Software engineering critical systems

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Critical Systems

Transcript of Software engineering critical systems

  • 1. Critical Systems Loganathan R.Prof. Loganathan R., CSE, HKBKCE 1
  • 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, reliability, safety and security. To explain that, to achieve dependability, you need to avoid mistakes, detect and remove errors and limit damage caused by failure. Prof. Loganathan R., CSE, HKBKCE 2
  • 3. Critical Systems If the system failure results in significant economic losses, physical damages or threats to human life than the system is called critical systems. 3 types of it are: 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 accounting system in a bank; Prof. Loganathan R., CSE, HKBKCE 3
  • 4. System dependability The most important emergent property of a critical system is its dependability. It covers the related system attributes of availability, reliability, safety & security. Importance of dependability Systems that are unreliable, unsafe or insecure are often rejected by their users(refuse to the product from the same company). System failure costs may be very high.(reactor / aircraft navigation) Untrustworthy systems may cause information loss with a high consequent recovery cost. Prof. Loganathan R., CSE, HKBKCE 4
  • 5. Development methods for critical systems Trusted methods and technique must be used. These methods are not cost-effective for other types of system. The older methods strengths & weaknesses are understood Formal methods reduce the amount of testing required. Example : Formal mathematical method Prof. Loganathan R., CSE, HKBKCE 5
  • 6. Socio-technical critical systems Failures 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 errors in its specification, design or implementation. Human Operator failure Fail to operate correctly. Now perhaps the largest single cause of system failures. Prof. Loganathan R., CSE, HKBKCE 6
  • 7. A Simple safety Critical System Example of software-controlled insulin pump. Used by diabetics to simulate the function of insulin, an essential hormone that metabolises blood glucose. Measures blood glucose (sugar) using a micro- sensor and computes the insulin dose required to metabolise the glucose. Prof. Loganathan R., CSE, HKBKCE 7
  • 8. Insulin pump organisation Insulin reservoir Needle Clock assembly Pump Sensor Controller Alarm Display1 Display2 Power supply Prof. Loganathan R., CSE, HKBKCE 8
  • 9. Insulin pump data-flow BloodBlood Blood sugar parameters Blood sugar sensor analysis Blood sugar level Insulin requirement computation Insulin Pump control Insulin commands Insulin requirement Insulin delivery pump controller Prof. Loganathan R., CSE, HKBKCE 9
  • 10. Dependability requirements The system shall be available to deliver insulin when required to do so. The system shall perform reliably and deliver the correct amount of insulin to counteract the current level of blood sugar. The essential safety requirement is that excessive doses of insulin should never be delivered as this is potentially life threatening. Prof. Loganathan R., CSE, HKBKCE 10
  • 11. System Dependability The dependability of a system equates to its trustworthiness. A dependable system is a system that is trusted by its users. Principal dimensions of dependability are: Availability :- Probability that it will be up & running & able to deliver at any given time ; Reliability :-Correct delivery of services as expected by user over a given period of time; Safety :-A Judgment of how likely the system will cause damage to people or its environment; Security :- A Judgment of how likely the system can resist accidental or deliberate intrusions; Prof. Loganathan R., CSE, HKBKCE 11
  • 12. Dimensions of dependability Dependability Availability Reliability Safety SecurityThe ability of the system The ability of the The ability of the system The ability of the systemto deliver services when system to deliver to operate without to protect itself against requested services as specified catastrophic failure accidental or deliberate intrusion Prof. Loganathan R., CSE, HKBKCE 12
  • 13. Other dependability properties Repairability Reflects the extent to which the system can be repaired in the event of a failure Maintainability Reflects the extent to which the system can be adapted to new requirements; Survivability Reflects the extent to which the system can deliver services while it is under hostile attack; Error tolerance Reflects the extent to which user input errors can be avoided and tolerated. Prof. Loganathan R., CSE, HKBKCE 13
  • 14. Dependability vs performance It is very difficult to tune systems to make them more dependable High level dependability can be achieved by expense of performance. Because it include extra/ redundant code to perform necessary checking It also increases the cost. Prof. Loganathan R., CSE, HKBKCE 14
  • 15. Dependability costs Dependability costs tend to increase exponentially as increasing levels of dependability are required There are two reasons for this The use of more expensive development techniques and hardware that are required to achieve the higher levels of dependability The increased testing and system validation that is required to convince the system client that the required levels of dependability have been achieved Prof. Loganathan R., CSE, HKBKCE 15
  • 16. Costs of increasing dependability Cost Low Medium High Very Ultra-high high Dependability Prof. Loganathan R., CSE, HKBKCE 16