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Re: New Catalog XML Draft



----- Original Message -----
From: "Joe Wielgosz" <address@hidden>
To: "John Caron" <address@hidden>
Cc: <address@hidden>
Sent: Friday, May 17, 2002 1:07 PM
Subject: Re: New Catalog XML Draft


> John Caron wrote:
> ...
>  > ----- Original Message -----
>  > From: "Joe Wielgosz" <address@hidden>
> ...
>
> >>1) Don't restrict service types to known values. It is certain that
> >>people will want to add new service types, so the catalog format should
> >>be extensible in this area.
> >>
> >>In order to prevent ambiguity (does "dods" equal "DODS" equals
> >>"distributed oceanographic data system"?) perhaps these types could
> >>somehow resolve to the url of the service's home page (e.g.
> >>DODS->http://unidata.ucar.edu/packages/dods). I don't know the most
> >>XML-savvy way to do this but perhaps the known mappings can be included
> >>in the DTD?
> >>
> >>2) Same suggestion for metadata types.
> >>
> >
> > In both these cases, the XML thing to do is to use a URI as a unique
> > identifier. The options are eg:
> >
> > 1.  xlink:arcrole="http://unidata.ucar.edu/packages/dods";
> >
> > vs
> >
> > 2.  metadataType="DODS"
> >
> > Pros of 1: allows services to be added by anyone, URI optionally point
to
> > explanation
> > Pros of 2: compact, explicitly documents allowable types
>
>
> John, flipping through my XML tome, I'm wondering if it wouldn't be
> appropriate to use the "notation" concept here.  Notations are supposed
> to give the parser a clue how a particular entity should be interpreted,
> which seems to be what we are doing with the metadataTypes and
> serviceTypes.
>
> Say the THREDDS DTD contained the following:
>
> <!NOTATION dods SYSTEM "http://unidata.ucar.edu/packages/dods";>
> <!NOTATION netcdf SYSTEM "http://unidata.ucar.edu/packages/netcdf";>
> <!ENTITY % standardMetadataTypes "dods | netcdf">
> <!ATTLIST metadataRef metadataType
>      NOTATION ( %standardmetadataTypes; ) #REQUIRED>
>
> What I didn't realize until last night is that it is possible for the
> client to extend the DTD in the actual document. So if i have a protocol
> that is not on the THREDDS standard list, I could potentially produce
> the following catalog for my server:
>
> <?xml version="1.0" encoding="UTF-8"?>
> <!DOCTYPE catalog SYSTEM
> "http://www.unidata.ucar.edu/projects/THREDDS/xml/InvCatalog.0.6.dtd"; [
>    <!NOTATION lgdp
>        SYSTEM "http://u-somewhere.edu/latest_greatest_data_protocol";>
>    <!ATTLIST metadataRef metadataType
>        NOTATION ( %standardMetadataTypes; | lgdp)
> ]>
> <catalog>
>     <service ID="myDODSService" type="dods" />
>     <service ID="myLGDPService" type="lgdp" />
>     ...
> </catalog>
>
> This means we keep our unambiguous standard list, and readability in the
> file, but it is still possible for people to accurately catalog their
> data if they are using a "non-standard" (i.e. unknown to THREDDS)
> service. Of course it is a bit more complicated, but only for people who
> are extending the standard types - for standard types it would be just
> like the 0.6 DTD from the data provider's p.o.v.
>
> What do you think?

Well Id say i dont really understand it. What are the square brackets? Does
that get added to the DTD? Its cool to be able to extend a Choice
declaration, but the complexity seems high. Im unclear as to whether the
NOTATION is actually seperate from the method of extending the Choice list.

NOTATION appears to be rarely used, I wonder if parsers will barf on it.

> From the XML spec:
[Definition: Notations identify by name the format of unparsed entities, the
format of elements which bear a notation attribute, or the application to
which a processing instruction is addressed.]

Your proposal does seem to fit in with the second meaning, but "XML in a
Nutshell" says "in practice, no common software actually supports this
scheme for indicating the element type". So Im a bit hesitant....





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