DTD versioning

John Wilson tug at wilson.co.uk
Tue Apr 7 13:54:35 BST 1998

I'm very new to this XML stuff, so perhaps I'm missing the point. Any advice
gratefully received.

I'm principally interested in using XML to pass data between computer
programs. One of the problems I foresee is that I may well want to extend or
change the DTD which represents the data exchanged as time goes by. However,
I will have installed code which expects to see the data as described by a
previous version of the DTD.

I would expect the DTD spec to deal with this in one of three ways:

1/ To have a set of rules that specifies what changes or additions I can
make to a DTD without breaking the original DTD(for example, adding an
attribute or a default value to an existing attribute) and to have an
automated way of checking that I've applied these rules.

2/ A way of specifying that the new DTD extends the old.

3/ A Relational database view type mechanism that gives a set of simple
mechanical transformations that turns the data which corresponds to one DTD
to another.

In case 1/ the new DTD would just say that it is a valid superset of the old
DTD, in case 2/ the DTD new DTD would specify the old and just contain the
changes and additions, in case 3/ the DTD would refer to the view to be
applied to transform the data to a form which is defined by the old DTD.

I would expect the XML parser to serve up the data  as described by the old
DTD and that it (the parser) would suppress attributes, apply
transformations, etc so that the data would appear in this form.

I suspect that I can do some or all these things with XSL, but that's really
hitting a small problem with a very big hammer.

John Wilson
The Wilson Partnership
5 Market Hill, Whitchurch, Aylesbury, Bucks HP22 4JB, UK
+44 1296 641072, +44 976 611010(mobile), +44 1296 641874(fax)
Mailto: tug at wilson.co.uk

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