DFDL includes an expression language based on XPath.
Turns out that there are some "issues" with XPath 1.0 and XML namespaces.
Things that don't work
If you have XML data that has a namespace, such as:
<data xmlns="urn:someNamespaceOrOther"><a>75</a></data>
Well, it turns out that there is no way to get this XPath to work:
/data/a
Apparently, there is no mechanism for associating an unqualified XPath path step, (such as 'data' or 'a' in the path) to be implicitly qualified by a namespace. This path can only work if the data has no namespace specified.
Things that work
There are a few ways to deal with this problem.
1) Avoid Namespaces
If you have all non-qualfied elements with none of these xmlns="..." things around like this:
<data><a>75</a></data>
well, this works fine with the XPath /data/a.
2) Prefix All Element Names:
You can qualify every element. This is in some sense the most robust approach. So if you write:
<tns:data xmlns:tns="urn:someNamespaceOrOther"><tns:a>75</tns:a></tns:data>
Then the path
/tns:data/tns:a
is meaningful and works.
3) Use Both Non-Qualfied Names and a Prefix
If you really can't stand the tns prefix clutter in the data, then you can add a prefix along side the non-prefix namespace. This prefix is there for XPath:
<data xmlns="urn:someNamespaceOrOther" xmlns:tns="urn:someNamespaceOrOther"><a>75</a></data>
This tns prefix definition enables qualified XPath expressions to also work. Notice that the unqualified namespace and the tns namespace prefix are bound to the same namespace, so this path:
/tns:data/tns:a
also works in this case.