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

2.5 Modelling Concepts 23
entirety. Therefore behaviour should be distributed over modules whose behaviour
can be understood without having to understand the environment they are placed in.
This requires modules to be self-contained, autonomous, relatively independent, and
weakly coupled entities. We will call these entities process objects, or processes in
short. Processes exist in their own right, have their own responsibilities, and they have
their own activities to perform. This is highly desirable since it enables processes to be
reused in contexts other than the one for which they were originally devised. Reuse is
achieved by organising objects into classes. A class is a template from which objects are
instantiated.
Behaviour models should be built from process objects. Process
objects are concurrent, self-contained, autonomous, relatively independent
and weakly coupled entities. Process objects have to
be grouped into classes.
Channels and Clusters
A very important characteristic of hardware/software systems is structure. Physical
or spatial distribution, physical topology, hardware/software partitioning and software
layering are all examples of phenomena that impose structure. The modelling of topology
requires that processes can be structured in a network of channels. The other forms
of structure require the possibility to group processes into higher-order entities, called
clusters. A cluster acts as an hierarchical abstraction of its internals. Hierarchy is an
important concept that helps to manage complexity. To enhance modularity and reuse,
clusters should be organised into classes.
Processes should be connected by channels. It must be possible
to group processes into clusters. Clusters have to be organised
into classes.
Message Passing
Processes (and clusters) are relatively independent and they can perform their activities
concurrently and at their own speed. Despite of their autonomy, processes will frequently
have to exchange information. To make the coupling between processes (and
clusters) as weak as possible, information exchange should be based upon message
passing. The essential feature of message passing is that information is exchanged by
means of an intermediate artefact, called a message. The purpose of a message is to
reduce the coupling between the message senders and the message receivers. To exchange
information, the sender and the receiver only have to share the format and the
general semantics of a message. They do not have to know anything about each other’s
internals, i.e. about private data. The internals of a process are said to be encapsulated
by a strong encapsulation boundary. The only way for other processes to access these
internals is through a clear-cut message interface.