eXtensible Markup Language (XML)

Syntax:

1. XML is a tagged, structured language like HTML. However, in XML, the tags are not predefined (with a very few exceptions). Users are free to define whatever tags are appropriate for their applications.

   XML is a tagged, structured language, designed to deliver data in application specific structures.

2. Example: xmlEx1.xml

   An example of basic XML document structure.

3. There are two correctness issues for an XML document:
   
   
   • is it well formed? That is, does it satisfy the basic rules of syntax for an XML document?
   • is it valid? That is, is it a valid element of a set of XML documents defined by rules established by the designer?
4. For this second case, there are two ways of establishing the rules for validation - through a Document Type Definition (DTD), or using a Schema.
   
   
   • DTDs have been around a long time. HTML is, for example, defined by a DTD. DTDs are tagged, structured documents themselves, but their language and structure do not conform to the rules of XML.
   • Schemas, on the other hand, are much newer. A final specification is about to be ratified by W3C. Schemas are themselves valid XML documents, and are expected to become the method of choice for specifying XML document classes.
5. Schemas would be used to:
   
   
   • verify that a document satisfies the rules of its parent class before being placed on a web server for distribution.
   • verify that data obtained from some new site conforms to the class you need for your application.
   • provide documentation, in human readable form, for data served to some application. The application could, in fact, base its help system on information contained in the schema.
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