[auscope-geosciml] A problem in BGS instance doc [SEC=UNCLASSIFIED]

Sen, Marcus A. mase at bgs.ac.uk
Tue May 10 04:15:21 EDT 2011

> -----Original Message-----
> From: auscope-geosciml-bounces at lists.arcs.org.au [mailto:auscope-

>    <cgu:qualifier
> xlink:href=http://resource.geosciml.org/classifier/cgi/ValueQualifier/2
> 00811/always xsi:nil="true"/>
> My understanding is that using xsi:nil=”true” means that you have no
> content for cgu:qualifier.  Ergo, you can’t deliver the qualifier value
> “always” as well as “nil”.  It’s one or the other, but not both?
This was also pointed out by Clemens Portele and is one of the reasons I made the task https://projects.arcs.org.au/trac/geosciml/ticket/43 to define best practice for nils. Our instance was generated by the Snowflake WFS with the xsi:nil attribute automatically added. It is valid in XML Schema because there is no element content but element attributes are allowed. Although schema valid it makes the semantic meaning of xsi:nil="true" not very useful for our GML applications where we indicate that there is "remote content" with an xlink:href attribute (although there is another issue with cases where we don't really have any remote content but just use the value of the URI itself...). In this sense, GML Applications overload the xsi:nil attribute with extra significance which doesn't exist in XML Schema.

I will need to ask Snowflake Software to change the behaviour of their WFS if our instances are not to include the xsi:nil="true" setting in these cases. This seems reasonable to do but I would like to have a defined best practice on handling nil values to point to to justify it. I haven't done this yet but I guess we need to update the https://www.seegrid.csiro.au/wiki/bin/view/AppSchemas/NilValues page a bit.


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