reliability quinn edition 5 chapter 8.ppt
TRANSCRIPT
8/18/2019 reliability QUINN Edition 5 Chapter 8.ppt
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Ethics for the Information AgeFifth Edition
by
Michael J. Quinn
Chapter 8:
Computer eliability
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Chapter !"er"ie#
• Introduction
• Data-entry or data-retrieval errors
• Software and billing errors
• Notable software system failures• Therac-25
• Comuter simulations
•
Software engineering• Software warranties
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8.$ Introduction
• Comuter systems are sometimes unreliable " #rroneous information in databases
" $isinterretation of database information
" $alfunction of embedded systems• #ffects of comuter errors
" Inconvenience
" %ad business decisions
" &atalities
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8.% &ata'Entry or &ata'etrie"al
Errors
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(#o )inds of &ata'related Failure
• ( comuteri)ed system may fail becausewrong data entered into it
•
( comuteri)ed system may fail becauseeole incorrectly interret data theyretrieve
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&isfranchised *oters
• November 2+++ general election
• &lorida dis,ualified thousands of voters
•
eason. /eole identified as felons• Cause. Incorrect records in voter database
• Conse,uence. $ay have affected
election0s outcome
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False Arrests
• Sheila ac3son Stossier mista3en for Shirleyac3son " (rrested and sent five days in detention
•
oberto 4ernande) mista3en for anotheroberto 4ernande) " (rrested twice and sent 12 days in ail
• Terry Dean ogan arrested after someone stole
his identity " (rrested five times6 three times at gun oint
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Accuracy of +CIC ecords
• $arch 2++!. ustice Det8 announces &%Inot resonsible for accuracy of NCICinformation
• #9emts NCIC from some rovisions of/rivacy (ct of 1:'
• Should government ta3e resonsibility for
data correctness;
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&ept. of Justice ,osition
• Imractical for &%I to be resonsible fordata0s accuracy
• $uch information rovided by other law
enforcement and intelligence agencies• (gents should be able to use discretion
• If rovisions of /rivacy (ct strictly followed6
much less information would be in NCIC• esult. fewer arrests
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,osition of ,ri"acy Ad"ocates
• Number of records is increasing
• $ore erroneous records → more falsearrests
• (ccuracy of NCIC records more imortantthan ever
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Analysis: &atabase of -tolen *ehicles
• < 1 million cars stolen every year " =wners suffer emotional6 financial harm
" aises insurance rates for all
• Transorting stolen car across a state line " %efore NCIC6 greatly reduced chance of recovery
" (fter NCIC6 nationwide stolen car retrieval
• (t least 5+6+++ recoveries annually due to NCIC•
&ew stories of faulty information causing false arrests• %enefit < harm → Creating database the right action
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8. -oft#are and /illing Errors
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Errors 0hen &ata Are Correct
• (ssume data correctly fed intocomuteri)ed system
• System may still fail if there is an error inits rogramming
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Errors 1eading to -ystem Malfunctions
• >west sent incorrect bills to cell hone customers
• &aulty ?SD( beef rice reorts
• ?8S8 /ostal Service returned mail addressed to /atentand Trademar3 =ffice
• Selling and grammar error chec3ers increased errors
• New @or3 City 4ousing authority overcharged renters
• (bout '5+ California rison inmates mista3enly released
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Errors 1eading to -ystem Failures
• (mbulance disatch system in Aondon
• Chicago %oard of Trade
• %$B limousine
•
aan0s air traffic control system• Aos (ngeles County ?SC $edical Center laboratory
comuter system
• Comair0s Christmas Day shutdown
• %oeing
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Comair Cancelled All Flights on
Christmas &ay2 %334
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(/ /hoto(l %ehrman6 &ile
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Analysis: E'etailer ,osts 0rong
,rice2 efuses to &eli"er
• (ma)on8com in %ritain offered i/a, for Einstead of E25
• =rders flooded in• (ma)on8com shut down site6 refused to
deliver unless customers aid true rice
• Bas (ma)on8com wrong to refuse to fillthe orders;
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ule 5tilitarian Analysis
• Imagine rule. ( comany must always honorthe advertised rice
• Conse,uences " $ore time sent roofreading advertisements
" Comanies would ta3e out insurance olicies
" 4igher costs→ higher rices
" (ll consumers would ay higher rices
" &ew customers would benefit from errors
• Conclusion " ule has more harms than benefits
" (ma)on8com did the right thing
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)antian Analysis
• %uyers 3new :85F mar3down was anerror
• They attemted to ta3e advantage of (ma)on8com0s stoc3holders
• They were not acting in Ggood faithH
•
%uyers did something wrong
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8.4 +otable -oft#are -ystem
Failures
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,atriot Missile
• Designed as anti-aircraft missile
• ?sed in 1::1 ulf Bar to intercet Scud missiles
• =ne battery failed to shoot at Scud that 3illed 27
soldiers• Designed to oerate only a few hours at a time
• Jet in oeration < 1++ hours
• Tiny truncation errors added u
• Cloc3 error of +8!'!! seconds→ trac3ing error of*7 meters
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,atriot Missile Failure
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&igure from SCI#NC# 255.1!'8 Coyright K1::2 by The (merican (ssociation for the (dvancement of Science8 erinted with ermission8
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Ariane 6
• Satellite launch vehicle
• '+ seconds into maiden flight6 roc3et self-destructed " L5++ million of uninsured satellites lost
• Statement assigning floating-oint value to integer raised
e9cetion• #9cetion not caught and comuter crashed
• Code reused from (riane ' " Slower roc3et
" Smaller values being maniulated " #9cetion was imossible
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A(7( 1ong'&istance +et#or
• Significant service disrution " (bout half of telehone-routing switches crashed
" + million calls not ut through
" *+6+++ eole lost all service " (TMT lost revenue and credibility
• Cause " Single line of code in error-recovery rocedure
" $ost switches running same software " Crashes roagated through switching networ3
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A(7( 1ong &istance +et#or Failure
1-25
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obot Missions to Mars
• $ars Climate =rbiter " Disintegrated in $artian atmoshere
" Aoc3heed $artin design used #nglish units
" et /roulsion Aab design used metric units
• $ars /olar Aander " Crashed into $artian surface
" #ngines shut off too soon " &alse signal from landing gear
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&en"er International Airport
• %(# built automated baggage handling system
• /roblems " (irort designed before automated system chosen
" Timeline too short " System comle9ity e9ceeded develoment team0s ability
• esults " (dded conventional baggage system
" 1*-month delay in oening airort " Cost Denver L1 million a day
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(oyo -toc E9change
• &irst day of trading for -Com
• $i)uho Securities emloyee mista3enly enteredorder to sell *1+6++ shares at 1 yen6 instead of 1
share at *1+6+++ yen• #mloyee overrides comuter warning
• (fter sell order osted on e9change0s dislay board6$i)uho tried to cancel order several times software
bug caused attemts to fail• $i)uho lost L225 million buying bac3 shares
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&irect ecording Electronic *oting
Machines
• (fter roblems with 2+++ election6 Congress assed4el (merica Oote (ct of 2++2
• 4(O( rovided money to states to relace unchcard voting systems
• $any states used 4(O( funds to urchase directrecording electronic PD#Q voting machines
• %ra)il and India have run national elections usingD# voting machines e9clusively
• In November 2++* 1! of ?8S8 voters used D#voting machines
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&iebold Electronic *oting Machine
1-!+
K (/ /hotoogelio Solis
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Issues #ith &E *oting Machines
• Ooting irregularities " &ailure to record votes
" =vercounting votes
" $isrecording votes• Aac3 of a aer audit trail
• Oulnerability to tamering
• Source code a trade secret6 can0t be e9amined
• /ossibility of widesread fraud through maliciousrogramming
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8.6 (herac'%6
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enesis of the (herac'%6
• (#CA and C built Therac-* and Therac-2+
• Therac-25 built by (#CA " /D/-11 an integral art of system
" 4ardware safety features relaced with software " eused code from Therac-* and Therac-2+
• &irst Therac-25 shied in 1:7! "
/atient in one room " Technician in adoining room
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Chronology of Accidents and
AEC1 esponses
• $arietta6 eorgia Pune 1:75Q
• 4amilton6 =ntario Puly 1:75Q
• &irst (#CA investigation Puly-Set8 1:75Q
•
@a3ima6 Bashington PDecember 1:75Q• Tyler6 Te9as P$arch 1:7*Q
• Second (#CA investigation P$arch 1:7*Q
• Tyler6 Te9as P(ril 1:7*Q
• @a3ima6 Bashington Panuary 1:7Q• &D( declares Therac-25 defective P&ebruary 1:7Q
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-oft#are Errors
• ace condition. order in which two or moreconcurrent tas3s access a shared variablecan affect rogram0s behavior
• Two race conditions in Therac-25 software
" Command screen editing
" $ovement of electron beam gun
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ace Condition e"ealed by Fast'
typing !perators
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ace Condition Caused by Counter
olling !"er to ;ero
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,ost Mortem
• (#CA focused on fi9ing individual bugs• System not designed to be fail-safe
• No devices to reort overdoses
•
Software lessons " Difficult to debug rograms with concurrent tas3s
" Design must be as simle as ossible
" Documentation crucial
" Code reuse does not always lead to higher ,uality• (#CA did not communicate fully with
customers
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Moral esponsibility of the
(herac'%6 (eam
• Conditions for moral resonsibility " Causal condition. actions Por inactionsQ caused the
harm
" $ental conditionR (ctions Por inactionsQ intended or willed -=-
R $oral agent is careless6 rec3less6 or negligent
• Therac-25 team morally resonsible
" They constructed the device that caused the harm " They were negligent
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,ostcript
• Comuter errors related to radiationmachines continue to maim and 3illatients
• Investigation by The New York Times
" Scott erome-/ar3s6 New @or3 P2++*Q
" (le9andra n-Charles6 New @or3 P2++*Q
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8.< Computer -imulations
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5ses of -imulations
• Simulations relace hysical e9eriments
" #9eriment too e9ensive or time-consuming
" #9eriment unethical
" #9eriment imossible
• $odel ast events
• ?nderstand world around us
• /redict the future
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-imulations ,redict ,ath and -peed
of =urricanes
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Courtesy of N(S(
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*alidating -imulations
• Oerification. Does rogram correctly imlementmodel;
• Oalidation. Does the model accurately reresent
the real system;• Oalidation methods
" $a3e rediction6 wait to see if it comes true
" /redict the resent from old data
" Test credibility with e9erts and decision ma3ers
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*alidation by Comparing ,redicted
and Actual !utcomes
1-'5
Courtesy of Daimler (
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*alidation by >,redicting the ,resent?
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8.@ -oft#are Engineering
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-pecification
• Determine system re,uirements
• ?nderstand constraints
•
Determine feasibility• #nd roducts
" 4igh-level statement of re,uirements
" $oc3-u of user interface " Aow-level re,uirements statement
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&e"elopment
• Create high-level design
• Discover and resolve mista3es6 omissionsin secification
• C(S# tools to suort design rocess• =bect-oriented systems have advantages
• (fter detailed design6 actual rograms
written• esult. wor3ing software system
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*alidation (estingB
• #nsure software satisfies secification
• #nsure software meets user0s needs
• Challenges to testing software
" Noncontinuous resonses to changes in inut " #9haustive testing imossible
" Testing reveals bugs6 but cannot rove none e9ist
•
Test modules6 then subsystems6 then system
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-oft#are Quality Is Impro"ing
• Standish rou trac3s IT roects• Situation in 1::'
" 1! roects cancelled before comletion
" 12 roects had time andor cost overruns " 1* roects comleted on time on budget
• Situation in 2++*
" 1* roects cancelled " 12 roects had time andor cost overruns
" 1! roects comleted on time on budget
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-uccess of I( ,roects !"er (ime
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8.8 -oft#are 0arranties
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-hrin#rap 0arranties
• Some say you accet software Gas isH
• Some offer :+-day relacement or money-bac3 guarantee
• None accet liability for harm caused byuse of software
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Are -oft#are 0arranties EnforceableD
• (rticle 2 of ?niform Commercial Code
• $agnuson-$oss Barranty (ct
•
Step-Saver Data Systems v. WyseTechnology and The Software Link
• ProD! "nc. v. #eiden$erg
•
%ortensen v. Tim$erline Software
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Moral esponsibility of -oft#are
Manufacturers
• If vendors were resonsible for harmful conse,uencesof defects " Comanies would test software more
" They would urchase liability insurance
" Software would cost more " Start-us would be affected more than big comanies
" Aess innovation in software industry
" Software would be more reliable
•
$a3ing vendors resonsible for harmful conse,uencesof defects may be wrong6 but
• Consumers should not have to ay for bug fi9es