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

7.2 A Brief Language Characterisation 231
Communication Between Process Objects
The grain of concurrency in POOSL is the process. Processes communicate by sending
each other messages via channels. When a process wants to send a message it explicitly
states to which channel this message has to be sent. It also explicitly states when and
from which channel it wants to receive a message. Immediately after a message has
been received, the sending process continues its activities (it does not have to wait for
a result). If a process receives a message, it does not execute a method as in traditional
object-oriented languages. Also, a possibly expected result is not automatically returned
to the sender. If a result of the message is expected, it has to be transmitted by means of
another rendezvous.
Methods of Process Objects
A process object can call and execute one of its methods. This does not require the
reception of a message. Methods are comparable with procedures of imperative programming
languages such as C or Pascal. However, procedures of imperative programming
languages are in general expected to terminate and can therefore only express
finite behaviour. Methods in POOSL, on the other hand, can be used to express infinite
(non-terminating) behaviour. Such infinite behaviour is specified by defining methods
in a (mutual) tail-recursive manner. The concept of tail recursion has proven to be
very useful for the specification of hardware/software systems with complex dynamic
(communication) behaviour.
Internal Data of Process Objects
Processes contain internal data in the form of data objects (also called travelling objects)
which are stored in instance variables. Data objects are private to the process that owns
them, i.e. process objects have neither shared variables nor shared data. A process
can interact with its data objects by sending messages to them. When a process sends
a message to one of its data objects, its activities are suspended until the result of the
message is returned by that data object. Data objects themselves cannot send messages
(except for replies) to a process object autonomously.
Parameter Passing of Process Objects
When two processes communicate, a message and a (possibly empty) set of parameters
is passed from one process to another. The parameters refer to data objects which are
private to the sending process. Because processes do not have any data in common,
it does not suffice to just pass a set of references to the data objects, as in traditional
object-oriented languages. Instead, it has to be mimicked that the objects themselves
are passed (whence the term travelling object). This means that a new set of objects has
to be created within the environment of the receiving process. These objects are (deep)
copies of the data objects involved in the rendezvous. The concept of travelling object
was first introduced in [Ver92].
Clusters
Next to processes, POOSL supports clusters. A cluster is composed of processes and
other clusters and acts as an abstraction of these. Clusters are used to create a hierarchical