XML Handbook, Third Edition, 3rd edition
Authors: Charles F. Goldfarb, Paul Prescod
Venue:  (2000)

From the Book:XML is taking over the world!I saw the proof a few days ago in the newsletter for a major mutual fund. A profile of one of the fund's top holdings praised the company's great prospects because of its leadership in XML technologyand the article didn't even explain what XML is!If a financial analyst thinks that XML is common knowledge, can world domination be far behind?I'm delighted at the analyst's enthusiasm, but I don't think the knowledge is all that common yetwhich is why Paul and I wrote this book. We knowand we want to share with youthe reasons why XML is taking over the world. We want you to understand how it is enabling all sorts of wonderful services for Web users and amazing new opportunities for website developers and businesses.HTMLthe HyperText Markup Languagemade the Web the world's library. XMLthe Extensible Markup Languageis its sibling, and it is making the Web the world's commercial and financial hub.In the process, the Web is becoming much more than a static library. Increasingly, users are accessing the Web for "Web pages" that aren't actually on the shelves. Instead, the pages are generated dynamically from information available to the Web server. That information can come from databases on the Web server, from the site owner's enterprise databases, or even from other websites.And that dynamic information needn't be served up raw. It can be analyzed, extracted, sorted, styled, and customized to create a personalized Web experience for the end-user. For this kind of power and flexibility, XML is the markup language ofchoice.You can see why by comparing XML and HTML. Both are based on SGMLthe International Standard for structured informationbut look at the difference:In HTML: <p>P200 Laptop <br>Friendly Computer Shop <br>$1438 In XML: <product> <model>P200 Laptop</model> <dealer>Friendly Computer Shop</dealer> <price>$1438</price> </product> Both of these may appear the same in your browser, but the XML data is smart data. HTML tells how the data should look, but XML tells you what it means.With XML, your browser knows there is a product, and it knows the model, dealer, and price. From a group of these it can show you the cheapest product or closest dealer without going back to the server.Unlike HTML, with XML you create your own tags, so they describe exactly what you need to know. Because of that, your client-side applications can access data sources anywhere on the Web, in any format. New "middle-tier" servers sit between the data sources and the client, translating everything into your own task-specific XML.But XML data isn't just smart data, it's also a smart document. That means when you display the information, the model name can be a different font from the dealer name, and the lowest price can be highlighted in green. Unlike HTML, where text is just text to be rendered in a uniform way, with XML text is smart, so it can control the rendition.And you don't have to decide whether your information is data or documents; in XML, it is always both at once. You can do data processing or document processing or both at the same time.With that kind of flexibility, it's no wonder that we're starting to see a Brave New Web of smart, structured information. Your broker sends your account data to Quicken using XML. Your "push" technology channel definitions are in XML. Everything from math to multimedia, chemistry to CommerceNet, is using XML or is preparing to start.You should be too!Welcome to the Brave New XML Web.What about SGML?This book is about XML. You won't find feature comparisons to SGML, or footnotes with nerdy observations like "the XML empty-element tag does not contradict the rule that every element has a start-tag and an end-tag because, in SGML terms, it is actually a start-tag followed immediately by a null end-tag."Nevertheless, for readers who use SGML, it is worth addressing the question of how XML and SGML relate. There has been a lot of speculation about this.Some claim that XML will replace SGML because there will be so much free and low-cost software. Others assert that XML users, like HTML users before them, will discover that they need more of SGML and will eventually migrate to the full standard.Both assertions are nonsense ... XML and SGML don't even compete.XML is a simplified subset of SGML. The subsetting was optimized for the Web environment, which implies data-processing-oriented (rather than publishing-oriented), short life-span (in fact, usually dynamically-generated) information. The vast majority of XML documents will be created by computer programs and processed by other programs, then destroyed. Humans will never see them.Eliot Kimber, who was a member of both the XML and SGML standards committees, says:There are certain use domains for which XML is simply not sufficient and where you need the additional features of SGML. These applications tend to be very large scale and of long term; e.g., aircraft maintenance information, government regulations, power plant documentation, etc.Any one of them might involve a larger volume of information than the entire use of XML on the Web. A single model of commercial aircraft, for example, requires some four million unique pages of documentation that must be revised and republished quarterly. Multiply that by the number of models produced by companies like Airbus and Boeing and you get a feel for the scale involved.I agree with Eliot. I invented SGML, I'm proud of it, and I'm awed that such a staggering volume of the world's mission-critical information is represented in it.I'm thrilled that it has been such an enabler of the Web that the Society for Technical Communication awarded joint Honorary Fellowships to the Web's inventor,Tim Berners-Lee, and myself in recognition of the synergy.But I'm also proud of XML. I'm proud of my friend Jon Bosak who made it happen, and I'm excited that the World Wide Web is becoming XML-based.If you are new to XML, don't worry about any of this. All you need to know is that the XML subset of SGML has been in use for a decade or more, so you can trust it.SGML still keeps the airplanes flying, the nuclear plants operating safely, and the defense departments in a state of readiness. You should look into it if you produce documents on the scale of an Airbus or Boeing. For the rest of us, there's XML.About our sponsorsWith all the buzz surrounding a hot technology like XML, it can be tough for a newcomer to distinguish the solid projects and realistic applications from the fluff and the fantasies. It is tough for authors as well, to keep track of all that is happening in the brief time we can steal from our day jobs.The solution to both problems was to seek support and expert help from our friends in the industry. We know the leading companies in the XML arena and knew they had experience with both proven and leading-edge applications and products.In the usual way of doing things, had we years to write this book, we would have interviewed each company to learn about its products and/or application experiences, written the chapters, asked the companies to review them, etc., and gone on to the next company. To save time and improve accuracy, we engaged in parallel processing. I spoke with the sponsors, agreed on subject matter for their chapters, and asked them to write the first draft.All sponsored chapters are identified with the name of the sponsor, and sometimes with the names of the experts who prepared the original text. I used their materials as though they were my own interview notesediting, rewriting, deleting, and augmenting as necessary to achieve my objective for the chapter in the context of the book, with consistent terminology and an objective factual style. I'd like to take this opportunity to thank these experts publicly for being so generous with their time and knowledge. The sponsorship program was directed by Linda Burman, the president of L. A. Burman Associates, a consulting company that provides marketing and business development services to the XML and SGML industries.We are grateful to our sponsors just as we are grateful to you, our readers. Both of you together make it possible for The XML Handbook to exist. In the interests of everyone, we make our own editorial decisions and we don't recommend or endorse any product or service offerings over any others.Our 27 sponsors are: Adobe Systems Incorporated, ...