When the first edition of this book was written, XML was a relatively new language but already gaining ground fast and becoming more and more widely used in a vast range of applications. By the time of the second edition, XML had already proven itself to be more than a passing fad, and was in fact being used throughout the industry for an incredibly wide range of uses. With the third edition, it was clear that XML was a mature technology, but more important, it became evident that the XML landscape was dividing into several areas of expertise. Now in this edition, we needed to categorize the increasing number of specifications surrounding XML, which either use XML or provide functionality in addition to the XML core specification.

So what is XML? It’s a markup language, used to describe the structure of data in meaningful ways. Anywhere that data is input/output, stored, or transmitted from one place to another, is a potential fit for XML’s capabilities. Perhaps the most well–known applications are web–related (especially with the latest developments in handheld web access—for which some of the technology is XML–based). However, there are many other non–web–based applications for which XML is useful—for example, as a replacement for (or to complement) traditional databases, or for the transfer of financial information between businesses. News organizations, along with individuals, have also been using XML to distribute syndicated news stories and blog entries.

This book aims to teach you all you need to know about XML—what it is, how it works, what technologies surround it, and how it can best be used in a variety of situations, from simple data transfer to using XML in your web pages. It answers the fundamental questions:

∗ What is XML?

∗ How do you use XML?

∗ How does it work?

∗ What can you use it for, anyway?

This book is for people who know that it would be a pretty good idea to learn XML but aren’t 100 percent sure why. You’ve heard the hype but haven’t seen enough substance to figure out what XML is and what it can do. You may be using development tools that try to hide the XML behind user interfaces and scripts, but you want to know what is really happening behind the scenes. You may already be somehow involved in web development and probably even know the basics of HTML, although neither of these qualifications is absolutely necessary for this book.

What you don’t need is knowledge of markup languages in general. This book assumes that you’re new to the concept of markup languages, and we have structured it in a way that should make sense to the beginner and yet quickly bring you to XML expert status.

The word “Beginning” in the title refers to the style of the book, rather than the reader’s experience level. There are two types of beginner for whom this book is ideal:

∗ Programmers who are already familiar with some web programming or data exchange techniques. Programmers in this category will already understand some of the concepts discussed here, but you will learn how you can incorporate XML technologies to enhance those solutions you currently develop.

∗ Those working in a programming environment but with no substantial knowledge or experience of web development or data exchange applications. In addition to learning how XML technologies can be applied to such applications, you will be introduced to some new concepts to help you understand how such systems work.

The subjects covered in this book are arranged to take you from novice to expert in as logical a manner as we could. This Fourth Edition is structured in sections based on various areas of XML expertise. Unless you are already using XML, you should start by reading the introduction to XML in Part I. From there, you can quickly jump into specific areas of expertise, or, if you prefer, you can read through the book in order. Keep in mind that there is quite a lot of overlap in XML, and that some of the sections make use of techniques described elsewhere in the book.

∗ The book begins by explaining what exactly XML is and why the industry felt that a language like this was needed.

∗ After covering the why, the next logical step is the how, so it shows you how to create well–formed XML.

∗ Once you understand the whys and hows of XML, you’ll go on to some more advanced things you can do when creating your XML documents, to make them not only well formed, but valid. (And you’ll learn what “valid” really means.)

∗ After you’re comfortable with XML and have seen it in action, the book unleashes the programmer within and looks at an XML–based programming language that you can use to transform XML documents from one format to another.

∗ Eventually, you will need to store and retrieve XML information from databases. At this point, you will learn not only the state of the art for XML and databases, but also how to query XML information using an SQL–like syntax called XQuery.

∗ XML wouldn’t really be useful unless you could write programs to read the data in XML documents and create new XML documents, so we’ll get back to programming and look at a couple of ways that you can do that.

∗ Understanding how to program and use XML within your own business is one thing, but sending that information to a business partner or publishing it to the Internet is another. You’ll learn about technologies that use XML that enable you to send messages across the Internet, publish information, and discover services that provide information.

∗ Since you have all of this data in XML format, it would be great if you could easily display it to people, and it turns out you can. You’ll see an XML version of HTML called XHTML. You’ll also look at a technology you may already be using in conjunction with HTML documents called CSS. CSS enables you to add visual styles to your XML documents. In addition, you’ll learn how to design stunning graphics and make interactive forms using XML.

∗ Finally, the book ends with a case study, which should help to give you ideas about how XML can be used in real–life situations, and which could be used in your own applications.

This book builds on the strengths of the earlier editions, and provides new material to reflect the changes in the XML landscape—notably XQuery, RSS and Atom, and AJAX. Updates have been made to reflect the most recent versions of specifications and best practices throughout the book. In addition to the many changes, each chapter has a set of exercise questions to test your understanding of the material. Possible solutions to these questions appear in Appendix