Preface

Learning the fundamentals of XML might take a programmer a week. Learning how to use XML effectively might take a lifetime. While many books have been written that teach developers how to use the basic syntax of XML, this is the first one that really focuses on how to use XML well. This book is not a tutorial. It is not going to teach you what a tag is or how to write a DTD. I assume you know these things. Instead it's going to tell you when, why, where, and how to use such tools effectively (and, perhaps equally importantly, when not to use them).

This book derives directly from my own experiences teaching and writing about XML. Over the last five years , I've written several books and taught numerous courses about XML. Increasingly I'm finding that audiences are already familiar with the basics of XML. They know what a tag is, how to validate a document against a DTD, and how to transform a document with an XSLT stylesheet. The question of what XML is and why to use it has been sufficiently well evangelized. The essential syntax and supporting technologies are reasonably well understood . However, although most developers know what a CDATA section is, they are not sure what to use one for. Although programmers know how to add attribute and child nodes to elements, they are not certain which one to use when. Although programmers know what a schema is, they don't know which schema language to choose.

Since XML has become a fundamental underpinning of new software systems, it becomes important to begin asking new questionsnot just about what XML is but also how to use it effectively. Which techniques work and which don't? Less obviously, which techniques appear to work at first but fail to scale as systems are further developed? When I teach programming at my university, one of the first things I tell my students is that it is not enough to write programs that compile and produce the expected results. It is as important (indeed more important) to write code that is extensible, legible, and maintainable. XML can be used to produce robust, extensible, maintainable , comprehensible systems; or it can be used to create masses of unmaintainable, illegible, fragile, closed code. In the immortal words of Eric Clapton, "It's In The Way That You Use It."

XML is not a programming language. It is a markup language, but it is being successfully used by many programmers. There have been markup languages before, but in the developer community XML is far and away the most successful. However, the newness and unfamiliarity of markup languages have meant that many developers are using it less effectively than they could. Many programmers are hacking together systems that work but are not as robust, extensible, or portable as XML promises. This is to be expected. Programmers working with XML are pioneers exploring new territory, opening up new vistas in software, and accomplishing things that could not easily be accomplished just a few years ago. However, more than a few XML pioneers have returned from the frontier with arrows in their backs.

Five years after the initial release of XML into the world, certain patterns and antipatterns for the proper design of XML applications are becoming apparent. All of us in the XML community have made mistakes while exploring this new territory, the author of this book prominently among them. However, we've learned from those mistakes, and we're beginning to develop some principles that may help those who follow in our footsteps to avoid making the same mistakes we did. It is time to put up some caution signs in the road. We may not exactly say "Here there be dragons," but we can at least say, "That road is a lot rockier than it looks at first glance, and you might really want to take this slightly less obvious but much smoother path off to the left."

This book is divided into four parts , beginning with the lowest layer of XML and gradually working up to the highest.

• Part 1 covers XML syntax, those aspects of XML that don't really affect the information content of an XML document but may have large impact on how easy or hard those documents are to edit and process.

• Part 2 looks at XML structures, the general organization and annotation of information in an XML document.

• Part 3 discusses the various techniques and APIs available for processing XML with languages such as C++, C#, Java, Python, and Perl and thus attaching local semantics to the labeled structures of XML.

• Part 4 explores effective techniques for systems built around XML documents, rather than looking at individual documents in isolation.

Although this is how I've organized the book, you should be able to begin reading at essentially any chapter. This book makes an excellent bathroom reader. You may wish to read the introduction first, which defines a number of key terms used throughout the book that are frequently misused or confused . However, after that feel free to pick and choose from the topics as your interest and needs dictate . I've made liberal use of cross-references throughout to direct you along other paths through the book that may be of interest.

I hope this book is a beginning, not an end. It's still early in the life of XML, and much remains to be discovered and invented. You may well develop best practices of your own that are not mentioned here. If you do, I'd love to hear about them. You may also take issue with some of the principles stated here. I'd like to hear about that too. Discussion of many of the guidelines identified here has taken place on the xml-dev mailing list and seems likely to continue in the future. If you're interested in further discussion of the issues raised in this book, I recommend that you subscribe and participate there. Complete details can be found at http://lists.xml.org/. On the other hand, if you find outright mistakes in this book (the ID attribute value is missing a closing quote; the word "cat" is misspelled ), you can write me directly at [e-mail]. I maintain a Web page that lists known errata for this book, as well as any updates, at http://www.cafeconleche.org/books/effectivexml/.

Finally, I hope this book makes your use of XML both more effective and more enjoyable.