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

11.4 Essential Specification Modelling 363
requesters. Another issue is the use of identifiers for the destination of messages.
Sharing a channel usually requires that objects can be identified. The communication is
in general weakly distributed. So channel design decisions can immediately influence
the style of the behaviour description of a system.
11.4.7.2 Channel Names
ISDs show instances that are connected by channels that must be named. The instances
are instantiated from classes that have message interfaces. A message interface defines
the inner channel names of an entity. The channels that connect instances must be named
with so-called outer channel names. These outer names can be modified by applying
one or more behaviour-preserving transformations (see Chapter 10).
Like object names, channel names should in principle be given readable names according
to the problem domain. In practice however, this can only be done for a minority of
the channels in a complex system. In general it is possible to give expressive names to
channels that clearly play a role in the system architecture. The same holds for lower
level shared channels that clearly have a role in structuring the system.
The majority of channels in a complex system is not subject of explicit design. After
all, the communication of process objects is always modelled by using channels, even if
there is no physical need for channels. A part of the channels corresponds to a structure
that we can derive almost automatically from message flows between pairs of process
objects. In some cases it is possible to keep the inner and outer names identical when a
pair of instances is designed with an appropriate message interface.
In general, it does not make sense to attempt to find expressive names for ’automatically’
derived channels. In general, it is rather difficult to find expressive names because
there are many channels like these. In future tools we expect to offer automatic name
generation. For the time being we recommend an approach that is illustrated in Figure
11.18. Most channel names are composed of four characters. These characters correspond
to the capitals of the words that are used to compose the name of the classes of
the instances at both sides of a pair-wise channel. In general, some creativity will be
necessary to create a unique name for each channel.
11.4.7.3 Modelling Cluster Properties
Instances are visualised in ISDs and in Message Flow Diagrams. Both diagrams offer
the possibility to annotate clusters with subsystem properties in the form of boundaries.
Examples are distribution boundaries, concurrency boundaries, abstraction boundaries
and implementation boundaries. Both diagrams offer this information consistently. The
creation of these boundaries is a design activity that can be performed best in ISDs. After
all, ISDs give a more abstract image and are meant to be used for structure formalisation.
Message Flow Diagrams can be cluttered by numerous flows. ISDs give a more clear
overview because they contain less information.