Modelling multiplicity in choreography models

Distributed reactive systems provide services where active concurrent components collaborate, may take independent initiatives and interact with each other and their environment. The modelling of such systems is complex. In this paper, we continue previous research where an approach is proposed to model distributed reactive systems. This approach enables us to define precise and complete global behaviour of reactive services through flow-global choreographies. These flow-global choreographies are mapped to flow-localized choreographies and further to distributed component designs in a systematic way. The complete and precise choreographies help service engineers to concentrate on choreographies only and to identify problems early at requirements specification. The proposed approach has the potential to become highly automated. While defining global behaviour, one normally focuses on just one instance of each role in a service to simplify the model. In some cases however, it is important to consider multiplicity issues early while defining requirements. The question then is when and how to deal with multiplicity of role instances and service sessions? Solutions to this problem in different contexts are proposed and discussed in this paper.