Entities are shortcuts similar to keyboard macros. They are used within a DTD and within the document instance. Within a DTD they can be used to summarize a long series of elements, attributes, and other entities, so they can be grouped and reused without significant typing. They are used within the document instance to represent special characters such as the Greek character beta. They can be used to represent a long data or text string that is used many times within the document instance. The parser inserts the correct character or text string on rendering.
One limitation of an XML DTD is that it is not a well-formed XML document instance. XML schemas are well-formed and are functionally equivalent to DTD’s. However, schemas are more powerful than XML DTD’s because their functionality has been extended. A schema can include extensive processing instructions and parameters used by software for any number of reasons. Data can be manipulated, generated, reorganized, and checked for errors. Element and attribute values can be better controlled fostering greater data quality.
Given the greater functionality of XML schemas, processing initiatives have been developed. Two processing initiatives are Digital Object Model (DOM) and Simple API for XML (SAX). DOM has a Level 1 W3C recommendation in circulation. DOM defines a programmatic interface for traversing the document instance hierarchy, and manipulating elements and attributes. DOM places the entire schema structure into memory, parsing it into a tree structure. Using a DOM implementation, programs can treat each node of the tree as an object and perform endless manipulations. SAX is being utilized to overcome the memory requirements of DOM, which can be onerous, for very complex schema applied to long documents. SAX as the name implies is simpler than DOM. Processing initiatives are the interface between your data and your software environment. DOM would layer between your XML repository, or database, and your photocomposition and fulfillment software.
Transformations allow multiple renderings, or views, of your data. An example of an XML transformation initiative is Extensible Stylesheet Language (XSL). XSL transform XML into HTML and other presentation formats including other XML. It allows extensive reordering, generating text, and calculations, but does not modify the source data. Because XSL uses XML syntax an XSL is well-formed. It can be sent with its document instance. Even multiple XSL can be sent with a document instance. Other transformation initiatives are underway. Your data requirements and the dominant trends in your field should guide your choice of initiatives.
A visit to an XML WEB site like www.xml.org reveals a bewildering array of XML schema, processing, and transformation initiatives. Where do you start? If helpful, go to the faq’s and tutorial pages. There is even an article titled, “XML for Dummies.” Once you’re comfortable with XML jargon, visit the vertical market pages. These represent initiatives in just about every field of endeavor from Agriculture to Zoology, from profit to nonprofits, from large to small. Stick to what is closest to home because such a visit can be overwhelming. A danger of XML is that it could become a “Tower of Babel.” Too many competing schema, requiring too much overhead to properly cross-reference using namespaces or anything else.
Herein lay areas of much needed research and development. What constitutes valid and useful taxonomies of meta data? Can meaningful ontologies of meta data taxonomies create archetypes and knowledge representations? Little is known about human-to-meta data interfaces?