Our approach is based on the top-down methodology presented in the previous section. As a starting point we define service according to our methodology as a set of interactions. But this is by far not enough to determine all aspects of a service. To narrow the definition we define the term service more precisely through the existence of the roles user, customer as well as provider and through their associations to the service. These roles and associations are defined in our service model depicted in figure .
The understanding of a service must be the same for customer and provider side. We follow the concept of service orientation which postulates the implementation independent description of the service from the perspective of the customer side. Furthermore, in a customer oriented world, the side independent information shared by both sides must be presented from the point of view of the customer side. The side independent aspects can be found in figure between the two domains symbolizing customer side and provider side. This information is an integral part of service agreements.
remainder of this section describes the service model in more detail. It is accompanied by a simplified example service, a virtual store service to sell products over the Internet (see figure ). This service is offered by an e-commerce provider to dealers to sell their products. The user of such a service is potentially every Internet user. He actually does not know that he uses a virtual store of the e-commerce provider. [Example service]Example service (0pt,80pt)[r][b] He thinks he uses the sales service of a dealer to get the wanted product. For the sales service the Internet user is also the customer. But for the virtual store he is just a user. The dealer is the customer because he outsourced the virtual store, which is a part of his sales service, to the e-commerce provider.