Scope of registry

[Tony Linde]

Tom,

Well put. That is what I was getting at with my view of each project's
ability to determine its own registry structure, granularity and method
of implementation but which must respond to IVOA standard queries (which
may require a fine-grained response).

Cheers,
Tony.

On Thu, 06 Feb 2003 09:31:32 -0500, "Tom McGlynn"
<Thomas.A.McGlynn at nasa.gov> said:
> One thing that has come to me in thinking about this
> issue is that there is potentially a difference between
> the granularity of a registry and that of a registry service.
> 
> Consider a registry as being
> a table having only a high-level (low-granularity) information
> about services.  The services themselves provide some protocol
> that gives fine-grainded information.  To give a concrete
> example, the registry might contain a reference to the
> Chandra archive, the NTT archive, and so forth.  Part of the
> information the registry has about the Chandra archive
> is its coverage service, which a user can invoke to get
> fine grained information about the position of Chandra observations.
> In some sense we might think of this as a registry hierarchy:
> an observation catalog is a 'registry' of the observations
> described.
> 
> However, there is no reason why a registry service that a user
> (or other software) might invoke, couldn't take advantage of
> both of the registry and the coverage services.  I could see this working
> something like DNS services on the Web.  When a domain name server
> is queried about some name it goes and queries a chain of services
> until it resolves the name.  When it's queried a second time for
> the same name, it uses a local cache.  Users tend to communicate
> with only a subset of internet nodes, so the relatively small local
> cache gives a local user almost the same benefit as if it had the
> full listing of all X billion web addresses.
> 
> Similarly when a registry service
> is queried about about observations in a given region the first
> time it looks at in coarse information to determine possible services
> and based upon that and other user criteria is goes off
> and gets fine grained information from the appropriate services.
> Since this information doesn't change rapidly, and people tend
> to be interested in the same regions of the sky, the registry
> service caches the fine grained information for a period of
> time of the order of hours or days perhaps longer for
> unchanging data sets.
> 
> We don't get static coupling of the various services with
> the registry, which we'd have if the registry itself contained
> the fine grained information, but the user is likely to get
> most of the speed advantage of having the data all in one place.
> I'd envisage the particular case of registries and coverage
> services as being a specialization of some more generic support
> for registry hierarchies.
> 
> If we can agree upon a standard protocol by which a archive
> gives the detailed information, then I think this approach
> will be easier for all sides: the data providers who provide only
> overview information to the registry, the registry
> builders who don't need to worry about synchronization of data
> and the users who get the latest information soonest.  I don't
> even think it will be very hard to implement in the registry services.
> 
> 	Tom McGlynn
> 
> Clive Page wrote:
> > On Wed, 5 Feb 2003, Arnold Rots wrote:
> > 
> > 
> >>So, it becomes a matter of degree.
> > 
> > 
> > Yes.  And I think the question of the granularity of information in the
> > Registry is a matter of debate.  It could be that different registries
> > have different policies.  The AstroGrid project has decided in principle
> > that a fine-grained registry is something to aim for (but maybe not in
> > version 1.0).  Others may have different aims.
> > 
> > It is clear that any query can be answered more definitively by firing
> > actual queries to each resource around the world, but the number of such
> > resources is getting quite large.  And we already know what happens when
> > you try that even on a limited scale: just use Astrobrowse to query the
> > set of sites they currently have listed and you find that even after a
> > minute or so not all the replies have come in.   A fine-grained registry
> > could, in principle, reduce the number of queries you need to send out
> > by quite a considerable factor (few observatories have observed more than
> > a tiny fraction of the sky, unless they have done systematic surveys).  I
> > think that would be nice to have, but I fully accept that it is not easy
> > to provide, so must be a matter for debate.  At least the debate has now
> > started.
> > 
> > 
> 
> 
> 
__
Tony Linde                       Phone:  +44 (0)116 223 1292
AstroGrid Project Manager        Fax:    +44 (0)116 252 3311
Dept of Physics & Astronomy      Mobile: +44 (0)7753 603356
University of Leicester          Email:  ael at star.le.ac.uk
Leicester, UK   LE1 7RH          Web:    http://www.astrogrid.org

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