XML trade off 1 - DTD vs XML Schema

[Rick Jelliffe]

From: Mark Birbeck <Mark.Birbeck at iedigital.net>

>In previous discussions on DTDs versus XML approaches to schemas I have
>argued that this ability to dynamically generate only enough of the
>schema as you need, (and the ability to cope with namespaces, which I
>haven't covered here) is my major reason for preferring XML schemas
over
>DTDs
>
>Does this confuse or clarify the point, Rick? :-)

On the other hand:

* Whether a schema is in one place or many places, you still need to
download all of it if your document has all of those elements;

* Under your system, all possible child element types are downloaded.
If your document starts at the root, you will download all the schema
anyway.

* XML Namespaces raises the possibility that elements from
different namespace can have content models that essentially
are the same, but which require separate schemas: for example,
one schema uses HTML 4 strict paragraphs and another
schema uses HTML transitional, or whatever. I think it is
important to have a commonly accepted basic vocabulary
to prevent this: HTML is a good start, but it is not managable
under any schema proposal I have seen yet.

So your system relies on each individual schema being small,
so that no fluff gets sent, and that people use well-known
content models rather than make their own.

In any case, I do not see why your system does not apply equally
to DTDs: what difference does the syntax make?  It seems to me
that some amount of the "you cannot do this with DTDs" argument
would vanish if we bothered to define a DOM for DTDs,
with XML Schemas a transformation and serialization of that
DOM. When the W3C spec-makers say "you cannot do this
with DTDs" that only would require a DOM mapping to be
specified, they are really saying "you cannot do this with W3C
specifications" not because of the intrinsic capabilities of
DTDs syntax. A little misleading.

Downloading branches of trees does not look either syntax-
dependent or semantics dependent: you don't need instance
syntax or XML Schema semantics. You just need a tree API
(e.g. DOM) and a serializer in whatever syntax.

Rick Jelliffe


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