The MOF metadata framework is typically depicted as a four layer architecture as
shown in Table 27.2. MOF is important for the UML to ensure that each UML model
type is defined in a consistent way. For example, MOF ensures that a ‘class’ in a class
diagram has an exact relationship to a ‘use case’ in a use case diagram or an ‘activity’
in an activity diagram.
(3) XML Metadata Interchange (XMI) maps the MOF to XML. XMI defines how XML
tags are used to represent MOF-compliant models in XML. An MOF-based metamodel
can be translated to a Document Type Definition (DTD) or an XML Schema
and a model is translated to an XML document that is consistent with its DTD or XML
Schema. XMI is intended to be a ‘stream’ format, so that it can either be stored in a
traditional file system or be streamed across the Internet from a database or repository.
We discuss XML, DTDs, and XML Schema in Chapter 30.