The evolution of software models requires collaboration of several designers i.e. collaborative designing involve the shared use of some artefact of models by a number of designers. Changes are essential features of the evolutionary development of software models. Applying changes to models concurrently results in having several versions which differ from each other and recognizing the differences is essential to understand how and why a model has reached its current state from previous one.

Therefore, it is very important for software designers to understand differences between consecutive versions of software models. For these intentions, there is a need for sophisticated versioning approach to model versioning.

Currently, most version management tools provide a text-based approach to represent differences as plain text documents, but do not refer to modeling concepts. Software models have rich data structure with constraints and behaviour. Thus, text-based version control systems do not provide detailed and complete techniques to represent differences suitable for model evolution. For this reason, appropriate techniques and tool support for the representation of version differences of software models are required.

In order to provide the collaborative work of development teams and keep track of previous versions of models, model version control approaches have to provide several services, such as calculating, visualizing, representing, merging, and analyzing differences between consecutive versions of software models.

At the moment, this PhD thesis intends to present an approach to the representation of model differences. These differences are usually stored in Delta document which consists of only differences between model versions instead of complete versions. The proposed approach is a meta-model-generic operation-based approach to representation of modeling deltas. The model modifications are carried out by means of modeling Deltas which identify sequences of modification operations such as add, delete and change. As long as meta-models give a collection of concepts within a certain (domain-specific) modeling language, applying the aforementioned basic operations to model elements results in a sequence of operations within a formal domain-specific language (DSL), which represents model differences. This allows for representing model differences in terms of the actual modeling concepts.

These delta operations are generated from the meta-models of modeling languages. In the same way, the meta-model-generic approach can be applied to other meta-models to generate appropriate domain-specific languages, as well. Consequently, the operations of the DSL are represented by a set of transformation rules. Sequences of these transformation rules form an executable description of model differences. Instead of making multiple copies of the same model artifacts, transformation-based versioning techniques only require to store an initial model and several modeling deltas. Another version of a given model will be derived by applying these rules to a given model.