A metamodeling language (or a meta-metamodel) is a specialized language used to define metamodels, which are abstract frameworks or “models of models” that specify the structure, semantics, and constraints of other models. In Model-Driven Engineering (MDE), metamodeling languages provide the formal foundation for creating modeling languages, defining the elements, relationships, and rules that will govern any models created using those languages.

Key Concepts in Metamodeling Languages

1. Modeling Language Definition: Metamodeling languages allow you to define the syntax and structure of a modeling language, establishing the rules and constructs that can be used to build models. This is often done by specifying “classes” (or elements), their attributes, and the relationships between them.
2. Abstract Syntax: A metamodeling language defines the abstract syntax of a modeling language. This is the underlying structure or schema of the language that specifies what elements and relationships can exist without considering how they will be visually represented.
3. Constraints: Metamodeling languages often include mechanisms to define constraints on models. These constraints enforce rules within the metamodel, such as cardinality, mandatory attributes, or relationships. Object Constraint Language (OCL) is often used alongside metamodeling languages to specify such rules.
4. Domain-Specific Language (DSL) Creation: Metamodeling languages enable the creation of Domain-Specific Modeling Languages (DSMLs) by allowing modelers to define modeling languages that are tailored to specific domains, such as business processes, embedded systems, or data science.

Examples of Metamodeling Languages

1. Ecore (Eclipse Modeling Framework): Ecore is the core metamodeling language in the Eclipse Modeling Framework (EMF). It allows users to define metamodels for custom modeling languages by creating classes, attributes, references, and constraints.
2. MOF (Meta-Object Facility): Defined by the Object Management Group (OMG), MOF is a standard for metamodeling that provides the foundation for defining languages like UML. It supports four levels of abstraction (M0 to M3), with M3 being the meta-metamodel level where MOF itself is defined.
3. KM3 (Kernel MetaMetaModel): KM3 is a simple, textual metamodeling language that allows for the definition of metamodels, often used in the context of model transformation tools and research.
4. MetaGME: Part of the Generic Modeling Environment (GME), MetaGME is a metamodeling language that allows users to define domain-specific modeling languages and is widely used in embedded and cyber-physical systems.

How Metamodeling Languages are Used in MDE

1. Defining Modeling Languages: Metamodeling languages provide the “blueprint” for creating modeling languages. For example, UML itself has a metamodel, defined in MOF, that specifies elements like classes, attributes, associations, and constraints, which can then be used to create UML models.
2. Supporting Model Interoperability: By defining metamodels in standard metamodeling languages like MOF, models can be more easily shared and integrated across tools and platforms that support the same metamodel standard.
3. Guiding Model Transformation: Metamodels define the structure and constraints of source and target models in model transformations. Knowing the metamodels involved allows transformation tools to ensure that the resulting models adhere to the specified syntax and semantics.
4. Enabling Validation and Consistency Checks: Metamodels defined in metamodeling languages make it possible to validate models against their metamodels, checking that they conform to the expected structure and rules. This validation is critical for maintaining consistency and correctness in complex models.

Example of Using a Metamodeling Language

Consider creating a Domain-Specific Modeling Language (DSML) for modeling workflows in a business process. Using a metamodeling language like Ecore, you could define a metamodel with elements such as “Task,” “Decision,” and “Event,” each with specific attributes and relationships. Constraints could enforce that a “Decision” must lead to at least two “Task” elements, each representing different outcomes. This metamodel can then be used as the foundation for creating workflow models that conform to these rules.

Summary

A metamodeling language provides the tools needed to define the abstract structure, elements, and constraints of other modeling languages, effectively enabling the creation of new modeling languages or DSLs. By using metamodeling languages like Ecore or MOF, modelers can establish the formal syntax and rules that will govern models within a particular domain, supporting interoperability, model transformation, and validation across MDE environments.