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

4.4 Data Objects and Process Objects 73
have instance variables and methods that can formalise attributes and messages found
during analysis. An important difference with data objects is that a pair of process objects
can exchange messages without becoming passive. Process objects are autonomous
concurrent entities. They can take initiative for sending messages with or without
waiting for a reply. They can continuously perform rich and diverse behaviour. In
contrast to data objects messages do not always lead to the same form of behaviour at
any time. The behaviour of a process object strongly depends on the past. Behaviour
can further be interrupted or aborted on reception of particular messages.
4.4.3.2 Concurrency
Process objects are concurrent entities. The internal behaviour of process objects is
sequential. This is a method design decision. Concurrency can be described in terms
of threads. (A thread is defined as ’a primitive executable processing element within a
process.’ A thread consists of a locus of control and a stack which represents its ’state’
and is initially empty[Weg87a]). A concurrent entity has an interface of executable
operations or entry points, and one or more threads of control that may be active or
suspended. There are three types of processes:
Sequential process, with one thread of control.
Quasi-concurrent process with at most one active thread of control.
Concurrent process which may have multiple active threads of control.
We choose to define the process object as the grain of concurrency. Internally they
are sequential processes with one thread of control. A reason for this choice is that
internally sequential objects are easier to reason about for human beings. Other reasons
are described in Subsection 6.2.2.
4.4.3.3 Instances
Class name
(Process
identifier)
Figure 4.9: Process Symbol for Object Instance Models
The visualisation of structure is one of the main issues in this thesis. Therefore we must
give it a balanced role during analysis. Object Class Diagrams do not show how many
instances are drawn from a class. They do not show who communicates with who. We
decided to give object instance modelling a major role for systems analysis. We define
two sorts of diagrams that together form an Object Instance Model of a system. Process