Page 86 - Building Digital Libraries

General-Purpose Technologies Useful for Digital Repositories

items from the XML document to retransform the data to something new.
When dealing with XML-based metadata formats, XPath and XQuery rep-
resent the two primary methods for working with metadata. Currently, most
metadata crosswalking is done utilizing XSLT stylesheets. These stylesheets
make liberal use of XPath statements to process elements within the XML
document in order to extract or transform the XML document into some
other useful format. The Library of Congress, for example, is producing a
number of XSLT stylesheets which make liberal use of XPath statements
to provide crosswalking functionality between MARC21XML (and XML
representations of MARC21) and other metadata formats like MODS and
Dublin Core. These crosswalks can be found at the Library of Congress’s
MARC website at www.loc.gov/marc/.

XForms

XForms was designed by the World Wide Web Consortium (W3C) as a
technology to replace the current form methodology found in HTML 4.0.
It was designed to work as a companion technology with XHTML. Unfor-
4
tunately, there are very few real-world implementations utilizing XForms,
in part because many current-generation browsers only lightly support
the protocols, and current development tools do not take advantage of the
XForms syntax. With that said, XForms, like XHTML, represents the future
of HTML and web development, so the technology should be monitored as
it continues to mature and becomes more universally supported.

XSLT

XSLT (eXtensible Stylesheet Language Transformations) represents one of
the technologies that can be used to generate display elements from XML. 5
XSLT’s primary function is to translate XML metadata to other document
types—whether those are HTML or another XML format. Essentially, an
XSLT document is made up of XPath statements, constructed in the form
of “templates” that are used to process specific elements within an XML or
XHTML document. In chapter 7, we will take a closer look at the mechan-
ics of XSLT and how it can be used in the crosswalking of XML metadata.

XLink

The XLink specification defines the methodology for the creation and
description of links between XML documents. In essence, the XLink syntax
6
simply provides HTML linking between XML documents. However, unlike
traditional HTML links, XLink treats links as objects that can have proper-
ties and actions associated with the link itself.

71