Scope of registry

[Arnold Rots]

Yep, I'll go for that.

  - Arnold

Tom McGlynn wrote:
> 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.
> > 
> > 
> 
> 
--------------------------------------------------------------------------
Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head-cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------