Lord of the Schemas, Part 1: Fellowship of the Schema
By Sean Mc Grath
This week, XML expert Sean McGrath begins an epic tale of dark document definitions, brave heroes, and a quest to save markup languages.
In the Land of Markup where the Schema languages lie One DTD to rule them all, One DTD to find them, One DTD to reify them all and to the objects bind them, In the Land of Markup where the Schema languages lie.
This part of our tale chronicles some of the events in MiddleMark that occurred during the Great Years following the Third Age. Our focus is on the emergence of XML during the Great Profiling and, in particular, the growth in power and danger of One Schema Language (known in the Common Tongue as "DTD").
The First Age ended with the Great Battle when the Office Document Architecture (ODA ISO 8613:1988) was smitten by SGML (ISO 8879:1986). SGML was crafted by an ISO committee of high Elves, Dwarves, and Hobbits who worked with runes in the ancient Elven tongues. Their utterances were written in pure Mithril from the Caves of Moria. The lore therein was only viewable when moonlight shone on the parchment of the sacred ISO-bound volumes.
These mighty tomes were stored, high upon the sturdy shelves of specialist bookstores from whence only deep dollars could retrieve them. Even the great Gandalf, wielding the sword of HTTP, could not dislodge SGML from behind the ancient gates of http://www.iso.ch.
SGML's promise was plain to see but its magic was buried deep, and only available to the sages of Eldar. These sages made good money as consultants, especially working for the deep-pocketed great armies of the protectors of the Western Way.
The history of the Second Age concerns the birth of XML. Its history is much debated in taverns and other haunts of the idle such as Geek Conferences and NNTP. Some races, notably, the Elves from Markwood, refer to this time by its common title: The "Great Profiling" of SGML -- a time in which the great works (SGML, HyTime, and DSSSL) where subsetted into a form suitable for use by simple folk of the Web.
Others, such as the Hobbits and Dwarves from West of the Misty Mountains, say it differently. These simple folk, steeped as they are in tree processing lore, speak of a war against the servants of complexity and turgid prose (known in the Common Tongue as "smart asses"). To this way of reckoning, the proud Halfling races that value simplicity above power and indeed seek power only through simplicity, song, and good cheer overthrew SGML and its kind.
For well nigh a thousand days, the world was at peace. The peace was due, in no small part, to the inactivity of what is now known as the One Schema, also known in the Common Tongue as "DTDs". Why the One Schema was inactive is not recorded in the annals of MiddleMark but some say that it was due to the blessing of "Well Formedness" that occurred during the Great Profiling. Well Formedness is a powerful incantation upon XML instances that shields them from the potential ravages of the DTD wielders. It had long been known that Orcs and Balrogs, crafting HTML browsers in the darkest caves of Moria, made use of an abominable work-around known as "structure inference rules" in their code. XML killed such foul rules-based processing and made structure information easily available to the fairer folk of MiddleMark.
Now some well-intentioned MiddleMark folk made use of DTDs without regard to their dangers. By simply slipping on a DTD, they discovered that some complex problems of data validation simply disappeared before their very eyes. However, the One Schema is subtle and considered ill-intentioned at its heart. Its seductive powers leads the unwary into the dubious lands of external general entities, the swamps of conditional sections and, after prolonged exposure, down to the bottomless pit of parameter entity trickery from whence none can escape.
The Elders watched in dismay as DTD darkness spread across the land. They feared a revolt from the proud Datatype Wielding Men of Rohan -- experts in the lore of database management systems. In order to keep the peace between the great ideologies of Doc-lore and Data-lore, they elected to destroy the One Schema. To this end, at a meeting of Halflings, Men, and Corporations at Rivendell (known in the modern tongue as MIT, Boston), a Quest was hatched to rid the world of the DTD by casting it into the deepest dungeons of Deprecation.
Little did they know that this would be the beginning of a long journey. A journey through the Schema War years, which dominated the rest of the Third Age. The Schema Wars is the topic to which we must now turn our attention.
To be continued....
Sean is co-founder and Chief Technology Officer of Propylon and is an industry–recognised XML expert.