1. Emergent properties
Ian Sommerville
Emergent properties, 2013
Slide 1

2. Socio-technical system
characteristics
•
Emergence
–
•
Non-determinism
–
•
The system, as a whole, has properties that only ‘emerge’ once it
has been created from its components
The system does not always produce the same output when
presented with the same input
Subjective behaviour
–
The success or failure of the system in supporting organisational
and user objectives depends on the interpretation of system
stakeholders.
Emergent properties, 2013
Slide 2

3. Emergent properties
• Properties of the system as a whole rather
than properties that can be derived from
the properties of the individual system
components
• A system is more than the sum of its parts
Emergent properties, 2013
Slide 3

4. Emergent properties, 2013
Slide 4

5. • Emergent properties are a consequence of
the relationships between system
components
• They can therefore only be assessed and
measured once the components have been
integrated into a system
Emergent properties, 2013
Slide 5

6. Functional emergent properties
• These appear when all the parts of a
system work together to achieve some
objective.
• The aim of system development is to
create a system with the desired
functional emergent properties
Emergent properties, 2013
Slide 6

7. Emergent properties, 2013
Slide 7

8. • For example, a bicycle has the
functional property of being a
transportation device once it has been
assembled from its components.
Emergent properties, 2013
Slide 8

9. Non-functional emergent properties
•
These relate to the behaviour of the system in its
operational environment.
•
They are often critical for computer-based systems as
failure to achieve some minimal defined level in these
properties may make the system unusable.
•
Examples are reliability, performance, safety, and
security.
Emergent properties, 2013
Slide 9

10. Non-functional properties
•
Volume
–
•
The volume of a system (the total space occupied) varies
depending on how the component assemblies are arranged
and connected.
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, and modify or replace these components.
Emergent properties, 2013
Slide 10

11. Non-functional properties
•
Reliability
–
•
System reliability depends on component reliability but
unexpected interactions can cause new types of failures and
therefore affect the reliability of the system.
Security
–
Security is a complex property that cannot be easily
measured. Attacks may be devised that were not anticipated
by the system designers and so may defeat built-in
safeguards.
Emergent properties, 2013
Slide 11

12. Reliability as an emergent
property
• System failures often occur because of
unforeseen inter-relationships between
components.
• It is practically impossible to anticipate all
possible component relationships.
Emergent properties, 2013
Slide 12

13. Emergent properties, 2013
Slide 13

14. Hardware reliability
• What is the probability of a hardware
component failing and how long does it take
to repair that component?
• Hardware components have a finite lifetime
and will always eventually fail
Emergent properties, 2013
Slide 14

15. Software reliability
• How likely is it that a software
component will produce an incorrect or
undesirable output?
• Software failure is distinct from
hardware failure in that software does
not wear out.
Emergent properties, 2013
Slide 15

16. Emergent properties, 2013
Slide 16

17. Operator reliability
• How likely is it that the operator of a
system will make an error?
Emergent properties, 2013
Slide 17

18. Still taken from CCTV images
Emergent properties, 2013
Slide 18

19. Failure propagation
Emergent properties, 2013
Slide 19

20. Summary
•
Distinguishing characteristics of STS are emergence,
non-determinism and subjective behaviour
•
Emergent properties are properties of the system as
a whole
•
They ‘emerge’ once a system has been integrated
•
Dependability properties are all emergent properties
Emergent properties, 2013
Slide 20