[DFDL-WG] DFDL implementation support for element refs

[Mike Beckerle]

I'm interested in what DFDL implementations support element references?

IBM ACE?
IBM zTPF?
DFDL4Space?

Can you let me know whether these implementations support element refs?

The reason I ask is below, which may be of interest or perhaps TL;DR.

We support element references in Daffodil, but I'm coming around to the
view that element refs are a bad idea in DFDL schemas.

They're not needed for any specific data format expressive power. That
suggests we should have left them out of DFDL, but for some reason we
didn't.

The problem is that most data languages have nothing like element
references and the associated element namespace management complexity
available.

So as soon as you want to use a DFDL schema but not use it to interchange
data as XML, element refs become a problem.

I'm playing around with a best practice/subset/profile suggestion where:

* The only global element declarations in the schema are for root elements.
* Element references are disallowed
* The root elements are declared in a root schema file that contains ONLY
the root elements
* Root elements should always be declared by one-liners like this:
`<element name="rootElement" type="prefix:rootElementType"/>`
* The root elements schema file has no target namespace.
* All group, type, and DFDL format/escapeScheme/variable definitions must
be declared in different schema files that may (and probably should) have a
target namespace.

The benefit of these restrictions is that the elements in the nest of a
DFDL infoset never have any namespaces.
This makes them compatible with non-namespaced data systems like JSON,
Apache Drill, Apache NiFi, Generated C code, etc.
This makes integration with those things *massively* simpler.

Such schemas are still easily reused by reusing the type of the root
element, so there is no need to ever use an element reference, and a nice
composition property occurs - you don't need element references to assemble
schemas from component schemas, and the assembled component has the same
characteristic.

There are a few other things this discipline also simplifies. Reusing test
data becomes simpler if namespace URIs aren't getting embedded in every
test infoset XML file, for example.

All comments are welcome.

Mike Beckerle
Apache Daffodil PMC | daffodil.apache.org
OGF DFDL Workgroup Co-Chair | www.ogf.org/ogf/doku.php/standards/dfdl/dfdl
Owl Cyber Defense | www.owlcyberdefense.com
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