In Figure 2a) we model that a user works on a workstation which is connected to a central file server. That the workstation uses a file server is transparent to the user. In Figure 2b) we model that the file server controls a set of disks. We see that on the one hand, the file server has to serve workstations, while on the other hand, it has to control the disks in order to be an efficient file server. These two File Server roles are therefore not identical. In Figure 2c) we have made an object model based on the synthesis of the role models in Figures 2a) and 2b).
Circles in Figures 2a) and 2b) represent roles, while the circles in 2c) represent objects. Lines between roles or between objects indicate that messages flow between them. Crow feet indicate cardinality. We have for example that a user can use many workstations and a workstation can serve many users (a many-to-many relationship). A workstation is related to one file server while the file server serves many workstations (a one-to-many relationship). We have a one-to-one cardinality when a line is without any crow feet. Note that if a role has a one-to-many relationship to another role, this means that the former can be related to one, two, or many of the latter. For many-to-many relationships this comment applies in both directions.