[Auscope-geosciml] RE : service architecture [SEC=UNCLASSIFIED]

Simon Cox simon.cox at jrc.ec.europa.eu
Fri Sep 18 03:00:00 EDT 2009


> XPath does not know about polymorphism, as far as I know.

That is true, and it is a problem for WFS. 
Substitutability is a feature of XSD, and Xpath was defined before XSD was
thought of. 

So this is one (biggish) lump in WFS. 
Xquery is schema aware, so potentially can be used to write queries that
allow for substitutability. 
>From time to time there have been mutterings about replacing FES with
Xquery, but the will has not been strong enough yet. 

---
Background note: 

In general GML applications are almost a perfect example of the 'markup'
principle, which is that the model is interleaved with the data within
instance documents. 
That 'annoying' striped element pattern in GML is a Good Thing since it
provides both the role and type information - you almost don’t need a schema
in order to interpret a dataset. 
AFAICT there are just two aspects of a data model that are not explicit in a
GML instance:
(i) simple types (this ommission was corrected in the ISO 19139 encoding
rule, which wraps even simple types in their own element - e.g.
gco:CharacterString) 
(ii) substitutability/polymorphism

Simon


--------------------------------------------------------
Simon Cox

European Commission, Joint Research Centre, 
Institute for Environment and Sustainability, 
Spatial Data Infrastructures Unit, TP 262 
Via E. Fermi, 2749, I-21027 Ispra (VA), Italy 
Tel: +39 0332 78 3652
Fax: +39 0332 78 6325
mailto:simon.cox at jrc.ec.europa.eu 
http://ies.jrc.ec.europa.eu/simon-cox 

SDI Unit: http://sdi.jrc.ec.europa.eu/ 
IES Institute: http://ies.jrc.ec.europa.eu/
JRC: http://www.jrc.ec.europa.eu/
--------------------------------------------------------

-----Original Message-----
From: auscope-geosciml-bounces at lists.arcs.org.au
[mailto:auscope-geosciml-bounces at lists.arcs.org.au] On Behalf Of Ben
Caradoc-Davies
Sent: Friday, 18 September 2009 07:37
To: auscope-geosciml at lists.arcs.org.au
Subject: Re: [Auscope-geosciml] RE : service architecture [SEC=UNCLASSIFIED]

On 17/09/09 07:53, Boisvert, Eric wrote:
> Do we need to write a big ogc:Or request

Isn't it valid for a WFS to return a type you have never seen before, as
long as it is defined in the schemaLocation of the response? This is
definitely the case for a property. You may not know the concrete type name
when you write the query.

XPath does not know about polymorphism, as far as I know.

--
Ben Caradoc-Davies <Ben.Caradoc-Davies at csiro.au> Software Engineer, CSIRO
Exploration and Mining Australian Resources Research Centre
26 Dick Perry Ave, Kensington WA 6151, Australia
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