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

294 Process Part of POOSL
distributed system is able to direct a message towards some specific other process by
naming it. This process will only receive a message if this message is actually meant for
it. A typical other characteristic of weakly distributed systems is that names of processes
in weakly distributed systems can be passed from one process to another. This means
that the network of process acquaintances can change during the lifetime of a system.
Previous versions of POOSL [Voe94, Voe95a]) did not have the expressive power to
model weakly distributed systems. The solution to this problem was obtained when
message-receive statements ch?m(p1 ¥¡ ¡ ¡ ¥ pm) were generalised by extending them with
reception conditions. Using these reception conditions, weakly distributed systems can
be modelled by supplying each process object with a unique process identifier. Process
identifiers are modelled by data objects. Therefore identities can be passed from one
process to another. The sending of a message m (on channel ch) to a process with identifier
id can be expressed by statement ch!m(id). The corresponding conditional messagereceive
statement is then of the form ch?m(i ¡ i id). It states that the process with
identifier id is willing to receive message m from channel ch only if message parameter i
equals id.
It is interesting to note that conditional message reception is not only applicable for the
modelling of message passing based on process identifiers. The primitive can for example
also be used to model processes with priorities. This example clearly shows how
complex the relation between language primitives and method concepts can be. The
reception condition primitive enables the expression of the concept weak distribution as
well as the concept of priorities. The primitive is however not sufficient to model these
concepts. Other primitives such as message-receive statements and data expressions are
also required.
The replacement of message-receive statements by conditional message receive statements
had a great impact on the semantics that had been developed till then. More
about this is explained in the next subsection.
9.7.3 Layers of Semantics
The semantics of POOSL consists of two layers. At the layer of data objects, data expressions
and data statements may take lots of small steps to be evaluated respectively
executed. At the layer of process objects, these small steps are abstracted from and combined
into single steps. In this subsection we will explain why we decided to build the
semantics this way. For an alternative would have been to consider only one semantic
layer and to formalise this layer by a single labeled-transition system.
Consider guarded command § E S p whose execution is given by rule (e’) of Subsection
9.5.3: