Chapter 2 XML Model Basics

What is an XSM?

An XML model (XSM) is a graphical representation of an XML Schema Definition file (.XSD), a Document Type Definition file (.DTD) or an XML-Data Reduced file (.XDR).

XML (or eXtensible Markup Language) is increasingly used to hold application data because it:

describes and structures data, whereas HTML only displays data
uses a self-describing and personalized syntax
can be exchanged between incompatible systems, since data is stored in plain text format

Since XML structures can be very complex, it is much easier to visualize them through comprehensive and explicit diagrams, than to read XML-coded pages. With its Browser tree view and diagram, a PowerDesigner XSM gives you a global and schematic view of all the elements composing your XSD, DTD, or XDR:

Once you have created an XML diagram, you can generate an XSD, a DTD or an XDR file from it for use in your application.

A PowerDesigner XSM allows you to:

Build and check XML models
Map objects in, and create reports of XML models
Generate and reverse engineer XSD, DTD and XDR files
Generate an XML model from a Physical Data Model (PDM), Object Oriented Model (OOM), or another XSM

DTD, XSD or XDR

The structure of an XSM is described by a DTD, an XSD or an XDR file:

A DTD file is a basic way to describe the structure of an XML document. It is a raw list of all the legal elements making up an XML document. An extract of a DTD file follows:
An XSD file (or schema) is an elaborated way to describe the structure of an XML document. It can support namespaces, derivations, keys, simple and complex user-defined data types and a robust collection of predefined data types. An extract of an XSD file follows:

An XSD file always starts with the <schema> tag (root element). All objects created in the model will appear in the XSD file between the schema start-tag and end-tag
An XDR file is a simplified XSD file (or schema). It does not support simple and complex user-defined data types. An extract of an XDR file follows:

An XDR file always starts with the <schema> tag (root element). All objects created in the model will appear in the XDR file between the schema start-tag and end-tag