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

6.2 Concurrency and Synchronisation 149
We defined process objects as entities that can perform their behaviour concurrently to
each other: Process objects can model concurrent independent system parts (connected
in a static channel structure). The internal behaviour of process objects is sequential.
Collaborating data objects perform sequential behaviour. They model passive entities that
must be activated by a message. Only one data object from a collection of collaborating
data objects is actively processing. Together with a message the thread of activity goes
from sender to receiver.
In general, collaborating data objects model a part of the internal sequential behaviour
of a process object. Two data objects that belong to two different process objects can
operate concurrently. These objects are completely independent and cannot directly
communicate with each other.
A cluster is an abstraction boundary around a group of collaborating process objects and
clusters. In contrast to process objects, clusters can have internally concurrent behaviour.
Internally concurrent behaviour is specified in a cluster class by a parallel composition
of processes and clusters (subsystems). The internal concurrency of a cluster is solely
established by its process objects.
Clusters can be used to specify whether the internal behaviour of a subsystem is sequential
or concurrent. Such a cluster is defined to be a concurrency boundary. This
is an enclosure that prescribes that the cluster has an internal sequential behaviour (see
Paragraph 5.8.2.4). This means that collaborating processes inside the boundary form
a composite that must be modelled such that it performs a sequential behaviour. This
requirement determines the behaviour style of the individual processes. This style is determined
by the sort of communication and synchronisation that must be used to specify
the behaviour of these processes. A subsystem that must be implemented sequentially
can have only one process active simultaneously. In that case the whole subsystem has
only one internal thread of activity. Synchronisation between objects in such a subsystem
implies that process objects pass the thread of activity together with the message,
just like data objects do.
6.2.2.2 Asynchronous Concurrency
By defining process objects as independent concurrent entities, we appealed to a general
intuition of concurrency, which is so-called asynchronous concurrency. This form of
concurrency enables processes to be independent in time. Processes are defined to be
asynchronous concurrent if they perform their actions at their own speed. This approach
is natural for distributed systems that are implemented by a collection of interconnected
processing elements that may run at different internal speeds.
The intuitive notion of concurrency takes time into account. Two operations, in two
different process objects, are thought to take time to be completed. Concurrent operation
means that both operations run simultaneously, at least during some period of time. The