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

110 Abstraction of a Problem Domain
sequential. Process objects have instance variables that hold references to data objects
or to other process objects. The references to other process objects are typically needed
for weakly distributed forms of communication with the other process objects. Process
identifiers are used as ’address’ of process objects for delivering messages on channels.
Process identifiers are modelled as data objects. Data objects in a process object are
private, which means that they are never shared with other process objects. However,
a private data object can be copied and can thereby emulate travelling from process to
process.
4.15 Operations, Services, Methods
This section introduces elements for the description of behaviour of objects. It will
appear that we define the concept of ’method’ for process objects rather differently than
in the mainstream of other object-oriented methods. This subject is strongly related to
other important concepts such as of ’state’ and of ’behaviour’ (see Section 6.4). There is
also a strong relation between services, methods and messages. Messages are described
in the section about communication (Section 6.3). This section has been limited to the
definition of the concepts of service, operation and of method, and their role in system
analysis.
4.15.1 Operations
We define an operation as an action performed by an object. This may be a transformation
on its internal data or on data furnished by a message, it may also be an input or
output action to its environment. Rumbaugh et al. define an operation as ’a function or
transformation that may be applied to or by objects in a class’ [R
91]. Notice that we give a
broader definition that does not require a one to one relation between an operation and
reception of a corresponding message. Operations can be performed by autonomous
process objects on their own initiative, as a postponed response to communication with
other objects. Operations form a grain of behaviour. A sequence of operations may be
the response on a single message reception.
4.15.2 Services
We define a service as the response of an object, on reception of a message. The services
offered by an object are defined in the message interface of its class. Names of messages
correspond to services exported by the message interface to collaborating objects. The
response of an object to a message consists of accepting the message with its parameters,
followed by performing one or more operations. The response of the object becomes
visible when an object produces results in the form of sending reply- messages or other
messages. Visible responses give so-called externally observable behaviour.
It is useful to distinguish between various sorts of services. Determining sorts enables
communication between people involved in analysis. The various sorts can be checked,