[OAI-implementers] Re: RDF, OAI, and application within Libraries

Dave Reynolds der@hplb.hpl.hp.com
Thu, 17 May 2001 17:54:21 +0100


Many thanks for this - it helps me understand OAI's positioning much better.

Let me paraphrase and respond to your points.

> [1] It is plausible to support non XML Schema means of validating metadata
passed back from OAI queries but [2] too many options actually limits
flexibility by being too complex.

Agreed. You are right there are a lot of alternatives (Relax, Schematron,
RDFS, DAML ...) and supporting everything it just too complex and costly. In
my naive view of the world there are two useful extremes to support.

Firstly, one wants a tightly constrained validatable format suited to
typical metadata records. For that, XML Schema seems entirely appropriate
and it's not obvious that going further and supporting Relax or whatever
adds much.

Secondly, I believe that for future proofing we also need to support less
formal semi-structured data formats. Semi-structured data arises in many
ways - merging of multiple data sources, sparse user annotations, rapidly
evolving schemas specific to given communities. In my simple view of the
world RDF is a good foundation for a wide variety of semi-structured data so
supporting that would be a useful complement to the highly structured

If this meant changing OAI to support an extra format I can see that as an
extra complication. However, by using the sort of shallow (XML Schema
compatible) encoding we were discussing in this thread it seems like RDF
could be supported as an incremental addition, within the current framework,
and need not complicate implementations. There is the issue of using
RDFS/DAML schemas  for interpreting the RDF when you get it. However, even
these need not affect the OAI protocol since they are (a) somewhat optional,
(b) can sometimes be inferred out of band e.g. from the namespaces used in
the RDF properties, (c) could be included in the RDF payload.

To me these two extremes are sufficient between them to cover most needs.

>   An outstanding
> question for me is to understand where RDF sits in the OAI data provider
> and service provider dichotomy.

An excellent point. Perhaps I am confused on the distinction between data
providers and service providers but I can see repositories like DSpace
wanting to combine metadata from multiple sources and "smush" them together
and also to support less structured metadata such as user or small community
annotations. Their data provider role makes it appropriate for them to use
OAI to export metadata access but they also have attributes of a service
provider, or at least a clearing house for various metadata, and so have
some role for these flexible semi-tructured metadata formats.