Specification of Reactive Hardware/Software Systems - Electronic ...

11.4 Essential Specification Modelling 353
initialisation, normal operation, configuration, job preparation, shut down and failure
recovery. An essential model is preferably structured by initial focusing on a normal
operation mode scenario.
In contrast to functional views, multidisciplinary views can be chosen to be considered
in advance. Particular aspects of these views can be modelled with scenarios. At least
the views that can cause risks in a design project should be chosen. If too many views are
chosen to be modelled as scenarios the analysis process can become counterproductive.
We recommend to define a list of scenarios that must be worked out for all system parts.
As long as dedicated tools are to be developed the designer is responsible for keeping
MFDs consistent. Consistency concerns for instance names and types of symbols for the
same entity on various levels of abstraction and in various representations. For scenarios
it is useful to keep the placement of symbols in related diagrams consistent. After all,
scenarios that concern the same group of collaborating objects must be composed to a
scenario assembly. Composing an assembly cannot be done efficiently unless the basic
placement structure of the scenarios is matching. Available drawing tools that offer the
concept of layers are suitable to create consistent models.
Figure 11.12 and Figure 11.13 show simplified scenarios that match the scenario assembly
in Figure 11.14. The objects have fixed positions by placing them in a common layer. The
flows of a scenario are drawn in a layer that corresponds to that scenario. An assembly
shows all flows that enter or leave an object by making all layers visible simultaneously.
When the scenarios are assembled, it is in general necessary to adjust the course of some
flows to achieve well structured graphical representations.
Scenarios are created by playing a part of the behaviour of collaborating objects in our
imagination. Objects send and receive messages and can mutually activate each other.
In general, behaviour consists of specific traces of activities. These traces are described
textually in scenario narratives. They are important input for the description of the formal
behaviour in the Unified Model.
11.4.5.6 Message Flows
SHE models emerge from four strongly related key activities:
exploration of objects (and clusters) needed to model the problem domain;
abstraction of properties so that objects adequately represent their (real- world)
counterparts;
allocation of operations to objects so that they become reusable components;
modelling of dynamic behaviour, which means exploration of causal relations
between events and responses, order of events, concurrent behaviour and possible
interruptions.