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[Michael Kay <mike@saxonica.com>]

Meaning of “data model” in the XML world

I think that a data model in the first sense is the set of constructs (complex types, simple types, element declarations, attribute declarations, etc.) defined in the XML Schema specification.

I think that a data model in the second sense is a particular XML Schema. For example, the Geography Markup Language (GML) schema is a data model in the second sense.

Do you agree?

Yes.

In the 1980s a popular concept was the 4-layer IRDS model. Each layer is a mechanism for describing the layer below. Your "data model in the first sense" (e.g. the relational model or the XML schema model) is at layer 2, and can be described using formalisms such as mathematical set theory which constitutes layer 1. Layer 2 is used to describe layer 3, which is your "data model in the second sense", that is the database or document schema. Layer 4 is the user data.

Michael Kay

Saxonica