The disclosure generally describes methods, software, and systems, including a method for defining and using models. A model definition language is provided for defining models. The model definition language includes elements of a meta-model. The elements define, for a model, a root element of the model and plural participant instances of the model. Each participant instance is linked with the root element. Each participant instance defines at least one of plural participants of the model. Each participant instance is an instance of a participant class. A relation port for the model defines plural relations and flows among the plural participants. Each relation is defined by a relation instance being of a relation class and defining a relationship between participants.