The origin of XMI can be traced to November 1997, when MOF and UML were adopted as OMG standards. However, because of lack of time, SMIF was not specified. Thus, in December 1997, OMG issued an RFP for SMIF. This received three initial submissions - XMI, CDIF and UOL - that are now integrated into one, XMI. Figure 1 shows the position of SMIF in the OMG repository and modeling architecture.

Why an XML-Based Interchange?
XML, which is based on SGML (Standard Generalized Markup Language), is a simple, flexible, tagged format designed for information interchange. The key feature, which enables XML to be chosen over other formats, is its ability to separate data and metadata, presentation and content. This architecture of contextual separation enables generic tools (like XML parsers) to validate an XML document against its grammar, commonly known as data type definition (DTD).

Thus XML-based metadata interchange boils down to defining DTDs for MOF, UML and other standards. With this power it isn't surprising to see that the support for XMI is industry-wide. The key aspects of the architecture are:

1. A four-layered metamodeling architecture for general-purpose manipulation of metadata in distributed object repositories.

2. The MOF model is used as the meta-metamodel, while MOF is used to define and manipulate metamodels programmatically using fine-grained CORBA interface. This approach leverages the strength of CORBA distributed object infrastructure.

3. UML notation is adopted for representing models and metamodels and to describe the semantics of OA&D models.

4. XML enables SMIF.

Figure 2shows XMI in a nutshell and Figure 3 shows the OMG's four-layered metadata/metamodeling architecture. The current XMI architecture is now extended to include data warehouse metadata (Common Warehouse Metadata Interchange RFP) and other metadata by defining MOF-compliant metamodels.

What the Specification Provides
The current specification focuses on import and export of metamodels and models including extensions. As stated above, the current architecture is being extended for interchange of repository, data-warehouse data.

The current specification provides an unambiguous XML DTD for UML-based models (UML DTD) as well as MOF-based metamodels and their instances (MOF DTD). It also provides a precise MOF-to-XML mapping that allows interchange of any MOF-based metamodel and corresponding models (MOF...>XML Stream) and enables automatic generation of DTDs for any MOF-based metamodel (MOF...>XML DTD). In addition, it recommends the use of UML and MOF for metamodel design.

Figure 4 shows an example of a simple model and its XML document fragment.

Conclusion
Adoption of this technology opens up many benefits to the distributed object industry. For example:

This exciting, desirable technology advances our quest for open standards in distributed heterogeneous environments.