1. Software Reliabilty1 Software Reliability Advanced Software Engineering COM360 University of Sunderland © 1998

2. Software Reliabilty2 Software Reliability Definition The probability that the software will; operate as required (i.e., without fail), for a specified time, in a specified environment.

3. Software Reliabilty3 Software Reliability - features failures in software are design faults, reliability during test changes continually (new problems are found as old ones are fixed / new code is never perfect) phenomenon of software reliability growth environment is important (platform/inputs) - new envt. may require s/w retest

4. Software Reliabilty4 Hardware Reliability - features failure is usually due to physical deterioration hardware reliability tends, more than software, towards a constant value, hardware reliability usually follows the ‘bathtub’ principle, again, environment is important; a proportion of hardware faults are design faults

5. Software Reliabilty5 Reliability - General (1) Of all software quality characteristics, reliability is consistently identified as the most important. Reliability, for both hardware and software, is a probability - SYSTEM reliability can be derived from combining the hardware and software reliabilities. Reliability is directly related to the number of failures. [ p(R)=1- p(F) ]

6. Software Reliabilty6 Reliability - General (2) Reliability measures are much more useful than fault measures - from the user perspective. Measures of faults/defects are developer oriented. Change of environment (operating platform, operational data profile) changes reliability.

7. Software Reliabilty7 Fault, Error, Failure (defined by BS5760 part8: 1998) Fault a defect in a software component which, when triggered, causes one or many errors Error a discrepancy between its actual state and the ‘correct state’ which, if it propagates, means that the component ceases to perform as required Failure the component ceases to perform as required

8. Software Reliabilty8 Fault, Error, Failure FAULTS ERRORS FAILURE ENVIRONMENT OPERATOR INPUT OR REVEALING MECHANISM AND LEADS TO ZERO OR MANY POTENTIALLY LEADS TO ZERO OR MANY MISTAKES (PERSON MAKES) CAN BE ATTRIBUTED TO ONE OR MANY (This slide from BS5760 Part 8: 1998)

9. Software Reliabilty9 Reliability Measures When we talk of reliability measures the irony is that we invariably talk about failure measures. There are four general ways of measuring failures against time;  time of failure,  interval between failures,  cumulative failures experienced up to a given time,  failures experienced in a time interval.

10. Software Reliabilty10 Availability & Maintainability Availability –is the probability that the software is operating at a given time. It is equal to the mean time to failure divided by the mean time between failures. Maintainability –is the mean time to repair the software and restore to working order.

11. Software Reliabilty11 Uses of S/w Reliability Measurement System Reliability evaluation Monitoring operational performance –e.g., decisions on when to go for enhancement Evaluation of test status / test schedule Management of new features and design changes

12. Software Reliabilty12 Reliability Model Types Phenomenological –model the characteristics that influence reliability and work on those (have to prove the level of influence - empirical data) Parametric (Mathematical) –basic assumptions (faults cause failures) –empirical data –examples: Jelinksi-Moranda, Littlewood, etc.