The Fundamentals of DTD Design - DTD Who? (Page 2 of 8 )
Let's start with the basics: what's a DTD when it's home, and why do you care?
The first part of the question is easy enough to answer. A DTD, or document type definition, is a lot like a blueprint. Unlike most blueprints, however, it doesn't tell you where the kitchen goes or how the capsule containing the plutonium is to be wired up. Nope, this blueprint is a lot more boring - it tells you exactly how an XML document should be structured, complete with lists of allowed values, permitted element and attribute names, and predefined entities.
DTDs are essential when managing a large number of XML documents, as they immediately make it possible to apply a standard set of rules to different documents and thereby demand conformance to a common standard. However, for smaller, simpler documents, a DTD can often be overkill, adding substantially to download and processing time.
Most XML documents start out as well-formed data - they meet the basic syntactical rules described in the XML specification, and are correctly structured (no overlapping, badly-nested elements or illegal values). However, an XML document which additionally meets all the rules, conditions and structural guidelines laid down in a DTD qualifies for the far cooler "valid" status. Think of it like a free airline upgrade from business to first...except, of course, without the complimentary drinks.
Why do you need to know about this? Well, you don't.
If your day job involves carrying out covert operations for an unnamed intelligence agency or building houses, you'd be better off studying the other sort of blueprint. If, on the other hand, your job involves developing and using XML applications and data, you need to have at least a working knowledge of how DTDs are constructed, so that you can roll your own whenever required.{mospagebreak title=How's The Weather Up There?} In order to illustrate how DTDs work, consider this simple XML document:
<?xml version="1.0"?>
<weather>
<city>New York</city>
<high>26</high>
<low>18</low>
<forecast>rain</forecast>
</weather>
As of now, this file is merely well-formed - it hasn't yet been compared to a
DTD and declared valid. In order to perform this comparison, I need to link it to a DTD - which I can do by adding a document type declaration referencing the DTD.
<?xml version="1.0"?>
<!DOCTYPE weather SYSTEM "http://www.somedomain.com/weather.dtd">
<weather>
<city>New
York</city>
<high>26</high>
<low>18</low>
<forecast>rain</forecast>
</weather>
As a result of this addition, the document would be validated against the DTD
located at http://www.somedomain.com/weather.dtd
Let's see what this DTD looks like:
<!ELEMENT weather (city, high, low, forecast)>
<!ELEMENT city (#PCDATA)>
<!ELEMENT
high (#PCDATA)>
<!ELEMENT low (#PCDATA)>
<!ELEMENT forecast (#PCDATA)>
To the untrained eye - gibberish. But give it a couple of minutes...
Next: Rainy Days >>
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