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

9.7 The Development of POOSL 293
data class Bit
instance variables bit
instance methods
error setToZero setToOne invert
primitive bit 0; bit 1; if bit 0 then bit : bit 1
self self else bit : bit ¤ 1 fi;
if bit 0 or bit ¡ 1 then self error fi;
self
Primitive message error aborts the execution with an error message. Assume that a
variable b refers to a Bit which is setToZero and consider the concurrent evaluation of
expression (b invert) ¡ (b invert). It seems clear that the result of this evaluation must
be true 8 . Indeed, true is one of the possible outcomes. Another possibility, however, is
that the evaluation aborts with an error message, leaving b in the unexpected state where
instance variable bit refers to 2!
Problems of this kind are well-known in object-oriented languages, such as Smalltalk-80
[GR89], that support processes as an orthogonal language concept. These processes may
act on the same collection of objects. It is even possible that they are executing the same
method in the same object at the same time [AR89b]. This can result in problems of
synchronisation and mutual exclusion as in the case of our Bit example.
Of course, these problems of synchronisation and mutual exclusion can be solved if
concurrently evaluated expressions are required to be side-effect free. However, the
application of expressions with side-effects in object-oriented languages is not at all
unusual. In Eiffel [Mey88], a restricted use of functions with side-effects is even recommended
and exploited (see also Paragraph 6.6.3.5).
The problems described above also occur when other forms of concurrency within
process objects are allowed. We have therefore determined the grain of concurrency at
the level of the process object.
9.7.2 Conditional Message Reception
The basic process communication model in POOSL is based upon the synchronous pairwise
message-passing mechanism of CCS [Mil89]. In this model, processes communicate
by exchanging messages over (static) channels. If a process sends a message on a
channel, it has in principle no knowledge of the identity of the receiving process. Vice
versa, if a process receives a message, it has no knowledge of the senders identity. The
basic communication model of POOSL thus allows the modelling of strongly distributed
systems (see also Paragraph 6.4.4.2). However, many complex (object-oriented) systems
are characterised by weak distribution (see Paragraph 6.4.4.1). A process in a weakly
8 Note that the primitive equality message only determines whether two expressions refer to the
same object or not.