[OAI-implementers] XML Schema

Leigh Dodds ldodds@ingenta.com
Fri, 7 Dec 2001 11:01:21 -0000


> This issue is currently being discussed by the oai-tech group, as part of
> the ongoing revision/stabilization of the metadata harvesting
> protocol.  we hope to conclude that work with the release of a version 2.0
> the protocol around April/May 2002.

Thanks, it's useful to know that this is an open issue.

> it would be good to hear from other implementers on this list whether they
> see the need to allow for other schema languagues for metadata
> containers in the protocol.

Another way to ask this question is: should OA-MHP care about what
schema might be used to validate the metadata returned in a record?

I think a perfectly valid answer is, no. Some reasoning:

The protocol is designed to support multiple metadata formats, with
DC as a minimum. The metadata prefix is a handy way to request
that a Data Provider returns responses conforming to a particular metadata

As prefixes have an undefined scope (they may become standardised,
they may not), the only identifier for the metadata format that an
can currently rely on is the metadata schema (i.e. the URI of the schema).

For example,  one can envisage a form of negotiation where a Service
Provider attempts to identify whether a Data Provider is capable of
metadata in one format, and if not, fall back to DC. The application will
need to identify that a given prefix in this repository is 'bound' to a
it understands, so that it can make this decision.

An alternative, and well-defined way of identifying a particular vocabulary
is by it's Namespace URI (NS-URI). This identifier has the advantage of
agnostic to a particular schema language. Technologies such as RDDL [1]
other useful 'value-added' functionality with an NS-URI as a starting point.
E.g. human-readable documentation, and a machine processable directory of
resources (that may include multiple schema languages).

To conclude, one way to resolve this issue is to alter the definition of
ListMetadataFormats such that the metadataFormat/schema element
contains not the URI for the schema, but the NS-URI. A 'best practice'
recommendation to document namespaces using RDDL might also be useful.

This would make OA-MHP completely agnostic to the particular schema
language that may be used to validate a response (assuming validation
takes place at all), while retaining the ability to uniquely define the
of metadata required.

[1] http://www.rddl.org