Information Technology Reference
In-Depth Information
The notion of a procedure is, inter alia, characterized by the fact that a new
procedure (referred to as “called”) called in a given procedure (referred to as “call-
ing”) has usually the environment of the calling procedure as part of its environment.
It results in the fact that through the common environment the calling procedure,
as well as the called procedure are related to each other and they cannot proceed
independently. Particularly, the calling procedure cannot be completed before the
termination of the called procedure.
The introduction of the process notion was aimed, inter alia, at increasing the
independence of action. That was mainly caused by problems and demands created
in the process of creation of such systems as parallel processing systems with the
time division.
The concept of a process is distinguished by the fact that environments of processes
(two or more) are not mutually independent (enough) and the existence of one process
does not have to influence the proceeding of another process. Therefore, a given
process may exist and be proceeded independently of the process which contributed
to its creation.
The process itself does not possess complete information about its state, and
its formation requires that the environment should be complemented with certain
elements memorized by the system (as a global environment of the system). In
effect, the process may be realized in strictly defined conditions and in a given
environment and therefore it does not possess the full independence of action (is not
an autonomous element).
The notion of an object had to be introduced in the course of development
of distributed processing systems. The object was established as a result of the
development of the process notion. The significant part of information indispens-
able for creating the object is recorded in a given object, and because of so-called
encapsulation the object takes full control of it. The object is, therefore, to a large
extent independent of the environment and may operate in different conditions that,
i.e., in different environments. The initiative to operate—in other words, to create an
algorithm (or at least its part) comes from the outside of the object, most often from
another algorithm, therefore it depends on the communication process between the
objects.
The object as an independent entity may relocate in the computer networks
and may be used in different environments constituting a handy element for the
construction of distributed systems in practice. However, the object needs an exter-
nal signal of activation to undertake the action.
Introducing the concept of an object, despite encapsulation, the possibility of
access of one object to the features (data) of another object was secured with the use
of the mechanism of a certain kind of communication. The realization of data transfer
between the objects uses the concept of a method for communication with internal
structures of data of the object. It involves activating by one object an appropriate,
aim-oriented and accessible method of another object which guarantees the transfer
of data. Access to data of one object may be realized by another object in two ways.
One is based on the fact that the object collecting data possesses the initiative to start
the operation of collecting. In the other case, the initiative is on the side of the object
