Model-Driven Engineering - Model-Driven and Software Product Line Engineering - page 64

of descriptions that define the structure and semantics of
metadata. The MetaMetamodel layer is composed of the
descriptions of the structure and semantics of meta-metadata.
Figure 3.5 presents the four-layer metadata architecture.
Figure 3.5. The four-layer metadata architecture
As defined in its specification V2.0, MOF provides, among
others,the following four basic meta-metaconcepts for creating
metamodels:
- Classes are types. Metaconcepts that conform to Class
have identity, state, and behavior. The state of a Class
metaconcept is expressed by its Attributes and Constants ,
anditsbehaviorisgovernedby Operations and Exceptions ;
- Associations describe the binary relationships between
classes. They may express composite or non-composite
aggregation semantics. MOF associations have no object
identity;
- Packages are containers for modularizing and
partitioning metamodels into logical sub-units. Generally,
a non-nested package contains all of the elements of a
metamodel and packages can be nested to logically organize
the units of information;