Peter,
thanks for your comments.

> Semantic variation is a fact of life of any metasearch
> activity. Even harvesting does not reduce the semantic
> differences of the underlying documents.

the point is not whether harvesting is immune from problems of semantic
interoperability, for sure enough successfully exchanged data must still
be uniformly understood.
the point is *how much* semantic alignment between the mutually
autonomous parties of a federation is required by harvesting and *how
much* by distributed searching.
you will agree that the interop issues raised by metadata disclosure are
a subset of those raised by the specification and implementation of a
full-fledged end-user service over that metadata.I cannot see any
agreement required for harvesting which is not also required by
distributed computing, regardless of the particular protocol
implementations of the two modes. In contrast, I can see a lot of
specification and implementation details in full service provision which
may compromise iinterop and which do not surface when parties
participate only of metadata disclosure and service provisions has all
the good properties of local implementations.
It seems to me hard to deny that the complexity introduced by full
service provision over metadata disclosure introduces more potential for
semantic disalignment.

> An aggregated
> document set will be re-indexed and the indices will expand
> to include the union of the disparate originals or the
> originals will all be fitted into the controlled vocabulary
> of the aggregation. The former only gives a mechanical
> advantage and no search advantage (in fact a disadvantage as
> in the original databases - which are presumed to be
> semantically optimised in some way - the search can be mapped
> to the specifics of the individual database semantics) the
> latter fits disparate documents under 'awkward' terms and
> reduces the specificity of those terms as measured by the
> cohesiveness of the documents they index.

I cordially disagree here. Participation to a community based on
harvesting implies agreement on the semantics of the exchanged metadata,
the looseness of which is only predicated on the metadata model of
choice (loose for DC, tighter for MARC or IEEE, etc) and the cataloguing
QoS at participating parties. Both observables do not disappear with
distiributed searching and are in fact orthogonal to the architectural
issues we are discussing. At most, harvesting encourages metadata
enhancements in correspondence with 'weak' participations, whereas
similar mediating services are much harder or altogether impossible to
accommodate in real-time by services based on distributed searching.

In particular, I cannot see the value you see in the 'specificity' of
search terms with respect to local metadata collections, when the
semantics of the original search terms is defined by the remote
metasearch client and its users. The local interpretation of a non-local
search term -- which may appear more precise from that local perspective
-- can only but contribute to a loss of precision outside that local
context, i.e. in the merged results, and of this fact it seems to me the
literature provides abundant experimental evidence. Rather, the very
value you attribute to local indexing policies applies intact at the
metasearch client side, which offers a local service against originally
remotely distributed data. In particular, indexing and other QoS aspects
of the metasearcher -- whether poor or good -- are of concern to the
metasearcher and the metasearcher only

My point here is not to criticise distributed computing model, of
course, but try to understand the model invariants that limit its
suitability to metasearching in the federation.

regards,

fabio simeoni

***************************************
Fabio Simeoni
Senior Research Fellow
Centre for Digital Library Research (CDLR)
Computer and Information Sciences Department
University of Strathclyde
Tel:0044-(0)141-5485855



> I would argue that the user gets better retrieval from a
> metasearch across a number of smaller specialist databases
> than from the same documents aggregated in one database. The
> variable is the accuracy of mapping the user's search to the
> capabilities of the individual database. If it uses the
> semantics of the database (a 'term list for example) then the
> mapping is improved for each such database and the overall
> result is improved. (I am talking mostly precision here,
> given real life volumes recall is usually not the problem.)
>
> Peter Noerr
>
> -----Original Message-----
> From: Z39.50 Next-Generation Initiative
> [mailto:[log in to unmask]]On Behalf Of Fabio Simeoni
> Sent: Tuesday,
> April 20, 2004 10:28 AM
> To: [log in to unmask]
> Subject: FW: metasearch
>
>
> >Currently it is not easy with SRU/W to broadcast the same
> query to many
> >SRU/W servers because one has to take into account all the
> differences
> >between different servers.
> >
>
> >> Definitely true - this due to the fact that SRW is a client-server
> protocol, whereas metasearch broadcasting as you are
> describing is more a grid computing task.
>
> One wonders if such problems of semantic interoperability
> between server-side implementations are an invariant of the
> distributed computing model underlying SRW/SRU rather than
> the protocol itself, Z39.50 and indeed any other
> implementation of that model. In particular, one wonders
> whether is the very requirement that servers align their
> interpretation of service provision (here searching) that
> introduces assumptions about the autonomy of participating
> parties which cannot be advanced in a large-scale federated
> environment. In this sense, harvesting as implemented in
> OAI-PMH relieves servers from any other semantic alignment
> beyond metadata format and thus suffer considerably less from
> these problems. With distributed computing, and thus with
> SRW/SRU, each party which contributes its data must also
> participate of service provision and this amplifies
> requirements of mutual consistency and the problems these
> requirements raise in a loosely-coupled environment.
>
> regards,
>
> fabio simeoni
>
> ***************************************
> Fabio Simeoni
> Senior Research Fellow
> Centre for Digital Library Research (CDLR)
> Computer and Information Sciences Department
> University of Strathclyde
> Tel:0044-(0)141-5485855
>