[DFDL-WG] discriminators near PoU or not

[Mike Beckerle]

I thought this situation was worth discussing.

I have a bunch of messages. They all work like this:

<element name="messageName1">
<complexType>
<sequence>
   <element ref="boilerplateElt1" minOccurs="0"/>
   <element ref="boilerplateElt2" minOccurs="0"/>
   <sequence dfdl:initiator="MSG1">
       .... message specific elements go in here...
   </sequence>
   ... optional trailing boilerplates go here
</complexType>
</element>

There are hundreds of those.

These all are used in this way:

<element name="topLevel">
<complexType>
   <sequence>
     <element name="message" minOccurs="1" maxOccurs="unbounded">
       <complexType>
          <choice>
              <element ref="messageName1"/>
              <element ref="messageName2"/>
              ....
          </choice>
        </complexType>
    </element>
  </sequence>
</complexType>
</element>

So, there are two points of uncertainty here. The choice, and the unbounded
array surrounding it.

For each message, it is not until the sequence carrying the initiator,
which is down inside the message structure, that we know for sure we've got
this kind of message.

So currently each message element is designed to be used ONLY in the above
context of two surrounding points of uncertainty, like so (same message
format as above
but with TWO discriminators added).

<element name="messageName1">
<complexType>
<sequence>
   <element ref="boilerplateElt1" minOccurs="0"/>
   <element ref="boilerplateElt2" minOccurs="0"/>
   <sequence dfdl:initiator="MSG1">
      <annotation><appinfo...>
          <!-- discriminate enclosing choice -->
          <dfdl:discriminator>{ true() }</dfdl:discriminator>
          <!-- discriminate enclosing array -->
          <dfdl:discriminator>{ true() }</dfdl:discriminator>
      </appinfo></annotation>
       .... message specific elements go in here...
   </sequence>
   ... optional trailing boilerplates go here
</complexType>
</element>

I'd love to have a better solution to this issue. But in the absence of one
this works and achieves what I want which is that once it hits the
initiator, we discriminate both which message we have, and we discriminate
that we in fact have a message.

Interesting that I'd really prefer to discriminate them in the opposite
order of their nesting. I could discriminate the array the the very start
of the message. But I have no way to do that because the discriminators
apply in inward-out nesting order, and the nearest enclosing PoU is the
choice. So I have to discriminate that one first, and I can't discriminate
that one until I see the initiator, which is later into the message.

Possible fixes/improvements: allow a label/id on each PoU, and allow
referencing that label from a discriminator to say exactly which PoU you
are discriminating.  This makes the outward reference implied by a
discriminator explicit. You still have the context issue, but it's not
implied anymore, it is explicit. (This makes the problem like expressions
with "../../.." that reach up and out of a construct. They are reaching up
and out, but at least you can see directly that they are doing so.)

-- 
Mike Beckerle | OGF DFDL WG Co-Chair
Tel:  781-330-0412
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