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The XML Family
of Technologies
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schemata (sometimes also called schemas).The primary purpose of a schema is to allow machine
validation of a document's structure, and a schema is comprised of metadata designed to
describe the organisation and content of related XML documents, thus augmenting the
variety and usefulness of Web-delivered content.
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A schema will list the elements that a
document contains, and the order in which they appear, as well as rules governing the
permitted content of elements, and more advanced cardinality restraints than DTDs offer.
XML schemata offer much more functionality and versatility than DTDs. Schemata
allow more meaningful and in-depth data typing and provide much improved support for
XML Namespaces, an approach by which tags are associated with a unique identifier to
eliminate confusion between vocabularies.
Data typing allows quality assurance checks
to be embedded directly within the schema
itself, instead of being linked to external pro-
cesses, as is the case with DTDs. Namespaces
allow the intermingling of vocabularies
without the confusion that can plague the
use of numerous DTDs. Further benefits
and uses of namespaces are covered in more
detail below. Unlike XML in its purest form,
schemata are always application-dependent,
and unlike DTDs schemata are them-
selves defined using XML. A working XML
schema definition is therefore by definition
a well-formed XML document as well.
Without the careful use of DTDs or
schemata, there can be no XML-assisted
interoperability between systems.
Interoperability and Resource Discovery: Practice
A diversity of future uses for XML will be derived from its machine-readability. As XML
development becomes more refined and an increasing amount of Web content is created
according to XML guidelines, automated searching, collocation, and cross-referencing of
materials will become an increasingly fundamental facet of the Web's future.
Currently, however, there is much risk of confusion between identical tags that may
have different types and uses in different fields or applications. At the moment XML ele-
ments and attributes can be identified and universalised using XML Namespaces, thus
eliminating the potential for clashes between identically named tags. In conjunction with
a Universal Resource Identifier (URI), which functions as a unique identifier rather than a
separate resource, namespaces guarantee uniqueness across all applications, and provide a
simple and efficient means of distinguishing between vocabularies.
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The platform neutrality of XML ensures its potential for interoperability, and an appli-
Univ
ersity of Glasgow
The relationship between DTDs, schemata and XML namespaces
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For more on crosswalks and interrelations between schemata, please see Kimberly S. Lightle and Judith S.
Ridgway, "Generation of XML Records across Multiple Metadata Standards" in D-Lib Magazine, vol. 9,
no. 9, September 2003: http://www.dlib.org/dlib/september03/lightle/09lightle.html
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It is worth stressing that the URI does not have to point to a real Web page or resource, though namespaces may
develop in the future for holding something useful such as identifying the file format of a related resource.
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