XML Metadata Interchange
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The XML Metadata Interchange (XMI) is an OMG standard for exchanging metadata information via Extensible Markup Language (XML). It can be used for any metadata whose metamodel can be expressed in Meta-Object Facility (MOF). The most common use of XMI is as an interchange format for UML models, although it can also be used for serialization of models of other languages (metamodels).
In the OMG vision of modeling, data is split into abstract models and concrete models. The abstract models represent the semantic information, whereas the concrete models represent visual diagrams. Abstract models are instances of arbitrary MOF-based modeling languages such as UML. For diagrams, the Diagram Interchange (DI, XMI[DI]) standard is used. At the moment there are severe incompatibilities between different modeling tool vendor implementations of XMI, even between interchange of abstract model data. The usage of Diagram Interchange is almost nonexistent. Unfortunately this means exchanging files between UML modeling tools using XMI is rarely possible.
The purpose of XML Metadata Interchange (XMI) is to enable easy interchange of metadata between UML-based modeling tools and MOF-based metadata repositories in distributed heterogeneous environments.
XMI integrates four industry standards:
- XML - eXtensible Markup Language, a W3C standard.
- UML - Unified Modeling Language, an OMG modeling standard.
- MOF - Meta Object Facility, an OMG meta-modeling and metadata repository standard.
- MOF Mapping to XMI
The integration of these four standards into XMI allows tool developers of distributed systems to share object models and other metadata.
Several versions of XMI have been created: 1.0, 1.1, 1.2, 2.0 and 2.1. The 2.x versions are radically different from the 1.x series. However it brings XMI closer to idiomatic XML usage, but is unfortunately very rarely supported by modeling tools. The lack of good XMI support is slightly incomprehensible, since it is not a particularly complicated standard to adhere to. Then again good interoperability might not be in the best interest of the tool vendors themselves due to "vendor lock-in".
There are now other XML standards for representing metadata. One of the most recent is the Web Ontology Language (OWL). OWL is built upon the Resource Description Framework (RDF).
It is now an international standard: