Is there a way to enforce/preserve order of XML elements in an XML Schema?

Question

Let's consider the following XML Schema:

<?xml version="1.0" encoding="UTF-8"?>
<schema 
    targetNamespace="http://www.example.org/library" 
    elementFormDefault="qualified" 
    xmlns="http://www.w3.org/2001/XMLSchema" 
    xmlns:lib="http://www.example.org/library">

    <element name="library" type="lib:libraryType"></element>

    <complexType name="libraryType">
        <sequence>
            <element name="books" type="lib:booksType"></element>
        </sequence>
    </complexType>

    <complexType name="booksType">
        <sequence>
            <element name="book" type="lib:bookType" 
                     maxOccurs="unbounded" minOccurs="1"></element>
        </sequence>
    </complexType>

    <complexType name="bookType">
        <attribute name="title" type="string"></attribute>
    </complexType>
</schema>

and a corresponding XML example:

<?xml version="1.0" encoding="UTF-8"?>
<lib:library 
    xmlns:lib="http://www.example.org/library" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:schemaLocation="http://www.example.org/library src/library.xsd ">

  <lib:books>
    <lib:book title="t1"/>
    <lib:book title="t2"/>
    <lib:book title="t3"/>
  </lib:books>

</lib:library>

Is there a way to guarantee that the order of <lib:book .../> elements is preserved? I want to be sure that any parser reading the XML will return books in the specified oder, that is first the book with title="t1", then the book with title="t2", and finally the book with title="t3".

As far as I know XML parsers are not required to preserve order. I wonder whether one can enforce this through XML Schema? One quick solution for me would be adding an index attribute to the <lib:book .../> element, and delegate order preservation to the application reading the XML.

Comments? Suggestions?

Answer 1

According to Michael Kay who seems to be important person in XML world the order is preserved.

Yes, you can assume that the order of elements will be preserved. The writers of the XML specification neglected to say this explicitly, but that's because they thought it was obvious. A parser that didn't preserve order might be technically conformant, but no-one would ever use it; many, many XML applications depend on order being preserved, notably those that use XML to represent documents.

Source: http://lists.xml.org/archives/xml-dev/201003/msg00045.html