explanation of various Registries attempted and a new element needed

[Ray Plante]

Thanks, Kevin, for your summary.  I think in practice it's fairly 
accurate, but as definitions I would break things out more cleanly based 
on a registries role as being searchable and/or harvestable.  

The RI draft includes in section 1.1 what I hope is a very clear
definition of our registry terms, starting with this paragraph: 

  This specification is based on the general IVOA model for registries 
  [Plante et al. 2004], which builds on the RM model for resources. In the 
  registry model, the VO environment features different types of 
  registries that serve different functions. The primary distinction is 
  between publishing registries and searchable ones. A secondary 
  distinction is full versus local. 

Note the last two sentences.  Use of "local" is different than as Tony 
used it in the recent discussion of aggregation--but, since he has pointed 
out, his meaning, will a great concept in practice, does not need to be 
understood at the interoperable level.  

The rest of the text is appended at the end of this message.  That section 
notes (in paragraph by itself),

  Note that some registries can play both roles; that is, a searchable 
  registry may also publish its own resource descriptions.

cheers,
Ray

>From RI, Section 1.1. "Registry Architecture and Definitions":

This specification is based on the general IVOA model for registries
[Plante et al. 2004], which builds on the RM model for resources. In
the registry model, the VO environment features different types of
registries that serve different functions. The primary distinction is
between publishing registries and searchable ones. A secondary
distinction is full versus local.

A searchable registry is one that allows users and client applications
to search for resource records using selection criteria against the
metadata contained in the records. The purpose of this type of
registry is to aggregate descriptions of many resources distributed
across the network. By providing a single place to locate data and
services, applications are saved from having to visit many different
sites to just to determine which ones are relevant to the scientific
problem at hand. A searchable registry gathers its descriptions from
across the network through a process called harvesting.

A publishing registry is one that simply exposes its resource
descriptions to the VO environment in a way that allows those
descriptions to be harvested. The contents of these registries tend to
be limited to resources maintained by one or a few providers and thus
are local in nature; for example, a data center will run its own
publishing registry to expose all the resources it maintains to the VO
environment. Since the purpose is simply publishing and not to serve
users and applications directly, it is not necessary to support full
searching capabilities. This simplifies the requirements for a
publishing registry: not only does it not need to support the general
search interface, the storage and management of the records can be
simpler. While a searchable registry in practice will necessitate the
use of a database system, a publishing registry can easily store its
records as flat files on disk.

Note that some registries can play both roles; that is, a searchable
registry may also publish its own resource descriptions.

A secondary distinction is full versus local. A full registry is one
that attempts to contain records of all resources known to the VO. In
practice, this attribute is associated only with searchable
registries, as in the so-called full searchable registry. It is
expected that there will be several such registries available, perhaps
each run by a major VO project; this not only avoids the single point
of failure, but allows some specialization to serve the particular
needs of the project that maintains it. A local registry, on the other
hand, contains only a subset of known resources. In practice, all
publishing registries are local; however, we expect that there may be
local searchable registries that specialize in particular types of
resources, perhaps oriented toward a scientific topic.