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Emergent properties


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Emergent properties

  1. 1. Emergent properties Ian Sommerville Emergent properties, 2013 Slide 1
  2. 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. 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. 4. Emergent properties, 2013 Slide 4
  5. 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. 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. 7. Emergent properties, 2013 Slide 7
  8. 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. 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. 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. 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. 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. 13. Emergent properties, 2013 Slide 13
  14. 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. 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. 16. Emergent properties, 2013 Slide 16
  17. 17. Operator reliability • How likely is it that the operator of a system will make an error? Emergent properties, 2013 Slide 17
  18. 18. Still taken from CCTV images Emergent properties, 2013 Slide 18
  19. 19. Failure propagation Emergent properties, 2013 Slide 19
  20. 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