Everything You Need to Know about XML
Everything You Need to Know about XML
By: Mark Cyzyk
Dec. 3, 2001 12:00 AM
What an excellent little book!
In a scant 96 pages, Eckstein and Casabianca have managed to present everything you need to know to get up and running with XML. After the obligatory review of what XML is and why it's needed, as well as definitions of some of the key concepts of XML technology, the authors launch into a concise, though comprehensive, discussion of DTD (Document Type Definition) design and construction. A DTD specifies the overall structure and content of a valid XML document; it specifies the elements a document can contain as well as the allowed attributes of those elements. Element declarations, entities, and attribute declarations are well covered here, and the examples are clear and unambiguous.
The authors then embark on a discussion of XSL (Extensible Stylesheet Language). The two XML technologies that fall under this rubric are XSLT (Extensible Stylesheet Language Transformations) and XSL-FO (Extensible Stylesheet Language, Formatting Objects). Because XSL-FO is not yet a very mature technology, little attention is paid to it. However, the coverage of XSLT is the richest part of the entire book.
Essentially, XSLT allows you to transform one XML document into another XML format; for example, from a custom XML DTD to XHTML for display in a browser. It converts one set of tags into another set of tags via a mapping in a template file. And XSLT ends up being much more than a way to map tags to tags: the authors illustrate the looping and conditional constructs of XSLT that allow you to program simple logic into the transformation process. After several clear examples of XSLT, the authors provide a long reference section of XSLT elements. It would have been nice if the book contained a long example illustrating the proper use of several of these elements in a stylesheet, but you can get the gist just by looking at their generic, element by element, definitions.
XPATH is the next topic of discussion. XPATH actually underlies the transformation process in XSLT by providing the syntax that locates each node in the document to be transformed. As such, XPATH is sort of a "regular expression" technology of XML. XPATH also provides the direction or "axes" in which the document and its nodes should be traversed, as well as some built-in functions that can be used to do such things as count the number of nodes in a branch of the document, search and compare strings, return the sum of the numerical values contained in a set of nodes, and so on.
So the simple looping and conditional contructs of the XSLT elements, coupled with the rich syntax of XPATH used to locate and otherwise manipulate node values in an XML tree, make XSLT a very powerful tool for data formatting and transformation. Again, one long example illustrating as many of these distinct technologies as possible would have been enlightening.
The last part of the Pocket Reference is dedicated to a discussion of XPointer and XLink - technologies that allow for the interlinking of XML documents. As the authors point out, this is essentially the same as the use of anchors and internal links within HTML documents. Use of these technologies basically allows for the linking of one section of a document to another section, either in the same document or in an entirely different one. It's interesting to note that XLink provides for linking from a single link to multiple targets.
With this book O'Reilly has added yet another winner to their fine catalog. As with other O'Reilly publications, the writing style is eminently readable. The typographical conventions used consistently throughout the book make even a quick scan visually informative.
Finally, as the authors note in their introduction: "Some XML-related specifications are still in flux as this book goes to print. However, after reading this book we hope that the components that make up XML will seem a little less foreign."
As far as this reader is concerned, the authors have admirably achieved their objective. I recommend this book highly for those who want to incorporate XML technologies into their Web applications.
XML: Pocket Reference, 2nd Edition
Published by O'Reilly and Associates
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