriented databases, and other non-document applications. The Resource Description Framework (RDF) was designed to integrate a variety of web-based metadata activities including content ratings, search engine data collection, and digital library collections. Many applications can benefit from the development of schemas:
When XML is used to exchange technical information in a multi-vendor environment, schemas will allow software to distinguish data governed by industry-standard and vendor-specific schemas, and help applications know when it is safe to ignore information they do not understand, and when they must not do so. This means schemas may help make software more robust and systems more able to change and adapt to evolving situations.
Traditional document processing will also benefit from XML Schemas, because schema-aware document management systems will be better able to guide authors and editors in the creation and maintenance of documents.
XML, in accordance with its origin, provides methods of expressing syntactic validity constraints on document content, without attempting any formal specification of the meaning of markup. RDF, starting from the problem of describing Web resources in general, has focused more on the semantics of metadata, and less on the syntax. One of the most important design challenges for the ongoing work on XML Schemas and RDF is to ensure that their approaches and data models converge.
At present, rules about what kinds of information can appear in an XML document can be expressed only in the form of XML document type definitions (DTDs). DTDs use a special format to define the rules for using XML markup for different kinds of documents, but in practice there are some common rules that cannot be expressed at all in DTD form. XML Schemas are more powerful than DTDs, so they will be able to express some rules that DTDs cannot express.
Even more important for many purposes is that XML schemas are themselves XML documents. Using XML as the document format for schemas, instead of using a special-purpose form as DTDs do, will allow users and developers of XML schemas to use standard XML tools, the same ones they use for other structured information, instead of having to shift to specialized tools for work on schemas. It will also remove some of the shroud of mystery that have traditionally made DTD development a black art. Any existing XML processor can read an XML schema; interchange will be easy.
The XML Schema Working Group began its work by attempting to clarify the scope of its work. Requirements and suggestions have been gathered and discussed; eventually, these were refined into a short requirements document, which outlines some of the core usage scenarios and assumptions that are governing the design of the XML Schema language.
The XML Schema work builds on existing W3C specifications and experience in XML and database fields. The intense work of preparing these drafts has harnessed the expertise of key players among the W3C Membership in the fields of document management, database management, and electronic commerce. The diversity of representation within the W3C working group helps ensure that XML Schemas will be an open, vendor-neutral format that content creators can easily use and depend upon for information interchange over the Web. The purpose of these publications is to encourage public comments and contributions.
Further information on XML Schema work can be found in the XML Activity statement at http://www.w3.org/XML/Activity
The W3C was created to lead the Web to its full potential by developing common protocols that promote its evolution and ensure its interoperability. It is an international industry consortium jointly run by the MIT Laboratory for Computer Science (MIT LCS) in the USA, the National Institute for Research in Computer Science and Control (INRIA) in France and Keio University in Japan. Services provided by the Consortium include: a repository of information about the World Wide Web for developers and users, sample code implementations to embody and promote standards, and various prototype and sample applications to demonstrate use of new technology. To date, over 300 organizations are Members of the Consortium.
For more information about the World Wide Web Consortium, see http://www.w3.org/