IVOA Identifiers Working Draft

[Thomas McGlynn]

On Thu, 4 Sep 2003, Wil O'Mullane wrote:

> > 
> > Referring back to comments on mirrored resources, I think the only way to
> > proceed is that every resource in the VO is uniquely identified and whether
> > or not it is a mirror should be part of the metadata for that resource.
> > 
> I was thinknig of another model where the identifier was not tied to a physical entity. We intorduce a lookup service which returns physical identifiers/URIs for given Identifiers. The lookup for an id could return multiple physical represetations .
> 
> The service url otherwise is a unique identifier ...
> 
> wil
> 

It makes more sense to me to think in terms of a concrete example.
Suppose we have an SIA service for 2MASS images which is duplicated
at several sites.  What gets put in the registry...

Do we put in a separate cone search service for each of the mirrors?

Do we put in a single entry that refers to all of them in some collective
way?

If we do the first we need some way to describe the relationship among
the entries.  Maybe we want to have a database of service relationships.
This could include a sense of how exactly services are replicated.
Alternatively we could have an entry for a
MirroredService that is itself a micro-registry of all of the
mirrors.  There's a problem though...  How does the DIS service
(for example) know whether the MirroredService is a cone search
or SIA service that it should invoke?  Does it need to check
each of the MirroredServices?

So maybe we want
to register this service as an SIA service.  If so, then issues
about which mirror is invoked need to be addressed automagically
by the service.  An example is something like how the CPAN web
site distributes requests to any of its mirrors.  This is more
work for the data provider and implies that all of the mirrors
are maintained by the same organization.
Here the outside user doesn't actually see the individual mirrors.


Not sure that any of these solutions is ideal, but I'm not sure
that we need to choose one solution exclusively.   The persons
responsible for the mirrored services can choose which of these
solutions (or another) is appropriate.  We do need to consider
whether we would want an service type of MirroredService
or perhaps a new flag in the registry should be available.

        Tom