3.2.4 Reality compliant Modeling of Object Relationships

 

Concerning the relationships between object classes, the first version of the model contains the same deficiencies as the SNMP MIB: There is no inheritance and almost no containment relationships. In contrast to this, the goal of object-oriented design is to model the components and their relationships as realistic as possible. The inheritance hierarchy indicates that one component is a refinement of another one, e.g. a storage device is a special kind of device.
The System class becomes the root of the containment hierarchy because the end user system is composed of other devices. The definition of aggregation relationships is also straightforward: A System contains at least one Processor (represented by a 1:n relationship with n$\ge$1) but any number of permanentStorageDevices (in this case, a 1:n relationship with n$\ge$0). A Printer, in contrast, is not part of a System but a peripheral device; its connection to the system is expressed by a simple 1:n relationship with n$\ge$0. Figure 4 gives a overview of the relationships between some object classes.

  

Figure 5: Quota as OMT association class

A special case of relationship can be found between the classes Filesystem, Quota and Account. As the quota is a property of a relationship between a user account and a filesystem, the Quota object class becomes an association class of the Filesystem--Quota relationship (see figure 5).