Re: Schema definitions -- a discussion

["Paul Robinson" <ukcueman@xxxxxxxxxxx>]

> I thought the XSD was a transformation that you did against an XML
document
> to "translate" it from one schema to another ?
You're confusing this with XSLT (eXtensibleStylesheet Language
Transformation) - a means
of transforming one XML document to another one. If you like, an XSL file
prescribes how
to programmatically cut&paste an XML document to a different format.

An XSD or schema document defines the format of an XML file, including (if
you want it to)
the character sets or values that are allowed to be used. So you can say
things like
"there's an element called X that contains a positive integer".
Typically, the XML file
would contain a reference to the XSD so that the program that processes the
XML can
validate it against the schema.

An XSD is itself an XML file.

If you're looking for a program that reads XML and XSD files and does
validation etc, then
you could try XMLSpy - there's now a cut-down free version that does these
things.

In xPL land, people use the word "schema" to mean a definition of
what is allowed in an
xPL message in the same way that an XSD is a schema that defines what can
be in an XML
file. But since xPL messages are not XML,  that would mean the power of XSD
& XML and all
the existing tools cannot be brought to bear.

There should (if there isn't already) be a single, global XSD that defines
what is allowed
in an xPL plugin's XML descriptor, but I don't see that an XSD can be
created for xPL
messages. However, if we do want to make xPL schemas program-readable, then
they ought to
be XML files. Where it then gets confusing is that this XML file (being the
xPL schema
definition for a particular type of xPL message) would have to have an XSD
- a schema that
defines the layout of the XML file. This XSD schema would be a single
global thing.

This is what Gerry said, only in different words...I hope at least one of
us made sense
because it's confusing to talk about a schema having a schema.

Paul




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