e3value user manual, first release

4.2 Boundary elements

A boundary element represents the limit of the scope of a value model. It is represented by a cross connected to the interfaces whose business logic we are not going to analyze further (figure 4.2).

Figure 4.2:A boundary element.

In figure 4.3, the interface of the Catering activity is connected to a boundary element, which means that the model does not show how the value activity is able to offer its service to the Luxury rail transporting activity. The Catering activity needs to acquire the food from somewhere and needs to buy equipment to prepare it. So in reality it has additional interfaces to deal with this. The boundary element indicates that our model ignores these interfaces.

Figure 4.3:A dependency graph.

In addition, the railway company needs to buy rail cars, buy fuel to drive the cars, buy or rent space on railways, and acquire the right to transport passengers. All of these activities have a value interface to the environment of the company. Our model ignores them.

If we were interested in the long-term economic viability of the railway company, we would have to include all of these interfaces. In addition, in that extended model we would include actors that sell rail cars, provide fuel, provide space on rail tracks, etc. If we are not interested in the economic viability of those actors, we would place boundary elements in those actors.

Whatever interfaces we include, there will always be many interfaces that we have omitted. Boundary elements are used to limit the scope of the model. The decision where to put the model boundary depends on our modeling goal: If we want to analyze the economic sustainability of an actor, we model all its value interfaces. If we are not interested in the economic sustainability of an actor, we ignore some of its interfaces and put a boundary element in the actor.