Tuesday, March 14, 2017

XML Schema Primer

1 Introduction

Basic Concepts: The Purchase Order (§2) covers the basic mechanisms of XML Schema. It describes how to declare the elements and attributes that appear in XML documents, the distinctions between simple and complex types, defining complex types, the use of simple types for element and attribute values, schema annotation, a simple mechanism for re-using element and attribute definitions, and nil values.

Advanced Concepts I: Namespaces, Schemas & Qualification (§3), explains the basics of how namespaces are used in XML and schema documents.

2 Basic Concepts: The Purchase Order

The purpose of XML schemas

  • a schema defines a class of XML documents (is a description of an XML document)
  • an instance document is an XML document that conforms to a particular schema

The instance document, the po.xml file, describes a purchase order that may be generated by a product ordering application.

Example
The Purchase Order, po.xml
<?xml version="1.0"?>
<purchaseOrder orderDate="1999-10-20">
   <shipTo country="US">
      <name>Alice Smith</name>
      <street>123 Maple Street</street>
      <city>Mill Valley</city>
      <state>CA</state>
      <zip>90952</zip>
   </shipTo>
   <billTo country="US">
      <name>Robert Smith</name>
      <street>8 Oak Avenue</street>
      <city>Old Town</city>
      <state>PA</state>
      <zip>95819</zip>
   </billTo>
   <comment>Hurry, my lawn is going wild!</comment>
   <items>
      <item partNum="872-AA">
         <productName>Lawnmower</productName>
         <quantity>1</quantity>
         <USPrice>148.95</USPrice>
         <comment>Confirm this is electric</comment>
      </item>
      <item partNum="926-AA">
         <productName>Baby Monitor</productName>
         <quantity>1</quantity>
         <USPrice>39.98</USPrice>
         <shipDate>1999-05-21</shipDate>
      </item>
   </items>
</purchaseOrder>

The purchase order's elements have simple types or complex types

  • the purchase order consists of a main element purchaseOrder and the subelements {shipTo, billTo, comment, items}.
  • subelements can contain other subelements or data
  • elements that contain subelements or carry attributes are said to have complex types
  • elements that contain numbers, strings, dates and no subelements are said to have simple types, attributes always have simple types

Where to find the definitions of the types in the instance document

  • the complex types in the instance document are defined in the schema for purchase orders
  • the simple types in the instance document are defined either in the schema for purchase orders or are part of the built-in simple types of XML Schema

What is the association between the instance document and the the purchase order schema?

  • an instance document does not need to refer to a schema
  • the purchase order does not reference the purchase order schema