Critical Systems

 To explain what is meant by a critical system where system failure can have severe human or economic consequence.  To explain four dimensions of dependability - availability, reliability, safety and security.  To explain that, to achieve dependa

©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 1Critical Systems©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 2Objectives To explain what is meant by a critical systemwhere system failure can have severehuman or economic consequence. To explain four dimensions of dependability -availability, reliability, safety and security. To explain that, to achieve dependability,you need to avoid mistakes, detect andremove errors and limit damage caused byfailure.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 3Topics covered A simple safety-critical system System dependability Availability and reliability Safety Security ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 4Critical Systems Safety-critical systems• Failure results in loss of life, injury or damage to theenvironment;• Chemical plant protection system; Mission-critical systems• Failure results in failure of some goal-directed activity;• Spacecraft navigation system; Business-critical systems• Failure results in high economic losses;• Customer accounting system in a bank;©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 5System dependability For critical systems, it is usually the case that themost important system property is the dependabilityof the system. The dependability of a system reflects the user’sdegree of trust in that system. It reflects the extent ofthe user’s confidence that it will operate as usersexpect and that it will not ‘fail’ in normal use. Usefulness and trustworthiness are not the samething. A system does not have to be trusted to beuseful.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 6Importance of dependability Systems that are not dependable and areunreliable, unsafe or insecure may berejected by their users. The costs of system failure may be veryhigh. Undependable systems may causeinformation loss with a high consequentrecovery cost. ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 7Development methods for critical systems The costs of critical system failure are sohigh that development methods may be usedthat are not cost-effective for other types ofsystem. Examples of development methods• Formal methods of software development• Static analysis• External quality assurance©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 8Socio-technical critical systems Hardware failure• Hardware fails because of design andmanufacturing errors or because componentshave reached the end of their natural life. Software failure• Software fails due to errors in its specification,design or implementation. Operational failure• Human operators make mistakes. Now perhapsthe largest single cause of system failures.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 9A software-controlled insulin pump Used by diabetics to simulate the function ofthe pancreas which manufactures insulin, anessential hormone that metabolises bloodglucose. Measures blood glucose (sugar) using amicro-sensor and computes the insulin doserequired to metabolise the glucose. ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 10Insulin pump organisationNeedleassemblySensorDisplay1 Display2AlarmPump ClockControllerPower supplyInsulin reservoir©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 11Insulin pump data-flowInsulinrequirementcomputationBlood sugaranalysisBlood sugarsensorInsulindeliverycontrollerInsulinpumpBloodBloodparametersBlood sugarlevelInsulinPump controlcommandsInsulinrequirement©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 12Dependability requirements The system shall be available to deliverinsulin when required to do so. The system shall perform reliability anddeliver the correct amount of insulin tocounteract the current level of blood sugar. The essential safety requirement is thatexcessive doses of insulin should never bedelivered as this is potentially lifethreatening. ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 13Dependability The dependability of a system equates to itstrustworthiness. A dependable system is a system that istrusted by its users. Principal dimensions of dependability are:• Availability;• Reliability;• Safety;• Security©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 14Dimensions of dependabilityDependabilityAvailability Reliability SecurityThe ability of the systemto deliver services whenrequestedThe ability of the systemto deliver services asspecifiedThe ability of the systemto operate withoutcatastrophic failureThe ability of the systemto protect itelfagainstaccidental or deliberateintrusionSafety©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 15Other dependability properties Repairability• Reflects the extent to which the system can be repaired inthe event of a failure Maintainability• Reflects the extent to which the system can be adapted tonew requirements; Survivability• Reflects the extent to which the system can deliverservices whilst under hostile attack; Error tolerance• Reflects the extent to which user input errors can beavoided and tolerated. ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 16Maintainability A system attribute that is concerned with the ease ofrepairing the system after a failure has beendiscovered or changing the system to include newfeatures Very important for critical systems as faults are oftenintroduced into a system because of maintenanceproblems Maintainability is distinct from other dimensions ofdependability because it is a static and not adynamic system attribute. I do not cover it in thiscourse.©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 17Survivability The ability of a system to continue to deliverits services to users in the face of deliberateor accidental attack This is an increasingly important attribute fordistributed systems whose security can becompromised Survivability subsumes the notion ofresilience - the ability of a system to continuein operation in spite of component failures©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 18Dependability vs performance Untrustworthy systems may be rejected by theirusers System failure costs may be very high It is very difficult to tune systems to make them moredependable It may be possible to compensate for poorperformance Untrustworthy systems may cause loss of valuableinformation ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 19Dependability costs Dependability costs tend to increase exponentiallyas increasing levels of dependability are required There are two reasons for this• The use of more expensive development techniques andhardware that are required to achieve the higher levels ofdependability• The increased testing and system validation that isrequired to convince the system client that the requiredlevels of dependability have been achieved©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 20Costs of increasing dependabilityLow Medium High VeryhighUltra-highDependability©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 21Dependability economics Because of very high costs of dependabilityachievement, it may be more cost effectiveto accept untrustworthy systems and pay forfailure costs However, this depends on social and politicalfactors. A reputation for products that can’tbe trusted may lose future business Depends on system type - for businesssystems in particular, modest levels ofdependability may be adequate ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 22Availability and reliability Reliability• The probability of failure-free system operationover a specified time in a given environment fora given purpose Availability• The probability that a system, at a point in time,will be operational and able to deliver therequested services Both of these attributes can be expressedquantitatively©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 23Availability and reliability It is sometimes possible to subsume systemavailability under system reliability• Obviously if a system is unavailable it is notdelivering the specified system services However, it is possible to have systems withlow reliability that must be available. So longas system failures can be repaired quicklyand do not damage data, low reliability maynot be a problem Availability takes repair time into account©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 24Reliability terminologyTerm DescriptionSystem failure An event that occurs at some point in time whenthe system doe s not deliver a service as expectedby its usersSystem error An erroneou s system state that can lead to systembehaviour that is unexpec ted by system users.System fault A characteristic of a software system that canlead to a system error. For example, failure toinitialise a variable could lead to that variablehaving the wrong value when it is used.Human error ormistakeHuman behaviour that results in the introductionof faults intoa system. ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 25Faults and failures Failures are a usually a result of system errors thatare derived from faults in the system However, faults do not necessarily result in systemerrors• The faulty system state may be transient and ‘corrected’before an error arises Errors do not necessarily lead to system failures• The error can be corrected by built-in error detection andrecovery• The failure can be protected against by built-in protectionfacilities. These may, for example, protect systemresources from system errors©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 26Perceptions of reliability The formal definition of reliability does not alwaysreflect the user’s perception of a system’s reliability• The assumptions that are made about the environmentwhere a system will be used may be incorrect• Usage of a system in an office environment is likely to bequite different from usage of the same system in a universityenvironment• The consequences of system failures affects theperception of reliability• Unreliable windscreen wipers in a car may be irrelevant in adry climate• Failures that have serious consequences (such as an enginebreakdown in a car) are given greater weight by users thanfailures that are inconvenient©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 27Reliability achievement Fault avoidance• Development technique are used that either minimise thepossibility of mistakes or trap mistakes before they resultin the introduction of system faults Fault detection and removal• Verification and validation techniques that increase theprobability of detecting and correcting errors before thesystem goes into service are used Fault tolerance• Run-time techniques are used to ensure that systemfaults do not result in system errors and/or that systemerrors do not lead to system failures ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 28Reliability modelling You can model a system as an input-outputmapping where some inputs will result inerroneous outputs The reliability of the system is the probabilitythat a particular input will lie in the set ofinputs that cause erroneous outputs Different people will use the system indifferent ways so this probability is not astatic system attribute but depends on thesystem’s environment©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 29Input/output mappingIeInput setOeOutput setProgramInputs causingerroneous outputsErroneousoutputs©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 30Reliability perceptionPossibleinputsUser1User3User2Erroneousinputs [...]... 2004 Software Engineering, 7th edition. Chapter 3 Slide 4 Critical Systems  Safety -critical systems • Failure results in loss of life, injury or damage to the environment; • Chemical plant protection system;  Mission -critical systems • Failure results in failure of some goal-directed activity; • Spacecraft navigation system;  Business -critical systems • Failure results in high economic losses; • Customer... dependability  Systems that are not dependable and are unreliable, unsafe or insecure may be rejected by their users.  The costs of system failure may be very high.  Undependable systems may cause information loss with a high consequent recovery cost. ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 7 Development methods for critical systems  The costs of critical system... Software Engineering, 7th edition. Chapter 3 Slide 38 Normal accidents  Accidents in complex systems rarely have a single cause as these systems are designed to be resilient to a single point of failure • Designing systems so that a single point of failure does not cause an accident is a fundamental principle of safe systems design  Almost all accidents are a result of combinations of malfunctions  It... that results in the introduction of faults intoa system. ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 1 Critical Systems ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 2 Objectives  To explain what is meant by a critical system where system failure can have severe human or economic consequence.  To explain four dimensions of dependability - availability,... Business -critical systems • Failure results in high economic losses; • Customer accounting system in a bank; ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 5 System dependability  For critical systems, it is usually the case that the most important system property is the dependability of the system.  The dependability of a system reflects the user’s degree of trust in that system. It reflects... especially, in software controlled systems is impossible so achieving complete safety is impossible ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 39 Security  The security of a system is a system property that reflects the system’s ability to protect itself from accidental or deliberate external attack  Security is becoming increasingly important as systems are networked so that... development methods • Formal methods of software development ã Static analysis ã External quality assurance âIan Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 8 Socio-technical critical systems  Hardware failure • Hardware fails because of design and manufacturing errors or because components have reached the end of their natural life.  Software failure • Software fails due to... is sometimes possible to subsume system availability under system reliability • Obviously if a system is unavailable it is not delivering the specified system services  However, it is possible to have systems with low reliability that must be available. So long as system failures can be repaired quickly and do not damage data, low reliability may not be a problem  Availability takes repair time into... dependability, you need to avoid mistakes, detect and remove errors and limit damage caused by failure. ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 3 Topics covered  A simple safety -critical system  System dependability  Availability and reliability  Safety  Security ©Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 13 Dependability  The dependability . safety -critical systems Systems whose failure results in faults in other systemswhich can threaten people Discussion here focuses on primary safety-criticalsystems•. system; Mission -critical systems Failure results in failure of some goal-directed activity;• Spacecraft navigation system; Business -critical systems Failure