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What's a Document Type Definition (DTD) and where do I get one?
XML Interview Questions and Answers
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31. What's a Document Type Definition (DTD) and where do I get one?
A DTD is a description in XML Declaration Syntax of a particular type or class of document. It sets out what names are to be used for the different types of element, where they may occur, and how they all fit together. (A question C.16, Schema does the same thing in XML Document Syntax, and allows more extensive data-checking.)
For example, if you want a document type to be able to describe Lists which contain Items, the relevant part of your DTD might contain something like this:
<!ELEMENT List (Item)+>
<!ELEMENT Item (#PCDATA)>
This defines a list as an element type containing one or more items (that's the plus sign); and it defines items as element types containing just plain text (Parsed Character Data or PCDATA). Validators read the DTD before they read your document so that they can identify where every element type ought to come and how each relates to the other, so that applications which need to know this in advance (most editors, search engines, navigators, and databases) can set themselves up correctly. The example above lets you create lists like:
<List>
<Item>Chocolate</Item>
<Item>Music</Item>
<Item>Surfingv</Item>
</List>
(The indentation in the example is just for legibility while editing: it is not required by XML.)
A DTD provides applications with advance notice of what names and structures can be used in a particular document type. Using a DTD and a validating editor means you can be certain that all documents of that particular type will be constructed and named in a consistent and conformant manner.
DTDs are not required for processing the tip in question Bwell-formed documents, but they are needed if you want to take advantage of XML's special attribute types like the built-in ID/IDREF cross-reference mechanism; or the use of default attribute values; or references to external non-XML files (‘Notations’); or if you simply want a check on document validity before processing.
There are thousands of DTDs already in existence in all kinds of areas (see the SGML/XML Web pages for pointers). Many of them can be downloaded and used freely; or you can write your own (see the question on creating your own DTD. Old SGML DTDs need to be converted to XML for use with XML systems: read the question on converting SGML DTDs to XML, but most popular SGML DTDs are already available in XML form.
The alternatives to a DTD are various forms of question C.16, Schema. These provide more extensive validation features than DTDs, including character data content validation.
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