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

192 Modelling of Concurrent Reactive Behaviour
static links and excludes the use of dynamic links. Weakly distributed communication
uses dynamic links and excludes the use of autistic communication. In practice an object
can use a mixed form of communication. In such a case individual communications can
be characterised to be in a strongly distributed fashion or weakly distributed fashion.
6.4.5 Static and Dynamic Linking
The concepts of physical and logical distribution as well as the concepts of weak and
strong distribution do not specify whether the number of links of a process is static
or dynamic. In general, the design of hardware, software and mixed systems requires
attention for dynamic and static connectivity.
A process object is statically linked if it has a fixed collection of communication partners.
A process is dynamically linked if it has a dynamic collection of communication partners.
A dynamically linked process has static links available for all potential communication
partners. In addition however, the concept of dynamic linking specifies that dynamic
links are available and are dynamically used. This means that an object has a flexible
number of identifiers of the communication partners. When we relate weak and strong
Statically Linked Dynamically Linked
Weakly Distributed fixed set of dynamic links variable set of dynamic links
Strongly Distributed autistic communication (not applicable)
Table 6.1: Distributed Communication Fashions
distribution to static and dynamic linking we observe various types of collaboration (see
Table 6.1). Process objects are always statically interconnected via channels and their
message interfaces are also statically determined in the class descriptions. So for all
cases there is a fixed set of static links. A weakly distributed process can in principle
obtain or transfer identifiers of communication partners (process objects) so that it can
have a variable set of dynamic links. Such a process is dynamically linked. If its set
of dynamic links is constrained to be fixed the process is statically linked. A strongly
distributed module (subsystem) always communicates autistically. Dynamic linking is
then not applicable.
So the meaning of static linking appears to be context dependent:
Static linking implies a fixed set of identifiers in a weakly distributed case.
Static linking implies the connection by static channels in a strongly distributed
case.
So there are several definitions of forms of distribution. These forms are not always
directly visible in a model. Most complex systems will contain a mix of various forms
on various hierarchical levels. Physical distribution can be shown explicitly by a distribution
boundary. However, weak and strong distribution are not shown as distribution
boundaries. They are forms of logical distribution. These concepts can be used to reason
about the constructs to be used in the behaviour description of (a part of) the system.