Assertion as the Forth Level Constraints

XSD Tutorials - Herong's Tutorial Examples - Version 5.10, by Dr. Herong Yang

XSD Tutorials - Herong's Tutorial Examples

∟Assertion as the Forth Level Constraints

This section describes validation rules specified as assertions, which can be viewed as the forth level of constraints that you can force the XML document author to comply.

Validation rules specified as assertions can be viewed as the forth level of constraints that you can force the XML document author to comply:

First Level Constraints - XSD built-in datatypes applied on attributes and simple element contents.
Second Level Constraints - User-defined datatypes applied on complex elements.
Third Level Constraints - Identify constraints applied on repeating sub elements.
Forth Level Constraints - Assertion validation rules applied across attributes and/or elements.

For example, I receive an order XML message, order.xml, from a client system asking to price the order:

<?xml version="1.0"?>
<!-- order.xml
 - Copyright (c) 2014, HerongYang.com, All Rights Reserved.
-->
<order currency="USD">
 <line sku="XMLBOOK" quantity="1.0"/>
 <billto country="CA"/>
 <line sku="XMLBOOK" quantity="2"/>
</order>

Obviously, there are a number of areas in this order XML message that we can use XSD schema to improve the quality:

quantity="1.0" - The quantity should be a positive integer number. We can use first level constraints like the XSD built-in datatype "positiveInteger" to enforce this rule.
billto country="CA" - The "billto" sub element should be placed before all "line" sub elements. We can use second level constraints like defining our own complex element type to enforce the sequence of sub elements.
sku="XMLBOOK" - Any two order lines should not have the same SKU number. We can use third level constraints like defining a key identify constraint to enforce "sku" values to be unique.
currency="USD" and country="CA" - The currency "USD" should not be used in bill-to country "CA. We can use forth level constraints like defining a validation assertion to enforce "currency" values to be valid in the bill-to country.

The validation assertion could be expressed in XPath as "(billto/@country eq 'CA' and @currency eq 'CAD') or (billto/@country eq 'US' and @currency eq 'USD')". See the next section for the full XSD schema.

Last update: 2014.

Table of Contents

About This Book

Introduction to XML Schema

XML Editor and Schema Processor - XMLPad

Java API for XML Processing - JAXP

JAXP - XML Schema (XSD) Validation

Xerces2 Java Parser - Java API of XML Parsers

Using Xerces2 Java API

XML Schema Language - Basics

Introduction of XSD Built-in Datatypes

"string" and Its Derived Datatypes