DTD or XML Schema. Which one is better?
What are the pros / cons in DTD and XML Schemas (I'm not even sure what the official name of the latter is!)? Which is better? Why do we need two ways to do the same thing?
Edit: I found this in an article I was reading, which is what prompted me to ask the question:
Why W3C XML Schema Language?
The W3C XML Schema Language is not the only schema language. In fact, the XML specification describes document-type definitions (DTDs) as the way to express a schema. In addition, pre-release versions of the JAXB Reference Implementation worked only with DTDs -- that is, not with schemas written in the XML Schema Language. However, the XML Schema Language is much richer than DTDs. For example, schemas written in the XML Schema Language can describe structural relationships and data types that can't be expressed (or can't easily be expressed) in DTDs. There are tools available to convert DTDs to the W3C XML Schema Language, so if you have DTD-based schemas that you used with an earlier version of the JAXB Reference Implementation, you can use these tools to convert the schemas to XML Schema Language. http://java.sun.com/developer/technicalArticles/WebServices/jaxb/#binsch
I guess I would like examples that illustrate why XML-Schema is better (if it indeed is).
Answer
From http://weblogs.asp.net/rchartier/archive/2006/03/21/440782.aspx
DTD's are not namespace aware.
DTD's have
#define,#include, and#ifdef-- or, less C-oriented, the ability to define shorthand abbreviations, external content, and some conditional parsing.A DTD describes the entire XML document (even if it leaves "holes"); a schema can define portions.
XSD has a type system.
XSD has a much richer language for describing what element or attribute content "looks like." This is related to the type system.
You can put a DTD inline into an XML document, you cannot do this with XSD. This means DTD's are more secure (you only have to protect one bytestream -- the xml/dtd -- and not
multiple).The official definition of "valid XML" requires a DTD. Since this may be impractical, if not impossible, you often have to settle for schema-valid, which is not quite the same.
For my part, it's pretty straightforward to write a validator for some XML if you have an XSD. I haven't seen this with a DTD, although I'm sure it exists.