Scenarios

A user of a technical or organizational system usually does not need each and every possible behavior of the system. The work is often limited to a few activities that are repeatedly executed. Therefore, it is convenient to consider typical

scenarios

of a system. A scenario consists of a finite number of elementary actions and terminates in the same state in which it started. Because of this, multiple

instances

of a scenario can occur multiple times in the same run. A scenario often describes an interaction pattern between a process and its environment, or between two processes.

Typical scenarios of the systems considered thus far are: selling a packet of cookies, visiting one’s critical state once, and sending a message.

Technically, a scenario is constructed as a finite distributed run, whose final marking equals its initial marking. A run of a distributed, reactive system is often composed of many instances of only a few scenarios. If every run can essentially be constructed like this, the system is

scenario-based

. Understanding the scenarios of a system is often the easiest way to understand the entire system. We will illustrate this using the examples of the mutual exclusion system, the crosstalk algorithm and the cookie vending machine.