Cooperative and distributed object systems can in essence be specified in using the Unified Modeling Language (UML) that is nowadays recognized as a worldwide standard. In practice, UML fails due to its lack of accuracy. Scenarios in UML, a.k.a. Sequence Diagrams, are natural tools for modeling object interactions and collaborations. Unfortunately, there is no support to deal with combinatory explosion, no support to arrange scenarios in independent packages or to express formal relations between scenarios. Large complex applications and their specification however generate such expectations, and more generally, scalability issues impose a component-based software development. By introducing scenario invariants and contracts in conformance with Class Diagrams, we provide here an original design method. It is illustrated by means of a concrete large-scale networking application. Since implementation is also highlighted, we sketch how the UML models can be derived into components as for instance Enterprise Java Beans (EJBs).