Let’s now roll back time and, from the outset of our service’s lifetime, specify a forwards-compatible, extensible schema:

<?xml version=”1.0” encoding=”utf-8”?>

<xs:schema xmlns=”urn:example.com:productsearch:products”

                xmlns:xs=”http://www.w3.org/2001/XMLSchema”

                elementFormDefault=”qualified”

                targetNamespace=”urn:example.com:productsearch:products”

                id=”Products”>

  <xs:element name=”Products” type=”Products” />

  <xs:complexType name=”Products”>

    <xs:sequence>

      <xs:element minOccurs=”0” maxOccurs=”unbounded” name=”Product” type=”Product” />

    </xs:sequence>

  </xs:complexType>

  <xs:complexType name=”Product”>

    <xs:sequence>

      <xs:element name=”CatalogueID” type=”xs:int” />

      <xs:element name=”Name” type=”xs:string” />

      <xs:element name=”Price” type=”xs:double” />

      <xs:element name=”Manufacturer” type=”xs:string” />

      <xs:element name=”InStock” type=”xs:string” />

      <xs:element minOccurs=”0” maxOccurs=”1” name=”Extension” type=”Extension” />

    </xs:sequence>

  </xs:complexType>

  <xs:complexType name=”Extension”>

    <xs:sequence>

      <xs:any minOccurs=”1” maxOccurs=”unbounded” namespace=”##targetNamespace” processContents=”lax” />

    </xs:sequence>

  </xs:complexType>

</xs:schema>

This schema includes an optional Extension element at the foot of each product. The extension element itself can contain one or more elements from the target namespace: