XML: It's a cheese spread. No, it's a floor wax. No, it's two--two--two products in one! Or maybe it's everything but the kitchen sink? Say, have you heard the one about the XML Kitchen Sink Language?--see http://blogspace.com/xkitchensink/.
XML: What It's All About
It has been said that XML, the Extensible Markup Language, will become the ASCII of the twenty-first century because it is rapidly becoming ubiquitous. XML is expected to have an impact on both the Web and application development comparable to that of Java and JavaScript because it has opened up a wide variety of new capabilities and has been embraced by so many sectors of human endeavor.
XML is a metalanguage--a syntax for describing other languages. These languages span diverse vertical industries including accounting, advertising, aerospace, agriculture, astronomy, automotive products, biology, chemistry, database management, e-commerce/EDI, education, financial institutions, health care, human resources, mathematics, publishing, real estate, software programs, supply chain management, and many more (for the many more, see http://www.xml.org/ ml/industry_industrysectors.jsp). In one sense, XML is really a very trivial thing--just a markup syntax for describing structured text using angle brackets. But in another sense, XML is a basic building block--an enabling technology that makes it possible to develop more complex, more interesting, and more powerful tools.
In the Web arena, XML is facilitating exciting improvements such as user-controllable views and filtering of information, highly focused searching based on element hierarchies, creation of truly device-independent content that can be repurposed for vastly different devices, and more sophisticated and flexible linking mechanisms. In the business and application arena, XML makes it easier to deliver filtered content from databases, to more readily share data between applications and between companies, and to exchange EDI messages that describe complex transactions. In the scientific arena, XML is a natural fit for describing complex datasets, models, control of instruments, images, chemical compounds, and much more.
Just as Java made data processing platform-independent, XML has done the same for data, making the exchange of information much easier than ever before. But, no, XML is not the kitchen sink; it is not the solution to all of the world's problems in one tidy package; nor is it the solution to all your computer needs either, at least not alone. Rather, XML is a tool, or more accurately, a set of tools from the same toolbox. That toolbox is the XML family of specifications. This book will help you see what XML can and cannot do by describing how to use each tool.
Although XML shares a number of concepts with its ancestor, SGML (Standard Generalized Markup Language), XML is said to yield 80 percent of the benefits of SGML, but with only 20 percent of the complexity. It is precisely this 80/20 rule that has excited countless companies and developers, encouraging them to support the efforts of the World Wide Web Consortium (W3C) in the development of XML. A few of the more than 500 companies and organizations that actively support XML development as members of the W3C include IBM, Sun Microsystems, Microsoft, Oracle, Commerce One, and NASA.
Audience: Who Should Read This Book?
The book is intended for Web developers, which includes programmers, content writers, and designers. Depending on your background and interests, some chapters may be more relevant to you than others. It's intended for those who may be familiar with particular aspects of XML but who have not been formally exposed to all of the major W3C specifications, as well as those who have never dealt with XML before. Later in this preface, I provide a roadmap to help orient you.
I've assumed that most readers are familiar with HTML elements and syntax, although the XML and DTD syntax discussions in Chapters 3 and 4 pretty much cover the concepts of elements, attributes, types, entities, and content that carry over from HTML to XML. In other words, you can get by without knowing HTML, except the XHTML chapter, which will make much more sense to you if you do. For those who would like to brush up on HTML, see "For Further Exploration: HTML and Java" at the end of this preface.
Some examples require programming knowledge, but for most examples, anyone with general Web development skills will find them beneficial. Generally, scope and breadth of treatment is favored over depth. On the other hand, some readers will find that the depth is more than they expected, but they should still be able to "tread the water." My intent in writing this book was to cover a number of XML-related technologies in varying degrees of detail. I'd like to make it clear that although there are three chapters containing Java examples, this is not a book about Java and XML. You don't need a Java background for the vast majority of what's in this book.
Although I do assume the Windows operating system, this is not a statement of preference. My formative years were spent on UNIX (I still use UNIX utilities to maintain a ski club site) at the office and on a Mac at home. Rather, since Windows tends to be somewhat ubiquitous, it seems appropriate to show Windows command lines and mention some Windows-only tools. UNIX and Mac users are encouraged to share their experiences with fellow readers via the book's Web site. Personally, I have found cygwin--a UNIX environment for Windows developed by Red Hat--to be very handy (see http://cygwin.com/).
What's Special About This Book?
There are several features that contribute to making this book an invaluable resource for anyone beginning to plunge into the somewhat turbulent "seas" of XML.
- XML Family of Specifications Big Picture--Since early 1998, I've periodically updated a diagram I call "The Big Picture of the XML Family of Specifications." This unique diagram (front inside cover) depicts virtually all of the key W3C efforts related to XML, with colors to indicate each specification's status (maturity); it includes related non-W3C efforts as well. Physical positioning denotes a relationship among neighboring specifications, as explained in Chapter 2. Best of all, the Big Picture diagram appears as an imagemap on the CD-ROM and on this book's Web site, possibly as a more up-to-date version. The Big Picture imagemap on the Web site expands acronyms as your mouse hovers over a term. Clicking on the acronym or name connects you instantly to the actual specification or, in some cases, a collection of documents relating to that specification.
- History Timeline--A detailed "History of the Web and XML" in timeline form--the product of a considerable amount of research--is broken down into three time periods in Chapter 1, which should be interesting to many readers. Historical perspectives are also presented for particular specifications in their own chapters. A rather unique pullout at the back of the book shows, in bar chart format, the gestation periods of all of the XML specifications in this book, giving you a visual picture of what developments occurred in sequence and/or in tandem.
- Coverage--I've selected what are generally considered to be the most significant XML-related specifications from the W3C: XML/DTDs, XML Namespaces, XML Schema, the DOM, CSS, XSLT, XPath, XSLFO, XLink, XPointer, XHTML, and RDF. Several of the less frequently discussed specifications, such as XML Information Set, Canonical XML, XML Base, and XML Inclusions, are also covered. In addition, I've included four topics that are not under the purview of the W3C: RDDL, SAX, JDOM, and JAXP. The focus is on breadth rather than depth of coverage because if you have a general understanding of a lot of XML topics, you can better appreciate which are most relevant to your needs and you can "drill down" to the details by following the links I provide. The hope is that as you become more familiar with each of the topics I present, you'll know which areas you'll want to explore by buying more specialized Addison-Wesley or Prentice Hall books (e.g., about XSLT, XML with Java, or XHTML). I've tried hard to make the information current and have spent a good bit of time in the final months polishing and updating details here and there. All topics are as up-to-date as possible, except where noted otherwise.
- For Further Exploration--Each chapter ends with a section called "For Further Exploration," which presents quite a few links that serve not only as my bibliography but point to resources that contain more details than what can be provided here without killing way more than my fair share of trees. Links are provided to the specifications themselves, to articles that explain the specs in more everyday language than the precision required for formal specifications, and to articles describing subtleties or nuances of the specs. Links to tutorials, books, software, special references, and so on are also supplied. My intention is that readers will use the links, so they all appear in HTML form on the book's CD-ROM. Professors may wish to consider some of these links for students' research assignments.
- Tables--I'm a big fan of the use of tables. When I read a technical book, I seldom read it word for word, cover to cover. Often I want to locate some particular detail pretty quickly, so I look it up in the table of contents or index--I don't want to have to skim through paragraph after paragraph to find the little tidbit I need. Therefore, I feel that tables will help you do the same thing, maximizing the use of your time. The List of Tables that follows the Contents is something with which you might want to familiarize yourself; let a table be your friend.
