Daniel and all,
I think XML communities are moving forward with Schemas.... and DTDs are not dying...but no major development will be done using DTD (see METS, OAI, etc.)... I am seeing almost the same debate a couple of years ago : Is XML going to replace SGML...not at all of course but the reality is : XML is gaining terrain because of its simplicity and endorsement by the industry... The same thing with proprietary in-house databases (where companies put a lot of money and efforts) vs. Relational databases and Client-Server technologies.
Talking about Schemas I think in a near future (maybe 2-3 years from now), the EAD community will embrace Schemas for different reasons...
- XML Schema will be more mature (maybe an ISO Standard),
- XML Schema is already a W3C Recommendation,
- XML Schema has an XML structure where the DTD has not (I was having hard time explaining to a colleague programmer that a DTD is not an XML document?),
- As of today, we have pretty good software Schemas-aware,
- Data typing is absent in DTDs,
- XML namespaces is absent in DTDs,
- Major player like OAI are supporting Schemas,
- Major Vendors (Oracle, MS, etc.) are supporting Schemas,
- Major ILS vendors (Endeavor, Ex-Libris, etc.) are planning to support schemas in their Digital library products.
As of entities, Unicode will be the one way of handling this complex issue..tough Schemas has a weakness (some say it's not) does not permit referencing external entities (OASIS entities WG is working on this I think).
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>>> [log in to unmask] 02/14/02 09:44AM >>>
The W3C editors of XML Schema have made it quite clear that Schemas are not
intended to replace DTDs.
Here is a quote from Michael Sperberg-McQueen, co-chair of the W3C XML
"Please let us eliminate this misconception [that Schemas are intended to
replace DTDs] as quickly as possible.
The XML Schema WG is not chartered to replace DTDs, and while I
suspect many WG members would not have minded had we been able to do
so, it remains that it is not now, and never has been, a formal goal
of the XML Schema work to do so. It's not in our charter, it's not in
our requirements, and our specs have given a clear direction in this
area for the past twelve months."
The full text of the quotation source can be found at:
XML Schema supports some of the functionality that DTD syntax supports
(namely specifying elements, attributes, and structure), but does not
support other functionality that DTD syntax supports (in particular, Entity
declarations). Schema introduces many features that enable a greater degree
of flexibility in imposing and controlling the logical structure of
documents than is available in DTDs. A key feature present in Schemas that
makes them attractive to many is the ability to place data type constraints
on the content of elements and on attribute values. DTDs have only a
limited ability to place such constraints on attribute values, and not on
Much of the additional control enabled by Schema potentially will benefit
ongoing development of EAD. In particular, data typing will enable placing
more constraints on element and attribute value content. Of course, before
imposing constraints, we need to develop community consensus on what
constraints to impose. For example, with Schema, it will be possible, (if
we all agree that it would be desirable) to constrain the @normal on
<unitdate> to conform to the ISO 8601 date format standard.
The very diverse XML developer and user community is trying to understand
the full implications of Schema, and when and where Schemas are the right
choice, and when and where DTDs, and when and where both are appropriate,
as complements to one another. Schema was only advanced to recommendation
status by W3C last May, and so "sorting it all out" is really only just
underway. My sense is that the EAD user community should learn as much as
possible about Schema, and watch and listen carefully to the XML developer
community as it works out the appropriate use or uses of Schema.
In regard to learning, there is currently software available that will
translate the subset of DTDs covered by Schema into a schema. Of course
there will be no more constraints in the derived Schema than are in the
source DTD, but such conversions offer us a means to explore the semantics
and syntax of Schema with familiar structures and semantics, and to
experiment with the increasing number of Schema programs.
At 08:06 AM 2/14/02 -0800, you wrote:
> >From what I understand, schemas are supposed to be replacing DTD's
>because they are more two dimensional...and give a
>more nested relation among items.
>Do you think that someone will soon develop an EAD
>schema standard? Also, since the advantage of a schema
>is that it is extensible (not like a DTD), what would
>be the advantage for using it with a finding aid?
>Oakland Museum of California
Daniel V. Pitti Project Director
Institute for Advanced Technology in the Humanities
Alderman Library University of Virginia Charlottesville, Virginia 22903
Phone: 434 924-6594 Fax: 434 982-2363 Email: [log in to unmask]
AREA CODE IS NEW EFFECTIVE JUNE 2001