On Horizontal and Vertical Relationships between Models

Detecting, modeling and managing relationships between models are central tasks within model-driven engineering. By taking a simple view on software development, we distinguish in a vertical dimension between domain-specific models, core models, and executable models. A typical example for a vertical relationship is the refinement relationship beween a core model and an executable model. In the horizontal dimension, there may be several so-called property models which have the task to validate or verify particular properties of the core model. Software development coincides in our view with model development, and therefore finding the right models and their relationships is a crucial task. 1 Models in Software Development We follow the basic assumption expressed in [Béz05] that Everything is a model. Under this asumption, every artifact in software development (e.g., a requirement, a specification, executable code, or a test case) may be understood as a model. Figure 1 shows our view on the use of models in software development. Everything within a rectangle represents a model, and an arrow stands for a relationship between models. As a special case, relationships may be unior bidirectional transformations. One concrete development is a path through this graph. The path may go up and down the different layers and may turn left and right as needed. Thus a path representing a development may be labelled as a yoyo-left-right path. In Fig. 1 we have shown three layers for domain, core, and executable models and have indicated typical languages in which these models may be described. However, a development is not restricted to exactly these three layers. For example, there may be more than one core model layer with models being closer or farer from the executable layer. We work with UML models which incorporate OCL invariants as well as OCL preand postconditions for operations [GBR07]. Respective class diagrams and OCL invariants may be formulated in a loose way allowing for different implementations or may be very close to programming languages and determine a particular implementation.