Auxiliary Correspondence Model

This is a bx pattern.

Application Conditions

For bx transformations it is often essential to know which source and target elements are related to each other: for each target element, which source element(s) it has been derived from. This information is needed both for effective change-propagation and to resolve ambiguities when operating a transformation in the reverse direction.


Define auxiliary entity types and associations which link corresponding source and target elements. These auxiliary elements are used to record the mappings performed by a bx, and to propagate modifications from source to related target elements or vice-versa, when one model changes.


The pattern is a significant aid in change-propagation between models, and helps to ensure the correctness of a bx. Feature value changes to a source element s can be propagated to changes to its corresponding target element, and vice-versa, via the links. Deletion of an element in one model may imply deletion of its corresponding element in the other model.


The correspondence metamodel must be maintained (by the transformation engineer) together with the source and target languages, and the necessary actions in creating and accessing correspondence elements adds complexity to the transformation and adds to its execution time and memory requirements.

Related Patterns

This pattern is a specialisation of the Auxiliary Metamodel pattern.


This mechanism is a key facility of Triple Graph Grammars (TGG), and correspondence traces are maintained explicitly or implicitly by other MT languages such as QVT-R. This pattern could also be used to record sequences of rules applied, in the case of refactoring or other update-in-place transformations, in order to invert these transformations by applying inverse rules in reverse order (eg., reversing a Pull-up Attribute refactoring by a Push-down Attribute refactoring).

In UML-RSDS the pattern is applied by introducing auxiliary attributes into source and target language entity types. These attributes are primary key/identity attributes for the entity types, and are used to record source-target element correspondences. Target element t : Tj is considered to correspond to source element(s) s1 : S1, ..., sn : Sn if they all have the same primary key values: t.idTj = s1.idS1, etc. The identity attributes are usually String-valued.

The existence of identity attributes facilitates element lookup by using the Object Indexing pattern, which defines maps from String to each entity type, permitting elements to be retrieved by the value of their identity attribute: Tj[v] denotes the Tj instance t with t.idTj = v if v is a single String value, or the collection of Tj instances t with v->includes(t.idTj) if v is a collection.

The pattern can be used to define source-target propagation and incremental application of a transformation tau.