Towards sound, optimal, and flexible building from megamodels

The model-driven development of systems involves multiple models, metamodels and transformations. Transformations -- which may be bidirectional -- specify, and provide means to enforce, desired "consistency" relationships between models. We can describe the whole configuration using a megamodel. As development proceeds, and various models are modified, we need to be able to restore consistency in the megamodel, so that the consequences of decisions first recorded in one model are appropriately reflected in the others. At the same time, we need to minimise the amount of recomputation needed; in particular, we would like to avoid reapplying a transformation when no relevant changes have occurred in the models it relates. In general, however, different results are obtained depending on which models are allowed to be modified and on the order and direction of transformation application. In this paper we propose using an orientation model to make important choices explicit. We explain the relationship between software build systems and the megamodel consistency problem. We show how to extend the formalised build system pluto to provide a means of restoring consistency in a megamodel that is, in appropriate senses, flexible, sound and optimal.