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

6.4 Distribution 193
6.4.6 Distribution, Concurrency and Synchronisation
A distributed system can be characterised by the sort of concurrency of its subsystems
and its accompanying way of synchronisation and communication. Concurrency can
be synchronous or asynchronous. Communication can be synchronous or asynchronous
as well. However, mark that both notions of synchrony have a completely different
meaning, and the same holds for the notion of asynchrony.
We chose for asynchronous concurrency together with synchronous communication and
synchronisation. Asynchronous concurrency fits naturally to asynchronously physically
distributed systems. A synchronous communication primitive can express the communication
between subsystems on an abstract level very elegantly. It abstracts from the
underlying implementation which may very well use an asynchronous handshake for
instance. The synchronous primitive can be used to express asynchronous communication
as well, see also Subsection 6.3.4.
6.4.6.1 Asynchronous Physically Distributed Systems
In general, an asynchronous system has a strong form of physical distribution. We call
it a strong form of distribution because asynchrony makes modules independent. An
asynchronous distributed system has the following properties. There are no bounds on
the relative speeds of the modules and there are no bounds on message delays. Notice
that the characterisation of a system cannot be done from a hardware point of view only,
where synchrony has the specific meaning of to be driven by a global clock. The modules
themselves may be synchronous internally, and the distributed system may even be
synchronised on one clock. In this case the relative speed of processors and message
delays over network links will be bound. On top of this hardware, layers of software
may multiplex physical resources. They realise abstractions such as processes and
(reliable) communication channels. In this case the system may better be characterised
as asynchronous [BM93].
6.4.6.2 Synchronous Physically Distributed Systems
A synchronous system has a weak form of physical distribution. It has the following
properties. The relative speed of the modules is kept within strict bounds. Bounds on
message delays must be realised. An example of such a system is a hardware realisation
in the form of collaborating digital ASICs 6 or PLDs 7 with a common clock.
6.4.7 Distribution and Sharing
6.4.7.1 Code Sharing
Instances of the same class share the behaviour description of that class. This is socalled
specification sharing, and differs from implementation sharing which focuses on
sharing of code. In distributed systems the behaviour of instances from the same class
6 Application Specific Integrated Circuit.
7 Programmable Logic Device.