[DFDL-WG] referencing local element declarations from path expressions between two schema documents

[Tim Kimber]

I think it is important to recognise that there are three sets of 
namespace prefixes in play here:
- prefixes for the namespaces used in the xsds for the targetNamespace and 
other imported namespaces.
- prefixes for the namespaces used in the XML instance documents
- prefixes for the namespaces used in the XPath expression
Each set of prefixes is defined entirely separately, and the prefixes may 
be completely different.

In the case of an XPath expression, the prefixes must be defined 
externally and given to the XPath processor in some kind of 'context'. How 
this is done depends on the XPath processor.

I think the answers to the questions below follow from these facts. The 
XPath is querying an XML info set. Each item in that info set has a name 
and a namespace. It may also have a 'preferred' namespace prefix inherited 
from the XML document that created the info set, but that prefix is never 
consulted when evaluating an XPath expression. So the algorithm goes like 
this:
- expand every namespace prefix in the XPath to a full URL using the 
mappings given to the XPath processor
- match each namespace:name to the namespace:name in the XML info set.
and note that the xsd's namespace prefixes are not in play at all.

regards,

Tim Kimber, 
Technical Lead for IBM Integration Bus Healthcare Pack
Hursley, UK
Internet:  kimbert at uk.ibm.com
Tel. 01962-816742 
Internal tel. 37246742




From:   Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:     "dfdl-wg at ogf.org" <dfdl-wg at ogf.org>, "Costello, Roger L." 
<costello at mitre.org>
Date:   17/09/2014 09:37
Subject:        [DFDL-WG] referencing local element declarations from path 
expressions between two schema documents
Sent by:        dfdl-wg-bounces at ogf.org




I am looking for clarification about how path steps are to be interpreted 
with respect to the issue of qualified/unqualified element names, and 
default or no-default namespaces.

If there is a precedent we can follow set by other tools which embed paths 
into XSD (e.g., schematron?) or even just the selector/paths things in XSD 
key/uniqueness constraints then I'd like to understand it.

These are some of the conundrums:

Two Schema Files: A, and B

Schema file A has target namespace AA, and binds the AA namespace to 
prefix aa, and the BB namespace to prefix bb
Schema file B has target namespace BB, and the same prefix bindings.

Schema B has an element named b1 which has two local element declarations 
inside it named c1 and c2.

Schema A has an element named a1 which contains an element reference to 
bb:b1.

a1 has a DFDL annotation (e.g., dfdl:assert) which has a path expression 
in it.  this path contains bb:b1/c1

When is the path correct?

1a) when schema B has elementFormDefault='unqualified' (that's the 
default), and schema A does NOT have a default namespace. Why? the default 
namespace - if it existed for schema A - would implicitly qualify the c1 
step of the path, but since c1 is a local element in schema B with element 
form unqualified, it does not require (and cannot have) a namespace 
specified in path steps for c1. 

1b) when schema B has elementFormDefault='qualified', and schema A has 
xmlns='BB'. Why? Because in this case the path bb:b1/c1 is equivalent to 
bb:b1/bb:c1, and since c1 will have a namespace-qualified name in schema 
B, that path will properly describe the path to the c1 element.

1c) Variation: when schema B has no target namespace (which I believe 
makes elementFormDefault irrelevant), and schema A has no default 
namespace. Because then c1 can't be qualfied as it lives in no namespace, 
and schema A has no implied qualifiers. 

When is the path incorrect?

2a) when schema A has xmlns="BB". Why? Because the default namespace means 
the bb:b1/c1 is equivalent to {BB}b1/{BB}c1, but there is no {BB}/c1 since 
c1 is just a local element declaration, and B has elementFormDefault 
unqualified.

2b) when schema B has elementFormDefault='qualified', and schema A has no 
default namespace. In that case the step c1 needs a namespace qualifier 
either by default, or explicitly via a prefix, but it doesn't have one. 

So I am interested in opinions on whether the above analysis is correct or 
not, and especially in precedent for how this stuff works in other tools. 

I have made a big assumption above which is that if xmlns="BB", then any 
path step inside something carrying that default binding is interpreted as 
qualified by that namespace. I picked this because it seemed reasonable, 
and it seems one can always modify one's schema to properly address 
something in another schema without having to change that other schema. 
I.e., it seems to have good composition properties. 

Thanks for any thoughts.


Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | 
www.tresys.com
Please note: Contributions to the DFDL Workgroup's email discussions are 
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