Datasheet

Using SYSTEM as shown allows the parser to load the DTD from the specified location. If you use PUBLIC,

the named DTD should be one that is familiar to the parser being used, which may have a store of

commonly used DTDs. In most cases, you will want to use your own DTD and use SYSTEM. This method

enables the parsing application to make its own decisions as to what DTD to use, which may result in a

performance increase; however, specific implementation of this is down to individual parsers, which might

limit the usefulness of this technique.

Because of the inherent disadvantages of DTDs, XML Schemas are the commonly used mechanism to

validate XML documents. XML schemas are discussed in detail in a later section of this chapter.

XDR

XML Data Reduced (XDR) schema is Microsoft’s own version of the W3C’s early 1999 work-in-progress

version of XSD. This schema is based on the W3C Recommendation of the XML-Data Note (

http://www

.w3.org/TR/1998/NOTE-XML-data

), which defines the XML Data Reduced schema.

The following document contains the same information that you could find in a DTD. The main difference

is that it has the structure of a well-formed XML document. This example shows the same constraints as

the DTD example, but in an XML schema format:

<?xml version=”1.0” ? >

</ElementType>

</Schema>

There are a few things that an XDR schema can do that a DTD cannot. You can directly add data types,

range checking, and external references called namespaces.

As useful as DTDs are, they also have their shortcomings. The major concern most

developers have with DTDs is the lack of strong type-checking. Also, DTDs are

created using a strange and seemingly archaic syntax. They have only a limited

capability in describing the document structure in terms of how many elements can

nest within other elements.

Introduction to XML

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