Is XPATH the way to search a data model?

[David Berry]

Obviously, the ability to search our data models will be very important,
but should we just assume that XPATH is the best way to do it? My question
is, should we be optimising our data models specifically so that they can
be searched using XPATH? This seems to be the general assumption, but I
have two questions with this:

1) Does not the fact that XPTAH is a specifically XML thing not mean that
it is more to do with data >formats< than data >models<? Fine if you
serialise your data as canonical XML but what if you use (e.g.)
stand-alone FITS files, or in some non-canonical XML format for which your
XPATH expressions are not valid?

2) Can it have astronomical knowledge built into it, or is it just a
sort of dumb regexp system for structured text? What I mean is, if for
instance you searched a StandardQuantity for a Frame (Frame "A") holding
the 3 axes:

(heliocentric radio velocity, ICRS RA, ICRS Dec)

could XPATH do anything sensible if the StdQ did not contain this exact
Frame, but instead contained a Frame (Frame "B") containing the 3 axes:

(Galactic longitude, geocentric frequency, galactic latitude)

?? Obviously Frame A and Frame B are not identical, but given a
position in Frame B it is possible to convert it into Frame A without
needing any extra information over and above that stored in the
Frames. What we want is a search system which has enough astronomical
knowledge to be able to do this. What you want from your search system is
for it to say "no I cannot find Frame A but Frame B looks very similar and
I can give you a Mapping which will convert positions in Frame B into the
corresponding positions in Frame A". Such recoverable mis-matches between
what the client wants and what the server can provide is bound to happen
over and over again in the VO. So can XPATH be used to do this sort of
searching? If not, then should we not drop XPATH in favour of building
customised intelligent searching into our code which searches the data
model itself rather than just some specific data format?

My questions may have revealed that I have little experience with
XPATH, but more experience with creating customised intelligent searching
code such as that outlined above. Hence the questions...

David