Constraint checking in UML modeling

Modeling aims to represent a system in a given formalism. As representations based on high level modeling languages can be interpreted in many ways, constraints are introduced to restrict the field of the possible. On the other hand, language semantics is defined using constraints as well. Within the context of the UML, a classification is proposed to clarify the nature of the constraints that must be fulfilled to ensure model correctness. Constraint violations are only warnings while others are serious. Depending on the kind and the context of the violation, help and advice can be supplied, and improvements may be conditionally done. In the UML, constraints are assertions described in a side-effect free language (OCL), while actions are not supplied. To extend capabilities, the constraints are translated intowell-using modeling rules that form the knowledge base of an expert system in modeling. This modeling companion is briefly outlined.