inputVariable optional on Invoke

["Yaron Y. Goland" <ygoland@bea.com>]

Executive Summary: Antony Miguel, in a private e-mail, pointed out that 
the inputVariable attribute on invoke is currently optional. In BPEL as 
it now is specified this doesn't make sense as all operations MUST have 
a WSDL message associated with them. But if issue 12 passes then the 
optional inputVariable makes sense in cases where the message has no 
parts. Unfortunately the WSDL 1.1 spec seems to contradict itself on the 
legality of partless messages.

Long Winded Version:

Antony Miguel, in a private e-mail with me, pointed out that the 
inputVariable attribute is optional on invokes. Given that all invoke 
MEPs MUST have an outgoing message does it make sense to have the 
attribute be optional?

The only scenario I can come up with where it makes sense to not have an 
inputVariable attribute is if the outgoing message is 'empty'. This is 
not a completely insane idea. Some protocols do have 'empty' messages 
which have an address header but no body. This could be used for things 
like pings.

But all BPEL messages have to be sent using a WSDL message structure and 
it isn't possible to define an operation that doesn't point at a 
message. However it MAY be legal to define a WSDL message with no parts. 
There is a contradiction between the text in WSDL 1.1 and the schema. 
The text in section 2.3 says "Messages consist of one or more logical 
parts.". But the psuedo-schema in 2.3 says:

<definitions .... >
     <message name="nmtoken"> *
         <part name="nmtoken" element="qname"? type="qname"?/> *
     </message>
</definitions>

Furthermore the XML Schema in the appendix says:

    <complexType name="messageType">
       <complexContent>
          <extension base="wsdl:documented">
             <sequence>
                <element ref="wsdl:part" minOccurs="0"
                         maxOccurs="unbounded"/>
             </sequence>
             <attribute name="name" type="NCName" use="required"/>
          </extension>
       </complexContent>
    </complexType>

Assuming the schema triumphs then this means that it is legal to define 
a message with no parts. An empty message.

But this still leaves the problem that BPEL requires that all messages 
be contained in a WSDL container. This alone would require that invoke 
MUST always have an inputVariable attribute.

But, if issue 12 passes, then it would make sense to make inputVariable 
optional to cover the case where the message for an operation has no parts.