Closing Keynote, XML 2006 Jon Bosak, Sun Microsystems Boston, 7 December 2006 Good evening, friends and fellow conference attendees. When David Megginson asked me to give you this closing keynote tonight, I think he knew that I couldn’t refuse an opportunity to come back to the same conference, in the same hotel, at which I had the honor of introducing the SGML community to a product initially known as “SGML for the Web” but by then repackaged under the name “XML.” It’s hard to remember now just how different this community was ten years ago. There were only a few hundred SGML experts in the world, a goodly number of whom came to this conference every year. Very few groups in existence today could boast the level of intelligence, the breadth of interest, and the depth of independence, not to say downright weirdness, evidenced by the SGML community of a decade past. In my role as master of reminiscence this evening, I’ve been trying to figure out how to give those of you who weren’t part of that early SGML community some sense of what it was like to work within it. I’m going to give you one extended example that sticks in my memory as emblematic of that period and its people. To understand this anecdote, you’re going to have to absorb two pieces of context. The first piece is the text of a very famous seventeenth-century poem by Ben Jonson titled “To Celia,” which many of us know set to music under the name “Drink to Me Only with Thine Eyes.” Here’s the text of the original poem for those of us who need a refresher: Drink to me only with thine eyes, And I will pledge with mine; Or leave a kiss but in the cup And I’ll not look for wine. The thirst that from the soul doth rise Doth ask a drink divine; But might I of Jove’s nectar sup, I would not change for thine. I sent thee late a rosy wreath, Not so much honouring thee As giving it a hope that there It could not wither’d be; But thou thereon didst only breathe, And sent’st it back to me; Since when it grows, and smells, I swear, Not of itself but thee! You’ll see why I’m reminding you of this in a moment. The second piece of context, which in itself demonstrates how different things were ten years ago, 1 was the issue that was raging in the extended group of SGML experts that was known as the W3C SGML Working Group. That issue was: Does every XML document have a DTD? Notice that I didn’t say: Can every XML document have a DTD (because that was never in question); the question was, Does every XML document have a DTD. By DTD we meant, of course, the traditional notation used in SGML to specify the legal structure and vocabulary of a document. This is what came generically to be called a schema, and probably most of you know that DTDs are still part of XML, even though most people today implement the function of DTDs using one of the constraint languages that are themselves expressed in XML, such as W3C Schema or Relax NG, rather than something that uses its own special syntax. The debate on this subject, which grew quite intense, hinged on the concept of validity. In the SGML world, documents were of two kinds: valid and invalid. Valid was good. Invalid was bad. This made a certain kind of logical sense if the DTD was considered part of the document, which technically it was. But very few people actually used DTDs that way, and very few people thought about them that way; in practice, a DTD was something that stood outside individual documents and could be applied to all documents of a given type. But still people clung to the idea that a DTD had to be associated with every document, because a document could only be valid if it had a DTD, and validity was good. Furthermore, it was possible to change the DTD in a way that gave you a different information set even though the document in question still validated. And there were some other peculiarly SGMLish reasons for requiring a DTD, such as the fact that a DTD was needed to tell you what the delimiters were and which elements had to have end tags. So the idea of a very tight association between a specific document and a specific DTD was ingrained in SGML consciousness at that point. It was not until XML started being widely deployed that people began to realize that “validity” is not a category that can properly be predicated of a document, if a document is considered to be something separate from a DTD, which everyone had long acknowledged in practice to be the case. In practice, validity is something that’s predicated of the relationship between two things, namely a specific document and a specific DTD with which it has been associated somehow. This was true even in SGML, notwithstanding the fact that the DTD was technically part of the document. So in reality, it was never conceptually (as opposed to technically) correct to say that a document was valid in an absolute sense; you could only say that it validated against this DTD rather than that DTD. In other words, the document was sometimes valid and sometimes not, depending on which DTD you tried to associate it with. Once considered fairly, this thought, while deeply upsetting to many SGML experts, led to the observation that there were actually two kinds of SGML documents: ones that could be valid against some DTD and ones that could never be valid against any DTD. I don’t recall that there was an official name for this second kind, though perhaps there was one somewhere in the SGML standard; perhaps the most common term was “broken.” As fate would have it, a number of people working with very early pre-world-wide-web SGML- based online publishing systems had already discovered a very useful aspect of this way of looking at validity. These were people who, like me, were responsible for implementing publishing systems based on early versions of DynaText, a product of Electronic Book Technologies. The story, as I remember it, is that Steve DeRose, the engineer responsible for the original EBT 2 code, didn’t have time to implement a full SGML parser in the engine that compiled SGML input to produce the indexed and structured files used at run time — so in that first release, he just left out the part that validated the document against the DTD. The makebook program accepted the DTD, and in versions later than the first it actually did check the DTD, but in that first release it just didn’t bother with that kind of validation at all. What that first version did check was a certain kind of structural consistency that was the unnamed constituent of every SGML document that could be validated against a DTD. This suchness that inhered in every validatable document was what we later called “wellformedness.” Those of us who worked with the early EBT products discovered that the lack of a true SGML validator didn’t make a damn bit of difference if the document had already been validated by a true SGML parser further up the line. And the reason it didn’t matter was that the SGML parser had incidentally found the document to be what we later called “well formed” in the process of validating it. So without thinking about it very hard, or in the terms that I’ve just laid out for you, some of us working in the trenches had gotten used to the thought that DTDs were something separate from documents and that documents had a goodness or badness independent of any particular DTD. To the SGML theorists, however, this decoupling of document from DTD was anathema. And no one struggled harder against it than Charles Goldfarb, the IBM computer scientist who is rightly called “the father of SGML” and who would be called “the father of XML” as well if history had gone just a little bit differently. (As I’m sure most of you know, XML did not invent anything; it was basically a subsetting and repackaging of the SGML standard, ISO 8879.) Charles knew everything there was to know about SGML, and for various technical and theoretical reasons, he found the concept of an SGML document without a DTD completely unacceptable. Realizing, however, that the people working on XML were determined to allow documents without DTDs explicitly attached, he finally was forced to adopt the position that every well- formed document does have a DTD, even if you can’t see it. The controversy on this point (and there were several issues that aroused equal passion) reached a peak when Charles posted a message that read in part as follows: Questions like this are more easily analyzed if we avoid the confusing term “optional DTD”. A missing DTD is really an implied DTD (like SGML’s implied SGML Declaration) in which all element types have mixed content consisting of “(#pcdata | any element type)*” and all attributes are CDATA #REQUIRED (except for special conventions for ID, etc. that we might adopt). To which I responded: There is no “implied DTD” of this nature in XML. It is possible for a well-formed XML document to be parsed in the absence of a DTD.