[wfs-dev] null values
John Herring
john.herring at oracle.com
Mon Oct 30 09:01:14 EST 2006
Andreas,
Actually, your example worries me more than the
original question at hand.
>> Assume you have a list of 2 properties in your data
model, null-values should be
>> allowed, but also the order of the values has a meaning.
Here you can not use
>> minOccurs="0" because you will loose the value's order.
Now I am not an immediate nor great believer in XML,
but I will be the first to admit that it has its good
points. One of the most important is that the semantics of
the data is clearly labeled in such a manner that it is hard
to misinterpret the data. (I won't say foolproof since we
are always coming up with more inventive fallacies and
better fools every day, ain't progress marvelous B^}) This
means, among other things, that it is antithetical to use
XML is a non-obvious manner. In this case in point, the use
of order to signify a hidden meaning not expressed in or by
the labels and their associated attributes.
Again, I am at a lost to understand why preserving
"offset" in a list (ordering is actually a different issue)
unless the target application is blindly loading the list
into a set of data slots with specific meanings not
reflected in the XML's structure. This almost sounds like
the distinction between encoding and encryption. If
something is going on that requires understanding offset in
a list to understand the data but is represented and not
present in the XML <tags/> and their associated attributes,
then we are speaking of the ultimate of tightly bound
applications and the antithesis of service oriented
architecture (SOA).
The advantage of using the GML "something or NULL"
data types is that the transparency of meaning is
maintained. This transparency support understanding and
suppressing misinterpretation to a degree. What you are
suggesting is cutting very close to the heart of XML's
ability to support interoperability through loose coupling
in an SOA environment.
But then maybe I misunderstood your meaning?
Regards,
John
You do what you can when you can because you can.
The opinions expressed in this email are purely my own and
do not necessarily represent the opinions of any
organization
or otherwise sane person or persons.
John R. Herring
Architect, Spatial Products
Oracle Corporation
One Oracle Drive
Nashua, New Hampshire 03062
ph: 1 603 897 3216
fx: 1 603 897 3334
Annue cœptis - Novus Ordo Seclorum
-----Original Message-----
From:
wfs-dev-bounces+john.herring=oracle.com at opengeospatial.org
[mailto:wfs-dev-bounces+john.herring=oracle.com at opengeospati
al.org] On Behalf Of Andreas v.Dömming
Sent: Monday, October 30, 2006 5:14 AM
To: wfs-dev at opengeospatial.org
Subject: Re: [wfs-dev] null values
John, list
thinking about use-cases where "nillable" is needed I found
this:
Assume you have a list of 2 properties in your data model,
null-values should be allowed, but also the order of the
values has a meaning.
Here you can not use minOccurs="0" because you will loose
the value's order.
<FeatureA>
<orderedEntry>value<orderedEntry/>
<!-- is the first or the second orderedEntry a null-value
?--> </FeatureA>
But you can enforce to keep the order by using the
nillable-attribute:
<xs:element name="orderedEntry" minOccurs="2" maxOccurs="2"
nillable="true"
type="xs:string">
<FeatureA>
<orderedEntry xs:nil="true"/>
<orderedEntry>value<orderedEntry/>
</FeatureA>
Nevertheless, I agree that in most use-cases the
minOccurs="0" is the better choice. But depending on the
model it is not allways possible.
Regards,
Andreas
Am Freitag, 27. Oktober 2006 16:52 schrieb John Herring:
> Bart,
> I have a problem with "nillable". See XML Schema,
Part 1, Section
> 3.3.3, bullet 2.2 at
> http://www.w3.org/TR/xmlschema11-1/#d0e5913
>
> If I am reading this right, if you use a "ref" (a
global element
> specification) to specify a subelement of a type in a
schema, then
> "nillable" cannot be used, and your only shot at NULL
versus NOT NULL
> is "minOccurs=0". Which means that any concept of a
nillable
> subelement of a feature must be included in any global
definition of
> the original element, not specified differently for
different uses.
>
> That sounds a little off, and consideration the
times we naturally
> use ref in application schemas, a bit dangerous.
>
> Regards,
> John
>
> You do what you can when you can because you can.
>
> The opinions expressed in this email are purely my own and
do not
> necessarily represent the opinions of any organization or
otherwise
> sane person or persons.
>
> John R. Herring
> Architect, Spatial Products
> Oracle Corporation
> One Oracle Drive
> Nashua, New Hampshire 03062
> ph: 1 603 897 3216
> fx: 1 603 897 3334
>
> Annue cœptis - Novus Ordo Seclorum
>
>
> -----Original Message-----
> From: Bart van den Eijnden (OSGIS)
[mailto:bartvde at osgis.nl]
>
> Sent: Friday, October 27, 2006 10:31 AM
> To: John Herring; 'Bart van den Eijnden OSGIS';
> wfs-dev at opengeospatial.org
> Subject: RE: [wfs-dev] null values
>
> Hi John,
>
> I think XMLSchema deals with this by having the minOccurs
as well as
> the nillable attribute on an element.
>
> So minOccurs="1" nillable="false" would mean NOT NULL.
>
> Best regards,
> Bart
>
> --
> Bart van den Eijnden
> OSGIS, Open Source GIS
> http://www.osgis.nl
>
>
> --------- Oorspronkelijk bericht --------
> Van: John Herring <john.herring at oracle.com>
> Naar: 'Bart van den Eijnden OSGIS' <bartvde at osgis.nl>,
> wfs-dev at opengeospatial.org <wfs-dev at opengeospatial.org>
> Onderwerp: RE: [wfs-dev] null values
> Datum: 27/10/06 11:36
>
> > Bart,
> >
> > Most systems have their own way of dealing with
>
> NULLs. In XML is
>
> > seems to be simply leaving out the element.
> > Which leads to an interesting philosophic question about
>
> your
>
> > proposal.
> >
> > If one writes:
> >
> > <MYFIRSTITEM>aaa</MYFIRSTITEM>
> > <PRIORITY/>
> > <MYTHIRDITEM>zzz</MYTHIRDITEM>
> >
> > And the original schema calls for PRIORITY's
>
> minOccurs to be 1, have
>
> > you violated the intent of the schema? In SQL you get to
>
> put in
>
> > "NOT NULL" and in XML you get to put in
> > "minOccurs=1" and force a value. In either of
>
> your
>
> > suggestions with a PRIORITY element, you are not in
>
> violation of a
>
> > "minOccurs=1" but you are in violation of the
>
> semantics of
>
> > not allowing a NULL.
> >
> > Personally, I thing that IONIC is right and the only
>
> way to do this
>
> > is the way the XML gurus meant us to do it, which is
with
>
> minOccurs=0
>
> > and missing elements. On the other hand, GML's multiple
>
> flavors of
>
> > NULL does something else altogether, but at least you
can
>
> force an
>
> > application schema not to use it (the equivalent of a
NOT
>
> NULL).
>
> > Regards,
> > John
> >
> > You do what you can when you can because you can.
> >
> > The opinions expressed in this email are purely my own
and
>
> do not
>
> > necessarily represent the opinions of any organization
or
>
> otherwise
>
> > sane person or persons.
> >
> > John R. Herring
> > Architect, Spatial Products
> > Oracle Corporation
> > One Oracle Drive
> > Nashua, New Hampshire 03062
> > ph: 1 603 897 3216
> > fx: 1 603 897 3334
> >
> > Annue cœptis - Novus Ordo Seclorum
> >
> >
> > -----Original Message-----
> > From:
> >
wfs-dev-bounces+john.herring=oracle.com at opengeospatial.org
>
>
[mailto:wfs-dev-bounces+john.herring=oracle.com at opengeospati
>
> > al.org] On Behalf Of Bart van den Eijnden (OSGIS)
> > Sent: Friday, October 27, 2006 9:06 AM
> > To: wfs-dev at opengeospatial.org
> > Subject: [wfs-dev] null values
> >
> > Hi list,
> >
> > how is a WFS supposed to deal with null values?
> >
> > I can see 3 options (for instance an attribute named
> > PRIORITY):
> >
> >
> > 1) just output an empty element (I guess not the
cleanest
>
> approach
>
> > since it cannot be distinguished from an empty
> > string)
> >
> > <MYFIRSTITEM>aaa</MYFIRSTITEM>
> > <PRIORITY></PRIORITY>
> > <MYTHIRDITEM>zzz</MYTHIRDITEM>
> >
> > I think this is how Mapserver WFS operates.
> >
> >
> > 2)
> > Leave out the respective element in the reponse.
> >
> > <MYFIRSTITEM>aaa</MYFIRSTITEM>
> > <MYTHIRDITEM>zzz</MYTHIRDITEM>
> >
> > This is what Ionic WFS does.
> >
> > 3)
> > An option I have not yet seen, output
>
> xsi:nil="true" for the
>
> > respective element?
> >
> > <MYFIRSTITEM>aaa</MYFIRSTITEM>
> > <PRIORITY
xsi:nil="true"></PRIORITY>
> > <MYTHIRDITEM>zzz</MYTHIRDITEM>
> >
> > Ofcourse the 1) and 3) behaviour is easier to process
(for
>
> instance
>
> > with
> > XSLT) than the second one.
> >
> > Should the WFS spec say something about this? Or is it
up
>
> to the
>
> > vendor? It would make life easier if they all did the
same
>
> thing IMHO.
>
> > And I guess 3) is the cleanest approach.
> >
> > Thanks in advance for any input on this.
> >
> > Best regards,
> > Bart
> >
> > --
> > Bart van den Eijnden
> > OSGIS, Open Source GIS
> > http://www.osgis.nl
> >
> >
> >
> >
> >
> > _______________________________________________
> > wfs-dev mailing list
> > wfs-dev at opengeospatial.org
> > https://mail.opengeospatial.org/mailman/listinfo/wfs-dev
>
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