1. Software Reliability “The most important dynamic characteristic of most software systems..” Sommerville (5th ed.) p365

2. Sommerville on Reliability Sommerville deals with reliability in several places –16.2 pp 359 -364 –17.1 pp 373 - 364 –21.2 pp 470 - 476 this lecture attempts to address all these aspects of reliability

3. Definition The probability of failure-free operation within a specific time when used in a specific environment for a specific purpose But, –use of system –perception of reliability

4. Faults and Failures faults in every system faults lead to failures only if those parts of the system are used failures are perceived as such reliability is about failures not faults related to use difficult to apply to unacceptable failures

5. Sources of Faults specification design implementation maintenance

6. Estimation of Reliability depends on –what is expected of the system does it operate continuously? –transaction systems, operating systems is it invoked when required –desktop systems –application domain potentially dangerous? potential high financial cost?

7. Reliability Specification failure class description appropriate measure for the system value for that measure

8. Statistical Testing define operation profile generate appropriate test cases apply tests –record time (in correct units) between failures –for AVAIL record down time calculate measure - ensure it is significant result - reliability at a point in time

9. Reliability Modelling purpose –give an estimate of when (or if) the system will meet its reliability specification method decide on a model measure time to each failure fit to model, predict time to meet specification cost

10. Reliability Models linear improvement random improvement (possibly -ve.) curve fitting

11. Summary reliability depends on use and perception use quantitative reliability specifications measure depends on the system use of statistical testing use of reliability models