<xs:schema xmlns:xs=“www.w3.org/2001/XMLSchema

xmlns="http://FOO" targetNamespace="http://FOO">

<xs:element name="foo">
        <xs:complexType>
                <xs:complexContent>
                        <xs:extension base="typeA">                                                                             
                                <xs:attribute name="barA_1" type="xs:string"  use="optional"/>                                  
                                <xs:attribute name="barA_2" type="xs:string"  use="required"/>                                  
                        </xs:extension>
                </xs:complexContent>
        </xs:complexType>
</xs:element>

<xs:complexType name="typeA">
        <xs:attribute name="barA_1" type="xs:string" use="required"/>                           
</xs:complexType>

</xs:schema>