What is a Document Type Definition?

• In XML, you invent your own tags.
  • Write a DTD to define them.

• Or you use somebody else's definition (DTD) of a set of tags used by an application.
  • For example, there are DTDs that define the sets of tags comprising HTML.

• An XML document can be processed without a DTD; the DTD is optional.

• If it's optional, why bother with the DTD?
  • It acts as a 'contract' between producers and consumers.
  • It provides a guaranteed interface.
  • Producers validate to ensure they are providing what they promised.
  • Consumers validate to protect themselves from defective producers.
  • Application authors use validating parsers to do much of their error detection and analysis.

• An XML document may be:

  Well-Formed

  Follows the XML syntax rules: matching start and end-tags, correct syntax for attributes and entities, a single root element, correctly nested elements, etc.

  Valid

  Follows the rules in the DTD.

• Where is the DTD?

  Internal

  Within the XML document itself. Example:

  <?xml version = "1.0"?> <!DOCTYPE catalog [ ...declarations go here... ]>

  External

  In a separate file, referenced by the XML document. Example:

  <?xml version = "1.0"?> <!DOCTYPE catalog SYSTEM "catalog.dtd">

• External DTDs can be local files or remote URLs. They can be public, shared, industry-standard.

• The labs for this course use external DTDs.