The Object Management Group's Meta Object Facility

The Meta Object Facility (MOF) is another standard from the ACM's Object Management Group (OMG) just as UML. MOF is defined like an standard that ``provides a metadata management framework, and a set of services to enable the development and interoperability of model and metadata systems'' [Adaptive et al., 2003].

Therefore MOF can be viewed both as a metadata interchange framework or as a metamodeling framework. Although MOF has not been formally used in this Thesis it is interesting to note the many similarities exist between this standard and the solutions provided in CLAM (see chapter 3) or the conceptual metamodel presented in the Object-Oriented Content Transmission Metamodel (see chapter 5).

MOF is platform and implementation independent and can be mapped to different languages such as XML or Java.

MOF defines separate concerns or capabilities for reuse by other models and metamodels. These concerns are organized into packages and the main packages addressing modeling and metadata management concerns are: Reflection, Identity and Extension.

The Reflection package extends a model with the ability to be self-describing, that is it provides all the necessary infrastructure to use an object without prior knowledge of its specific features. In MOF an object's class (i.e. its meta object) reveals the nature, kind and features of the object.

The Reflection package includes the Object class, which is the the superclass of all model elements. This abstract class has the following main operations:

• getMetaClass(): returns the class that describes this object
• container(): returns the parent container of the object if any
• equals(element): returns true if the element and the 'this' reference the same instance.
• get(property): returns the value of the given property
• set(property, element): sets the element to the given property of the object.
• isSet(property): true if value is different from the default value (if multiplicity is >1 is true if there is at least 1 element).
• unset(property): sets the value to default

The other important class in the Reflection package is the Factory class, which is in charge of creating MOF instances and offers the following interface:

• createFromString(datatype, string): creates an Element from the value of the String with a format defined by the XML Schema Simple Type corresponding to that data type.
• convertToString(datatype,element): creates a string representation of the Element
• create(metaClass): creates an object with all properties unset that is an instance of the metaclass.

The Identity package provides an extension for uniquely identifying metamodel objects without relying on model data subject to change. The main concept in this package is the identifier. The identifier of an object is a formal representation of the object identity (see section 1.1.1), it distinguishes a given object from all the rest. Identifiers allow the serialization of references to external objects, can be used to coordinate data updates where there has been replication, and can provide clear identification in communication. Identifiers also facilitate Model-Driven Development by providing an identifier immutable to model transformations (see section 1.2.2 for a brief summary on Model-Driven Development).

The Identity package provides the concept of extents. An Extent is a context in which an object can be identified. An object may be the member of zero or more extents. An Extent is not an object, it is part of a MOF capability. It has the following operations:

• useContainment(): returns true if it includes all objects contained by members of the objects.
• objects(): returns a ReflectiveSequence of the objects directly referenced by this extent.

Identity extends the Basic::Package with a URI that can be used for externally identifying it. It also extends the Basic::Property with the ability to designate a property as an identifier for the containing object.

An URIExtent is an Extent that provides URI identity. It has the following operations:

• contextURI(): returns a string specifying the URI established for the context of the extent.
• uri(object): returns the uri of a given object in the extent.
• object(string): returns the object identified by a given uri in an extent.

Finally, the Extension package offers a simple way of extending model elements with name/value pairs. MOF offers the ability to define metamodel elements like classes with properties and operations. But sometimes it is necessary to extend model elements with additional information such as information missing from the model or data required for a particular application.

The Extension package includes the Tag class. A Tag is a single piece of information that can be associated with model elements. It has the following properties:

• name: string
• value: string (MOF places no meaning on the value)