Scope of registry

[Clive Page]

On Thu, 6 Feb 2003, Roy Williams wrote:

> In the OAI model, there are exactly six verbs that the registry understands.
> The verbs are:
>
> Identify (who are you)
> List Metadata Formats (what schemas do you have for metadata)
> List Sets (collections hosted by this repository)
> List Identifiers (return just the identifiers and dates of most recent
> change)
> List Records (return full records)
> Get Record (return a full record from its identifier).
>
> This is a pretty basic set of services!

That seems to me rather more basic than will be suitable for what we want.
I confess to thinking of the registry in database terms.  Users might want
to send queries to the registry equivalent to this sort of thing
(expressed for convenience in pseudo-SQL):

SELECT * FROM registry WHERE WAVELENGTH BETWEEN "400 nm" AND "800 nm" AND
  POSITION("12:34:56,-01:23,10") AND TYPE = "photometry"

If we don't allow such selectivity in registry queries, won't all queries
just return all records?  I thought the idea of the registry was not just
to have a definitive list of resources, but also avoid the necessity for
users to send their queries to _all_ registered resources, just on the
off-chance that each of them might have some information of relevance.

Querying all resources will be very time-consuming, and will also have to
be repeated, because not all resources will be on-line at all times.
Users who suspect that the archives of the University of Erewhon
Observatory has something relevant, but find that it is frequently
off-line will have to try it many times.  If, on the other hand, their
initial query to the Registry shows that the answer concerning the Erewhon
site is "no" rather than "maybe", they can avoid such time-wasting
activities.

To those who doubt that this is an important factor I would suggest: go to
the Astrobrowse site, http://heasarc.gsfc.nasa.gov/ab/, and select a full
search, and see how long it takes to get an answer from all the linked
resources.  I find it takes typically 40 to 60 seconds, but that sometimes
one or two sites never respond (we all have occasional outages, after
all).  Astrobrowse currently only queries the principal sites, all
well-networked and well-managed.  When our Registry includes sites less
well endowed, things may not be as good.


-- 
Clive Page,
Dept of Physics & Astronomy,
University of Leicester,    Tel +44 116 252 3551
Leicester, LE1 7RH,  U.K.   Fax +44 116 252 3311