Unit8-SoftwareR.pptVIP

Software Reliability Categorizing and specifying the reliability of software systems Objectives To discuss the problems of reliability specification and measurement To introduce reliability metrics and to discuss their use in reliability specification To describe the statistical testing process To show how reliability predications may be made from statistical test results Topics covered What is software reliability? Definition Specified environment Failures and faults Reliability perception Reliability and efficiency How to measure software reliability? Metrics Measurement How to specify reliability? How to test reliability? Statistical testing Operational profiles How to predict reliability Reliability growth modeling Reliability prediction 1. What is software reliability What is reliability? -definition Probability of failure-free operation for a specified time in a specified environment for a given purpose e.g. Software installed on an airplane will be 99.99% (1/10000) reliable during an average flight of every hours Informally, reliability is a measure of how well system users think it provides the services they require What is reliability?  -Specified environment Fixed environment: software systems, e.g. Operating systems, application software, compilers hardware environment, e.g. CPU, memory, cache, monitor, input and output varied environment: described using OPERATIONAL PROFILE An operational profile What is software reliability -some notes Cannot be defined objectively Reliability measurements which are quoted out of context are not meaningful Requires operational profile for its definition The operational profile defines the expected pattern of software usage Must consider fault consequences Not all faults are equally serious. System is perceived as more unreliable if there are more serious faults What is reliability?  -Failures and faults A failure corresponds to unexpected run-time behavior observed by a user of the software