Auxiliary Metamodel

This pattern is one of the fundamental MT design patterns.


The pattern consists of the introduction of a metamodel for auxiliary data or operations, neither part of the source or target language, to support the processing of a model transformation.

Application conditions

Signs that this pattern is necessary include: (i) excessively complex expressions are read within rules; (ii) duplicated expressions occur across different rules; (iii) complex ad-hoc data structures are used within the transformation; (iv) tracing or other information is needed during the transformation execution in order to control the transformation processing.


Define the auxiliary metamodel as a set of (meta) attributes, associations, entity types, operations and generalisations additional to the source and/or target metamodels. The auxiliary data may be constructed by some rules (eg., the construction of explicit trace objects) and read by other rules.


This pattern helps to simplify the complexity of model navigations and constructions in a transformation, and to decompose the transformation into subparts/phases. Complex relationships between a source and target model can be decomposed into simpler relationships between these models and an intermediate model using the auxiliary metamodel. It can be used to store information about the transformation execution, to support optimisations or change-propagation.


The auxiliary data must be managed and kept up-to-date.

Applications and examples

This pattern is applicable to all categories of transformations. It is a strong candidate for inclusion as an in-built facility in model transformation languages, because of its wide applicability. In Triple Graph Grammars (TGG), correspondence graphs can be defined using auxiliary classes and associations. These record detailed traces to assist in the control of the transformation.

Auxiliary Metamodel would be applicable to examples such as the complex tuple structures used in the ATL solution to the class diagram to relational database problem SimpleClass2SimpleRDBMS.

Auxiliary Metamodel can be used to add artificial structure to a model, such as a root element, to assist in navigation. It can be used to store transformation parameter data, to enable parameterisation of a transformation. It can also be used to precompute expression values prior to a transformation execution, to avoid duplicated evaluations, or to simulate multiple element matching in rules by single element matching.

Auxiliary metamodels are also used to implement explicit tracing facilities. In transformation languages such as Viatra and Kermeta, the auxiliary entities and associations record information such as a history of rules applied and connections between target model elements and the source model elements they were derived from.

Related patterns

This pattern is used by Auxiliary Correspondence Model, Simulate Multiple Matching and Simulate Collection Matching.

The classic programming technique of memoisation also introduces auxiliary data structures in order to avoid redundant computations.

The pattern can evolve to the Auxiliary Models pattern if separate models of auxiliary or intermediate data are required to carry out the transformation.