datasets and registries

[amicol]

>The registry will tell you the ONE resource that matches 
>ivo://NASA/HST,
>since this is a unique resource identifier (where NASA is the 
>AuthorityID and HST is the ResourceKey)

I must have misunderstood the purpose of the identifier then!
Could you please clarify to me whether we speak of
an ID for a dataset or for a resource ?

What confuses me is that the same dataset can be offered
by different data centers. In the case of HST the same
dataset (which I assume should be described by one and the same
datasetID in a ivo space) can be offered by different resources,
where the possibility for HST are STScI, ST-ECF, CADC, and NAOJ.

Basically you could think of a number of mirrored sites,
but it is unfortunately more complex than that, since each
datacenter might or might not provide augmented products and services
for each (and the same) dataset.

For example, ST-ECF and CADC built associations of the WFPC2 datasets.
That is, the atomic datasets are always the same (both in metadata
and data products), but new metadata (the grouping into associations) 
made possible to build better products (the coaddition of all the
members of one association). 
Recently, also STScI has adopted this value-added product.

Now, would it make sense to have three different identifiers
(one for STScI one for CADC and one for STECF) for the same
WFPC2 association product ?

I would answer no in this case. 
In this case there would be three different services
listed in the registry (ie each of the three mentioned archives), 
and it would be up to the user to decide which one to use 
to retrive a wfpc2 product.

And the decision might be driven by mere network topology,
or involve the fact that user might
prefer to receive the data onto a medium (tape, DVD, magdisk, etc)
of his/her choice, and not all HST archives offer the same
capabilities in this respect.

In the end my basic questions are:

-What should I ask the registry to know which services
can offer HST products ?

-And what to ask to know which service can offer WFPC2 associations ?

I sincerely thought that at whatever site we would always
use ivo://NASA/HST<etc> because that is the UNIQUE identifier
for HST datasets ...
 
Alberto

--
Alberto Micol
ESA/RSSD/SN-M - ST-ECF