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

206 Modelling of Concurrent Reactive Behaviour
In our modelling approach, there are no entities other than objects that can hold a state.
State names such as normal operation mode, test mode, and maintenance mode are
names that describe behaviour on a system level. The only place to implement explicit
named states is the object. Subsection 6.6.4 describes how method names can be used as
state names in process objects. A process object can be used as a controller on the system
level. Such an object can implement named system states. Without such an approach
a system state is determined only by the joined state of the collaborating objects in the
system. The number of these states is huge in practice. It is often impractical to name
them explicitly.
Despite this problem the named behaviour state approach is very useful and feasible
when its use is restricted to:
collaborating objects that are synchronised such that they together perform a form
of behaviour that can be named. For instance, objects can perform concurrently a
system initialisation. The system as a whole as well as individual objects can then
be defined to be in the state initialising.
individual objects. The behaviour of objects can be described in a style that allows
clear distinction between different forms of behaviour that can be interpreted as
states.
The latter style is also useful for objects with a controller function. Such an object guards
and controls other objects. Its state is connected to the responsibility to enable the
appropriately named behaviour.
6.6.3.4 Variables State
The concept of behaviour state as we have proposed in the previous paragraphs is not
the common notion of state in the object-oriented world. The state of an object is usually
defined on values of its attributes. The number of states of an object is the Cartesian
product of the value ranges of all its variables. This implies that the number of states
explodes quickly even for a limited set of variables.
This notion of state is not a useful concept for analysis purposes. Such a state is theoretical
and cannot be given a name or used as communication item among people. When we
need to refer to this theoretical notion of state we call it the object variables state or briefly
variables state.
We can overcome a variable-state explosion by restricting the variables state to a ’variables
partial state’. One or more variables may be explicitly defined to determine an
object’s variables partial state. This way of modelling enables a practical definition of
states and related services (accessors) for inquiring about an object’s state.
We add behaviour state as a complementary concept to variables state. Although variables
state is the common object-oriented interpretation of state, this concept does not match