Table of Contents
| Preface | |
| Acknowledgments | |
| Introduction | |
| Pt. 1 | Syntax | 1 |
| Item 1 | Include an XML Declaration | 1 |
| Item 2 | Mark Up with ASCII if Possible | 5 |
| Item 3 | Stay with XML 1.0 | 10 |
| Item 4 | Use Standard Entity References | 19 |
| Item 5 | Comment DTDs Liberally | 21 |
| Item 6 | Name Elements with Camel Case | 29 |
| Item 7 | Parameterize DTDs | 32 |
| Item 8 | Modularize DTDs | 42 |
| Item 9 | Distinguish Text from Markup | 51 |
| Item 10 | White Space Matters | 52 |
| Pt. 2 | Structure | 59 |
| Item 11 | Make Structure Explicit through Markup | 59 |
| Item 12 | Store Metadata in Attributes | 69 |
| Item 13 | Remember Mixed Content | 75 |
| Item 14 | Allow All XML Syntax | 79 |
| Item 15 | Build on Top of Structures, Not Syntax | 82 |
| Item 16 | Prefer URLs to Unparsed Entities and Notations | 91 |
| Item 17 | Use Processing Instructions for Process-Specific Content | 95 |
| Item 18 | Include All Information in the Instance Document | 104 |
| Item 19 | Encode Binary Data Using Quoted Printable and/or Base64 | 109 |
| Item 20 | Use Namespaces for Modularity and Extensibility | 112 |
| Item 21 | Rely on Namespace URIs, Not Prefixes | 117 |
| Item 22 | Don't Use Namespace Prefixes in Element Content and Attribute Values | 121 |
| Item 23 | Reuse XHTML for Generic Narrative Content | 124 |
| Item 24 | Choose the Right Schema Language for the Job | 130 |
| Item 25 | Pretend There's No Such Thing as the PSVI | 139 |
| Item 26 | Version Documents, Schemas, and Stylesheets | 143 |
| Item 27 | Mark Up According to Meaning | 149 |
| Pt. 3 | Semantics | 155 |
| Item 28 | Use Only What You Need | 155 |
| Item 29 | Always Use a Parser | 162 |
| Item 30 | Layer Functionality | 166 |
| Item 31 | Program to Standard APIs | 170 |
| Item 32 | Choose SAX for Computer Efficiency | 178 |
| Item 33 | Choose DOM for Standards Support | 183 |
| Item 34 | Read the Complete DTD | 186 |
| Item 35 | Navigate with XPath | 192 |
| Item 36 | Serialize XML with XML | 201 |
| Item 37 | Validate Inside Your Program with Schemas | 203 |
| Pt. 4 | Implementation | 213 |
| Item 38 | Write in Unicode | 213 |
| Item 39 | Parameterize XSLT Stylesheets | 221 |
| Item 40 | Avoid Vendor Lock-In | 226 |
| Item 41 | Hang On to Your Relational Database | 230 |
| Item 42 | Document Namespaces with RDDL | 233 |
| Item 43 | Preprocess XSLT on the Server Side | 241 |
| Item 44 | Serve XML+CSS to the Client | 246 |
| Item 45 | Pick the Correct MIME Media Type | 249 |
| Item 46 | Tidy Up Your HTML | 253 |
| Item 47 | Catalog Common Resources | 257 |
| Item 48 | Verify Documents with XML Digital Signatures | 263 |
| Item 49 | Hide Confidential Data with XML Encryption | 275 |
| Item 50 | Compress if Space Is a Problem | 283 |
| Recommended Reading | 287 |
| Index | 289 |
Forewords & Introductions
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 introductory courses about XML syntax, APIs, and tools. 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 style sheet. The question of what XML is and why to use it has been sufficiently well evangelized. The essential syntax and rules 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.
Since XML has become a fundamental underpinning of new software systems, it becomes important to begin asking new questions, not just what XML is, but how does one use it effectively? Which techniques work and which don't? Perhaps most importantly, which techniques appear to work at first but fail to scale as systems are further developed? When I teachprogramming 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 (perhaps 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 one definition of a pioneer is someone with an arrow in their back, and 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 I covers XML syntax, those aspects of XML that don't really affect the information content of an XML document, but may have large impacts on how easy or hard those documents are to edit and process.
- Part II looks at XML structures, the general organization and annotation of information in an XML document.
- Part III discusses the various techniques and APIs that are available for processing XML with languages such as C++, C#, Java, Python, and Perl and thus local semantics to the labeled structures of XML.
- Part IV 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 that are used throughout the book, and which 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 to direct you along other paths through the book that may be of interest.
I hope that 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 you subscribe and participate there. On the other hand, if you find out and out mistakes in this book (The ID attribute value on p. 87 is missing a closing quote. The word "Cat" is misspelled on p. 46.) you can write me. I maintain a web page listing known errata for this book, as well as any updates. I hope this book makes your use of XML both more effective and more enjoyable.
—Elliotte Rusty Harold
Brooklyn NY
April 22, 2003
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