Query message format

[Ray Plante]

Hi Tony,

> I've just proposed to the AstroGrid group that we base the format of
> messages containing queries (eg from client, portal or workflow
> component to registry or dataset) on the W3C XQuery standard:
> 
> Note that I'm not proposing XQuery as the Astronomical Query Language
> (ie something that an astronomer would type into a query processor) but
> as the format in which such a query would be passed from one component
> to another.

In my opinion, the most important requirement of the query interchange 
format is that it be easily converted into other forms.  When distributing 
queries to many queryable services, you want the service implementer to be 
able to leverage off of existing capabilities as much as possible.  This 
will mean converting the query into the form currently supported by the 
repository.  This could include
   *  an SQL query to a relational database
   *  an XML Query to an XML database
   *  an ASU query to an ASU interface
   *  an SIA query to an SIA interface
   *  a JVO query to the JVO grid.  

The conversion is not just a matter of transforming the syntax, of course.  
To leverage these existing systems, it will also involve transforming 
the metadata schema used in the query from some standard schema (e.g. like 
SIA's use of POS=...) into the schema supported by the local database.  

In my mind, if you want a message to be easily converted into another 
form, it should be in XML.  This allows one to develop XSL stylesheets 
to do the transformation.  (In fact, the XSL stylesheets could be created 
automatically from an XML-based configuration file.)  The fact that an XML 
Query itself is not XML makes it difficult to transform into other forms.

Brian Thomas and Ed Shaya have been working on a white paper 
(http://nvo.gsfc.nasa.gov/QueryMediator/) exploring a query framework 
based on an XML-encoded query syntax which I think could meet the 
requirement of easy conversion.  We hope to experiment with this in the 
coming months.  

The use of an XML-encoded query as a query interchange format does not 
preclude AstroGrid's use of XML Query as its interchange format, 
particularly in the near-term.  The idea is that it would be 
straightforward, in theory, to transform the XML-encoded query 
into XML Query and thereby accept queries from "outside" AstroGrid.  

cheers,
Ray