xtype: similarity to VODataService's externalType

[Rob Seaman]

Hi Ray,

The statement: "It's expected that this name will only be understood  
by a special subset of applications" will raise the question in the  
reader of whether this is really a situation in which the 90/10 rule  
needs to be abandoned.  Presumably there are particular "special  
applications" in mind.  What are they?  If on the other hand, it is  
really thought that this will be a widely used feature, delete the  
statement as an unnecessary editorial comment.

In general the description could benefit from some clear and specific  
examples of how this would/should be used.  Perhaps there were slides  
presented that could be shared with those only virtually present?  (I  
only see the discussion in the introductory slides @ http://www.ivoa.net/internal/IVOA/InterOpMay2009VOTable/VOTable-09May.pdf)

Rob
---

On May 28, 2009, at 7:29 AM, Ray Plante wrote:

> Hi VOTablers,
>
> As discussed in the last VOTable session in Strasbourg, we are  
> considering adding a new FIELD attribute called xtype (or exttype or  
> externalType). As I mentioned, this is very similar to an attribute  
> availble in VODataService for describing a table column.
>
> For reference, here's how the VODataService Document (http://www.ivoa.net/Documents/latest/VODataService.html 
> ) describes them...
>
>  More descriptive information about the type can be provided via the
>  extendedType and extendedSchema, which provide an alternate data type
>  name. It's expected that this name will only be understood by a  
> special
>  subset of applications. The name given in the element content, then,
>  represents a more commonly understood "fall-back" type.
>
>    extendedType  Value type: 	     string: xs:string.
>                  Semantic Meaning:  the data value represented by this
>                                     type can be interpreted as of a
>                                     custom type identified by the  
> value
>                                     of this attribute.
>                  Occurrences: 	     optional
>                  Comments:          The name implies a particular
>                                     expected format for the data
>                                     value that can be parsed into a
>                                     value in memory.
>
>                                     If an application does not  
> recognize
>                                     this extendedType, it should  
> attempt
>                                     to handle value assuming the type
>                                     given by the element's value.
>                                     "string" (or its equivalent) is a
>                                     recommended default type.
>
>                                     This element may make use of the
>                                     extendedSchema attribute and/or  
> any
>                                     arbitrary (qualified) attribute to
>                                     refine the identification of the
>                                     type.
>
>  extendedSchema  Value type:        URI: xs:anyURI.
>                  Semantic Meaning:  An identifier for the schema that
>                                     the value given by the extended
>                                     attribute is drawn from.
>                  Occurrences:       optional
>                  Comments:          This attribute is normally  
> ignored if
>                                     the extended element is not  
> present.
>
> hope this is helpful for reference.
>
> cheers,
> Ray