Lori,

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
Schema WG.

"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:
http://lists.w3.org/Archives/Public/www-xml-schema-comments/2000OctDec/0172.html

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
elements.

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.

Daniel



At 08:06 AM 2/14/02 -0800, you wrote:
>Colleagues,
>
> >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?
>
>Lori Lindberg
>Oakland Museum of California


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Daniel V. Pitti Project Director
Institute for Advanced Technology in the Humanities
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