Question

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 1

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.