Emergent system prolflerties

The complex relationships between the components in a system mean that the system
is more than simply the sum of its parts. It has properties that are properties
of the system as a whole. These emergent properties (Checkland, 1981) cannot be
attributed to any specific part of the system. Rather, they emerge only once the system
components have been integrated!. Some of these properties can be derived directly
from the: comparable properties of sub-systems. However, more often, they result
from complex sub-system interrelationships that cannot, in practice, be derived from
the properties of the individual system components. Examples of some emergent
properties are shown in Figure 2.1.
There are two types of emergent properties:
I. Functional emergent properties appear when all the parts of a system work
together to achieve some objective. For example, a bicycle has the functional
property of being a transportation device once it has been assembled from its
components.
2. Non-functional emergent properties relate to the behaviour of the system in its
operational environment. Examples of non-functional properties are reliability,
performance, safety and securi1ty. These are often critical for computer-bas~d
systl~ms, as failure to achieve wme minimal defined level in these properties
may make the system unusable. Some users may not need some system function:;
so the system may be ac(:eptable without them. However, a system that
is unreliable or too slow is likely to be rejected by all its users.
To illustrate the complexity of emergent properties, consider the property of system
reliability. Reliability is a complex concept that must always be considered at
the system level rather than at the individual component level. The components in
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Volume
The volume of a system (the total space occupiee:) varies
depending on how the component assemblies are arranged Ind
connected.
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Reliability
System reliability depends on component reliability but
unexpected interactions can cause new types of failure and
therefore affect the reliability of the system.
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security
The security of the system (Its ability to resist attack) is a
complex property that cannot be easily measured. AttackS me,
be devised that were not anticipated by the system designers
and so may defeat built-in safeguard .
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Repairability
This property reflects how easy it is to fix a problem with the
system once it has been discovered. It depends on being able to
diagnose the problem. access the components that are faulty Mel
modify or replace these components.
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Usability
This property reflects how easy it is to use the system. It
depends on the technical system components, its operators and
its operating environment
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a system are interdependent, so failures in one component can be propagated
through the system and affect the operation of other components. It is often difficult
to anticipate how the consequences of component failures propagate through
the system. Consequently, you cannot make good estimates of overall system reliability
from data about the reliability of system components.
There are three related influences on the overall reliability of a system:
I. Hardware reliability What is the probability of a hardware component failing
and how long does it take to repair that component?
2. Software reliability How likely is it that a software component will produce an
incorrect output? Software failure is usually distinct from hardware failure in
that software does not wear out. Failures are usually transient so the system
carries on working after an incorrect result has been produced.
3. Operator reliability How likely is it that the operator of a system will make an
error?
AlI of these are closely linked. Hardware failure can generate spurious signals
that are outside the range of inputs expected by software. The software can thest
behave unpredictably. Operator error is most likely in conditions of stress, such as
when system failures are occurring. These operator errors may further stress the hard
ware, causing more failures, and so on. Thus, the initial, recoverable failure can
rapidly develop into a serious problem requiring a complete system shutdown.

Like reliability, other emergent properties such as performance or usability are
hard to assess but can be measured after the system is operational. Properties such
as safety and security, however, pose different problems. Here, you are not simply
concerned with an attribute that is related to the overall behaviour of the system
but are ~oncerned with behaviour that the system should not exhibit. A secure system
is one that does not allow unauthorised access to its data but it is clearly impossible
to predict all possible modes of access and explicitly forbid them. Therefore,
it may only be possible to assess these properties by default. That is, you only know
that a system is insecure when someone breaks into it.