XML Schema requires a type in addition to name to identify an element

This is implementation feedback on the Candidate Recommendation WSDL 2.0.
 
WSDL 2.0 uses the element name to identify a message. The example from the 
primer below uses the element names “ghns:checkAvailability” and 
“ghns:checkAvailabilityResponse” as references to messages.
 
<operation name="opCheckAvailability" pattern="
http://www.w3.org/2006/01/wsdl/in-out" style="
http://www.w3.org/2006/01/wsdl/style/iri" wsdlx:safe="true">
            <input messageLabel="In" element="ghns:checkAvailability"/>
            <output messageLabel="Out" element="
ghns:checkAvailabilityResponse"/>
            <outfault ref="tns:invalidDataFault" messageLabel="Out"/>
</operation>
 
The problem is that the name of an element is not sufficient to identify 
the message type when using XML Schema. The type of the element must also 
be supplied in addition to the name. This is because XML Schema allows the 
type to be overridden using the xsi:type construct.
 
In the below example the type of the element is overridden in the root 
node by the use of xsi:type.
 
<FpML version="4-2" xsi:type="DataDocument" xmlns:xsi="
http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="
http://www.fpml.org/2005/FpML-4-2 ../fpml-main-4-2.xsd" xmlns="
http://www.fpml.org/2005/FpML-4-2">
            ….
</FpML>
 
This is an example from the FpML schema http://www.fpml.org/. FpML is the 
XML Schema standard banks, brokers, and fund managers use to communicate 
within the Financial Services industry sector. FpML is frequently extended 
and used internally by the participants in the Financial Services sector, 
so these patterns are widespread within this sector.
 
FpML is an example of a schema using the Universal Root pattern 
http://www.xmlpatterns.com/UniversalRootMain.shtml. 
In the FpML schema the root element is FpML and is declared as being of 
type Document. This means that the hundreds of different types of FpML 
message all use the same element as their root. This makes the root 
element name useless in distinguishing between messages because all 
messages have the same root element.
 
FpML is declared of type Document.
 
<xsd:element name="FpML" type="Document">
            <xsd:annotation>
                        <xsd:documentation xml:lang="en">The FpML element 
forms the root for any conforming FpML instance document. The actual 
structure of the document is determined by setting the 'type' attribute to 
an appropriate derived subtype of the complex type Document.</
xsd:documentation>
            </xsd:annotation>
</xsd:element>
 
The Document complex type is abstract. This means that as the type of the 
universal root element is abstract, then all XML instance documents must 
override the type using xsi:type.  Every XML document has the same element 
name, and only the type changes.
 
<xsd:complexType name="Document" abstract="true">
            <xsd:annotation>
                        <xsd:documentation xml:lang="en">The abstract base 
type from which all FpML compliant messages and documents must be derived.
</xsd:documentation>
            </xsd:annotation>
            <xsd:attributeGroup ref="StandardAttributes.atts"/>
</xsd:complexType>
 
I suggest the type of the element is added to the Interface definition. 
This would support the usage of xsi:type in XML Schema. This would change 
the primer example to the below structure.
 
<operation name="opCheckAvailability" pattern="
http://www.w3.org/2006/01/wsdl/in-out" style="
http://www.w3.org/2006/01/wsdl/style/iri" wsdlx:safe="true">
            <input messageLabel="In" element="ghns:checkAvailability" type
="ghns:tcheckAvailability"/>
            <output messageLabel="Out" element="
ghns:checkAvailabilityResponse" type="xs:double"/>
            <outfault ref="tns:invalidDataFault" messageLabel="Out"/>
</operation>

This implementation feedback is the product of using WSDL 2.0 to define 
services using the FpML messaging schema defined in XML Schema. The aim is 
to use WSDL 2 in conjunction with XML Schema and WS-CDL within the 
International Standard for Financial Services Messaging, ISO 20022 
http://www.iso20022.org/.


Matthew Rawlings
+44 791 539 7824


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