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2 SOFTWARE HOLONS –
RATIONALE FOR LAYERED
ARCHITECTURES
The complexity of living systems by far exceeds the
complexity of any computer system, and yet living
systems are reliable and adaptive. Therefore, it
seems sensible to study the structure and behaviour
of living organisms in search for paradigms to be
used in the construction of software solutions. An
important way of thinking in behavioural science
(apart from reductionism and holism) is centered on
the notion of holon. To quote Koestler (1980,
p.447):
“A living organism is not an aggregation of
elementary parts, and its activities cannot be
reduced to reeling off a chain of conditioned
responses. In its bodily aspects, the organism is
a whole consisting of “sub-wholes”, such as
the circulatory system, digestive system, etc.,
which in turn branch into sub-wholes of a
lower order, such as organs and tissues - and so
down to individual cells. In other words, the
structure and behaviour of an organism ... is a
multi-levelled, stratified hierarchy of subwholes,
... where the sub-wholes form the
nodes, and the branching lines symbolise
channels of communication and control. ...
The point first to be emphasised is that each
member of this hierarchy, on whatever level, is
a sub-whole or “holon” in its own right - a
stable, integrated structure, equipped with selfregulatory
devices and enjoying a considerable
degree of autonomy or self-government.”
From studies of most complex systems that exist,
such as biological systems, it is known that complex
systems, which work well, have one thing in
common – they are hierarchies of elements; they are
not networks. Such systems exhibit layered
architectures without circular dependencies within
and between layers.
Holons are: “...subordinated as parts to the
higher centres in the hierarchy, but at the same time
function as quasi-autonomous wholes. They are
Janus-faced. The face turned upward, toward the
higher levels, is that of a dependent part; the face
turned downward, towards its own constituents, is
that of a whole of remarkable self-sufficiency.”
(Koestler, 1980, p.447)
Accordingly, in object-oriented system
modeling, the holon abstraction can be simulated by
the notion of composition. This notion has been
documented and offered as a design pattern (Gamma
et al., 1995). The composite pattern represents part-
MANAGING COMPLEXITY OF ENTERPRISE INFORMATION SYSTEMS 31
whole hierarchies of objects and allows treating
parts and wholes in a uniform way (by narrowing the
difference between the components and
compositions of components). Figure 1 is a UML
graphical representation of the composite pattern
(Gamma et al., 1995).
Component
Leaf Composite
Figure 1: The structure of the composite pattern.
As stated in Gamma et al. (1995, p.163):
“The key to the Composite pattern is an abstract
class that represents both primitives and their
containers.” To be precise, the Component class is
a partially implemented abstract class that declares
an interface for objects in the composition. The
behavior of primitive objects is defined
(implemented) in the Leaf class and the behavior of
components having children in the Composite
class. Default implementations of the behaviors
common to all classes in the composition can be
provided in the Component class (hence, this class
is “partially implemented”). Client programs use the
interface of the Component class to communicate
with objects in the composition. If a recipient object
is a Composite, then the client’s message will
usually trigger requests to its child components
before the requested operation can be completed.
Consistently with the holon abstraction, the
composite pattern reflects the “openess” property of
holons and supports “design for change”. In living
organisms, complex systems evolve from simple
systems and there is no absolute “leaf” holon or
“apex” holon. Wholes are composed of parts that
can be either leaves or composites.
The composite pattern matches the holon
hypothesis. Complex software systems consist of
several layers of abstraction. Each one of them is a
quasi-autonomous construction visible as a
dependent part to the higher layer of abstraction and
at the same time exerting some control over lower
layers of abstraction. A layer of abstraction has its
own semantics which is complete according to its
specification and which is independent from the
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