datasets and registries

[Doug Mink]

I'm not totally clear about the boundary between data collections
and datasets.  It is my understanding that a dataset is something
which can be returned by a data service.  Is the data collection
or the dataset (or both) a resource?  Is there a VO unit  below
a dataset?

My understanding is that pretty much all interaction with a data
collection (resource? dataset?) would be through data services.
An individual dataset could be returned in different formats by
different data services.  A data collection browser could be one
of the services.

To what level does a registry have to know about the data?
As I understand the hierarchy, it would go registry->
data collection->dataset.  The registry would only have to know
about the data collection and the data services which access it.

For a set of examples as I currently understand things, see

http://tdc-www.harvard.edu/cgi-bin/vo/arcprint?name=dmink&arcnum=3

for an entry in our mini-registry for a collection of spectra
from one of our instruments.  The catalog of spectra can be
searched, FITS or ASCII files can be returned, and web pages with
information on an individual spectrum, including graphs and links
to other information, can be displayed.  A registry would have
to know about all of the available data services releated to
a resource.  Our biggest problem has been getting people to
describe their data services in a uniform way.

Doug Mink
Telescope Data Center
Harvard-Smithsonian Center for Astrophysics
Cambridge, Massachusetts USA