software faults -& reliability

8/18/2019 Software Faults -& Reliability

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Software faults &Software faults &

reliabilityreliabilityPresented by:Presented by:

Pooja JainPooja Jain


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What is Software ReliabilityWhat is Software Reliability

T T he probability of failure-freehe probability of failure-freesoftware operation for a speci edsoftware operation for a speci ed

period of time in a speci edperiod of time in a speci eden ironmenten ironment!t denotes a product"s!t denotes a product"s

trustworthiness or dependability#trustworthiness or dependability#


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Software ReliabilitySoftware Reliability

Software reliability not caused due toSoftware reliability not caused due toa$in$ but due to bu$sa$in$ but due to bu$s

The more the bu$s% the lesser the The more the bu$s% the lesser thereliability of the softwarereliability of the softwareStill failures seem random% henceStill failures seem random% hencereliability theory can be appliedreliability theory can be applied


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Software faultsSoftware faults

Software is said to contain fault if forSoftware is said to contain fault if forsome set of input data the output issome set of input data the output isnot correct#not correct#


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Software ReliabilitySoftware Reliability

Software systems often are one-o Software systems often are one-o 'easurin$ reliability in lab not practical'easurin$ reliability in lab not practicalas too much failure data is needed(as too much failure data is needed(

re)uires timere)uires time*ailures often result in fault remo al%*ailures often result in fault remo al%leadin$ to reliability impro ementleadin$ to reliability impro ement

Predictin$ future reliability fromPredictin$ future reliability frommeasured reliability is hardermeasured reliability is harder+ence di erent models needed+ence di erent models needed


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Software Reliability 'odelsSoftware Reliability 'odels

Jelins,i and 'oranda 'odel Jelins,i and 'oranda 'odelReali es each time an error is repairedReali es each time an error is repaired

reliability does not increase by areliability does not increase by aconstant amount#constant amount#Reliability impro ement due to .in$Reliability impro ement due to .in$of an error is assumed to beof an error is assumed to beproportional to the number of errorsproportional to the number of errorspresent in the system at that time#present in the system at that time#


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/loc, co era$e model/loc, co era$e model0oel 1 2,umoto 30-24 !mperfect0oel 1 2,umoto 30-24 !mperfect

debu$$in$ modeldebu$$in$ model025+PP025+PP

'usa 1 2,umoto 3'-24 6o$arithmic'usa 1 2,umoto 3'-24 6o$arithmic

Poisson 7.ecution Time modelPoisson 7.ecution Time model


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Software Reliability 0rowthSoftware Reliability 0rowth'odels'odels

8ssume that reliability is a function8ssume that reliability is a functionof the defect le el and as defects areof the defect le el and as defects areremo ed% reliability impro esremo ed% reliability impro es

'odel parameters determined from'odel parameters determined frompast data on failures and .espast data on failures and .es


9/19

Software *ailure 'echanismsSoftware *ailure 'echanisms

Failure causeFailure cause : Software defects are mainly: Software defects are mainlydesi$n defects#desi$n defects#Wear-outWear-out : Software does not ha e ener$y related: Software does not ha e ener$y relatedwear-out phase# 7rrors can occur without warnin$#wear-out phase# 7rrors can occur without warnin$#Repairable system conceptRepairable system concept : Periodic restarts: Periodic restartscan help . software problems#can help . software problems#Time dependency and life cycleTime dependency and life cycle : Software: Softwarereliability is not a function of operational time#reliability is not a function of operational time#Environmental factorsEnvironmental factors : 9o not a ect Software: 9o not a ect Softwarereliability% e.cept it mi$ht a ect pro$ram inputs#reliability% e.cept it mi$ht a ect pro$ram inputs#Reliability predictionReliability prediction : Software reliability can: Software reliability cannot be predicted from any physical basis% since itnot be predicted from any physical basis% since itdepends completely on human factors in desi$n#depends completely on human factors in desi$n#


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8fter ttin$ a model describin$ the8fter ttin$ a model describin$ thefailure process we can estimate itsfailure process we can estimate itsparameters% and the )uantities suchparameters% and the )uantities suchas the total number of faults in theas the total number of faults in thecode% future failure intensity andcode% future failure intensity andadditional time re)uired to achie e aadditional time re)uired to achie e a

failure intensity objecti e#failure intensity objecti e#


11/19

Software fault toleranceSoftware fault tolerancetechni)ues:techni)ues:

are desi$ned to allow a system toare desi$ned to allow a system totolerate software faults that remain intolerate software faults that remain inthe system after its de elopmentthe system after its de elopment

pro ide mechanisms to the softwarepro ide mechanisms to the software

system to pre ent system failure fromsystem to pre ent system failure fromoccurrin$occurrin$


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'ultiple data representation en iroment:

9ata di erse techni)ues are used in a multiple datarepresentation en ironment

utili e di erent representations of input data to pro idetolerance to software desi$n faults

'ultiple ersion software en iroment:

9esi$n di erse techni)ues are used in a multiple ersionsoftware en ironmentuse the functionally of independently de eloped software

ersions to pro ide tolerance to software desi$n faults


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9esi$n di ersity9esi$n di ersity

Popular techni)ues which are basedPopular techni)ues which are basedon the desi$n di ersity concept foron the desi$n di ersity concept forfault tolerance in software are:fault tolerance in software are:

• Reco ery /loc,Reco ery /loc,

•

5- ersion Pro$rammin$5- ersion Pro$rammin$

• 5-Self-;hec,in$ Pro$rammin$5-Self-;hec,in$ Pro$rammin$


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9ata 9i ersity Techni)ues9ata 9i ersity Techni)ues

While the desi$n di ersity approaches to pro ideWhile the desi$n di ersity approaches to pro idefault tolerance rely on multiple ersions of thefault tolerance rely on multiple ersions of thesoftware written to the same speci cations% the datasoftware written to the same speci cations% the datadi ersity approach uses only one ersion of thedi ersity approach uses only one ersion of the

software#software#

This approach relies on the obser ation that a This approach relies on the obser ation that asoftware sometime fails for certain alues in thesoftware sometime fails for certain alues in theinput space andinput space and

this failure could be a oided if there is a minorthis failure could be a oided if there is a minorperturbation of input data which is acceptable to theperturbation of input data which is acceptable to thesoftware#software#


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Software *ailure 'echanismsSoftware *ailure 'echanisms

RedundancyRedundancy : ;an not impro e Software: ;an not impro e Softwarereliability if identical software components arereliability if identical software components areused#used#

InterfacesInterfaces : Software interfaces are purely: Software interfaces are purelyconceptual otherconceptual other & not& not isual#isual#

Failure rate motivatorsFailure rate motivators :


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Testin$ Testin$

Testin$ remains main eri cation Testin$ remains main eri cationacti ity 1 most reliance on itacti ity 1 most reliance on it;onsumes as much as half of the;onsumes as much as half of thetotal e ort in a sw producttotal e ort in a sw product

Testin$: test case desi$n% e.ecution% Testin$: test case desi$n% e.ecution%chec,in$ the results% thenchec,in$ the results% thendebu$$in$% .in$% retestin$debu$$in$% .in$% retestin$7ach step is e.pensi e7ach step is e.pensi e


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;onclusions;onclusions

Software reliability is a ,ey part inSoftware reliability is a ,ey part insoftware )ualitysoftware )ualitySoftware reliability impro ement is hardSoftware reliability impro ement is hard

There are no $eneric models# There are no $eneric models#Statistical testin$ should be used but itStatistical testin$ should be used but itis not easy a$ainis not easy a$ain to implement themto implement them


19/19

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software faults -& reliability

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