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

232 Introduction to the Semantics of POOSL
structure of modules that hide their internal structure. The constituents of a cluster
are composed by parallel composition, channel hiding and channel renaming. These
combinators are based upon similar combinators originally used in CCS [Mil80, Mil89].
Message Interfaces
Every process and cluster has a message interface describing which messages can be
sent and received on the specified channels. The set of message interfaces of a behaviour
description determines which instances can possibly communicate through which channels.
It does not state when possible communications take place.
Process and Cluster Classes
Processes and clusters are grouped in classes. A class of a process (a process class)
specifies the behaviour of a collection of similar processes objects. A class of a cluster
(a cluster class) specifies the behaviour and internal structure of a collection of similar
clusters. Processes of process classes and clusters of cluster classes are called instances.
All classes are parameterised by so-called expression parameters. Expression parameters
refer to data objects and are used to initialise an instance of a process class or an
instance of a cluster class.
Data Objects
Besides processes and clusters, POOSL supports data objects, also called travelling objects.
Data objects are comparable to objects in sequential object-oriented programming
languages such as Smalltalk [GR89], C++ [Str92], Eiffel [Mey88], and SPOOL [AB90]. A
data object consists of some private data and has the ability to perform operations on
this data. Data is stored in instance variables, which contain (references to) other objects
or to the object which owns the variables. The variables of an object are encapsulated;
they cannot be accessed directly by any other object. They can only be read and changed
by the object itself.
Communication Between Data Objects
Data objects can interact by sending messages to each other. A message consists of a
message name, and zero or more parameters. A message can be seen as a request to
carry out one of the object’s services. An object explicitly states to which object it wants
to send a message. When an object sends a message, its activities are suspended until
the result of the message arrives. An object that receives a message will execute a socalled
method. The name and the parameters of this method correspond in a one-to-one
fashion to the name and the parameters of the received message. A method implements
one of the object’s services. It can access all instance variables of its corresponding
object. In addition, a method may have local variables of its own. The result of a
method execution is returned to the sender.
Data Classes
Data objects are grouped into data classes. A data class describes a set of objects that
have the same functionality. The individual objects in a class are called instances.