分布式操作系统第二版答案.docx

分布式操作系统第二版答案.docx

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分布式操作系统第二版答案

SYSTEMS

PRINCIPLESANDPARADIGMS

SECONDEDITION

PROBLEMSOLUTIONS

ANDREWS.TANENBAUM

MAARTENVANSTEEN

VrijeUniversiteit

Amsterdam,TheNetherlands

PRENTICEHALL

UPPERSADDLERIVER,NJ07458

SOLUTIONSTOCHAPTER1PROBLEMS

1.Q:

Analternativede.nitionforadistributedsystemisthatofacollectionof

independentcomputersprovidingtheviewofbeingasinglesystem,thatis,it

iscompletelyhiddenfromusersthatthereevenmultiplecomputers.Givean

examplewherethisviewwouldcomeinveryhandy.

A:

Whatimmediatelycomestomindisparallelcomputing.Ifonecould

designprogramsthatrunwithoutanyseriousmodi.cationsondistributedsystems

thatappeartobethesameasnondistributedsystems,lifewouldbeso

mucheasier.Achievingasingle-systemviewisbynowconsideredvirtually

impossiblewhenperformanceisinplay.

2.Q:

Whatistheroleofmiddlewareinadistributedsystem?

A:

Toenhancethedistributiontransparencythatismissinginnetworkoperating

systems.Inotherwords,middlewareaimsatimprovingthesingle-system

viewthatadistributedsystemshouldhave.

3.Q:

Manynetworkedsystemsareorganizedintermsofabackof.ceanda

frontof.ce.Howdoesorganizationsmatchwiththecoherentviewwedemand

foradistributedsystem?

A:

Amistakeeasilymadeistoassumethatadistributedsystemasoperating

inanorganization,shouldbespreadacrosstheentireorganization.Inpractice,

weseedistributedsystemsbeinginstalledalongthewaythatanorganization

issplitup.Inthissense,wecouldhaveadistributedsystemsupportingbackof

.ceproceduresandprocesses,aswellasaseparatefront-of.cesystem.Of

course,thetwomaybecoupled,butthereisnoreasonforlettingthiscoupling

befullytransparent.

4.Q:

Explainwhatismeantby（distribution）transparency,andgiveexamplesof

differenttypesoftransparency.

A:

Distributiontransparencyisthephenomenonbywhichdistributionaspects

inasystemarehiddenfromusersandapplications.Examplesincludeaccess

transparency,locationtransparency,migrationtransparency,relocationtransparency,

replicationtransparency,concurrencytransparency,failuretransparency,

andpersistencetransparency.

5.Q:

Whyisitsometimessohardtohidetheoccurrenceandrecoveryfromfailures

inadistributedsystem?

A:

Itisgenerallyimpossibletodetectwhetheraserverisactuallydown,or

thatitissimplyslowinresponding.Consequently,asystemmayhaveto

reportthataserviceisnotavailable,although,infact,theserverisjustslow.

2PROBLEMSOLUTIONSFORCHAPTER1

6.Q:

Whyisitnotalwaysagoodideatoaimatimplementingthehighestdegree

oftransparencypossible?

A:

Aimingatthehighestdegreeoftransparencymayleadtoaconsiderable

lossofperformancethatusersarenotwillingtoaccept.

7.Q:

Whatisanopendistributedsystemandwhatbene.tsdoesopennessprovide?

A:

Anopendistributedsystemoffersservicesaccordingtoclearlyde.ned

rules.Anopensystemiscapableofeasilyinteroperatingwithotheropensystems

butalsoallowsapplicationstobeeasilyportedbetweendifferentimplementations

ofthesamesystem.

8.Q:

Describepreciselywhatismeantbyascalablesystem.

A:

Asystemisscalablewithrespecttoeitheritsnumberofcomponents,geographical

size,ornumberandsizeofadministrativedomains,ifitcangrowin

oneormoreofthesedimensionswithoutanunacceptablelossofperformance.

9.Q:

Scalabilitycanbeachievedbyapplyingdifferenttechniques.Whatare

thesetechniques?

A:

Scalingcanbeachievedthroughdistribution,replication,andcaching.

10.Q:

Explainwhatismeantbyavirtualorganizationandgiveahintonhow

suchorganizationscouldbeimplemented.

A:

Avirtualorganization（VO）de.nesagroupofusers/applicationsthathave

accesstoaspeci.edgroupofresources,whichmaybedistributedacrossmany

differentcomputers,ownedbymanydifferentorganizations.Ineffect,aVO

de.neswhohasaccesstowhat.Thisalsosuggeststhattheresourcesshould

keepanaccountofforeignusersalongwiththeiraccessrights.Thiscanoften

bedoneusingstandardaccesscontrolmechanisms（liketherwxbitsinUNIX）,

althoughforeignusersmayneedtohaveaspecialaccount.Thelattercomplicates

mattersconsiderably.

11.Q:

Whenatransactionisaborted,wehavesaidthattheworldisrestoredtoits

previousstate,asthoughthetransactionhadneverhappened.Welied.Givean

examplewhereresettingtheworldisimpossible.

A:

AnysituationinwhichphysicalI/Ohasoccurredcannotbereset.For

example,iftheprocesshasprintedsomeoutput,theinkcannotberemoved

fromthepaper.Also,inasystemthatcontrolsanykindofindustrialprocess,it

isusuallyimpossibletoundoworkthathasbeendone.

12.Q:

Executingnestedtransactionsrequiressomeformofcoordination.Explain

whatacoordinatorshouldactuallydo.

A:

Acoordinatorneedsimplyensurethatifoneofthenestedtransactions

aborts,thatallothersubtransactionsabortaswell.Likewise,itshould

PROBLEMSOLUTIONSFORCHAPTER13

coordinatethatallofthemcommitwheneachofthemcan.Tothisend,a

nestedtransactionshouldwaittocommituntilitistoldtodosobythecoordinator.

13.Q:

Wearguedthatdistributiontransparancymaynotbeinplaceforpervasice

systems.Thisstatementisnottrueforalltypesoftransparencies.Givean

example.

A:

Thinkofmigrationtransparency.Inmnaypervasivesystems,components

aremobileandwillneedtore-establishconnectionswhenmovingfromone

accesspointtoanother.Preferably,suchhandoversshouldbecompletely

transparenttotheuser.Likewise,itcanbearguedthatmanyothertypesof

transparenciesshouldbesupportedaswell.However,whatshouldnotbehidden

isauserispossiblyaccessingresourcesthataredirectlycoupledtothe

user'scurrentenvironment.

14.Q:

Wealreadygavesomeexamplesofdistributedpervasivesystems:

home

systems,electronichealth-caresystems,andsensornetworks.Extendthislist

withmoreexamples.

A:

Therearequiteafewotherexamplesofpervasivesystems.Thinkoflargescale

wirelessmeshnetworksincitiesorneighborhoodsthatprovideservices

suchasInternetaccess,butalsoformthebasisforotherserviceslikeanews

system.Therearesystemsforhabitatmonitoring（asinwildliferesorts）,electronic

jailsbywhichoffendersarecontinuouslymonitored,large-scaleintegrated

sportssystems,of.cesystemsdeployingactivebadgestoknowabout

thewhereaboutsoftheiremployees,andsoon.

15.Q:

Sketchadesignforahomesystemconsistingofaseparatemediaserver

thatwillallowfortheattachmentofawirelessclient.Thelatterisconnected

to（analog）audio/videoequipmentandtransformsthedigitalmediastreamsto

analogoutput.Theserverrunsonaseparatemachine,possiblyconnectedto

theInternet,buthasnokeyboardand/ormonitorconnected.

SOLUTIONSTOCHAPTER2PROBLEMS

1.Q:

Ifaclientandaserverareplacedfarapart,wemayseenetworklatency

dominatingoverallperformance.Howcanwetacklethisproblem?

A:

Itreallydependsonhowtheclientisorganized.Itmaybepossibleto

dividetheclient-sidecodeintosmallerpartsthatcanrunseparately.Inthat

case,whenonepartiswaitingfortheservertorespond,wecanschedule

anotherpart.Alternatively,wemaybeabletorearrangetheclientsothatitcan

dootherworkafterhavingsentarequesttotheserver.Thislastsolutioneffectively

replacesthesynchronousclient-servercommunicationwithasynchronous

one-waycommunication.

4PROBLEMSOLUTIONSFORCHAPTER2

2.Q:

Whatisathree-tieredclient-serverarchitecture?

A:

Athree-tieredclient-serverarchitectureconsistsofthreelogicallayers,

whereeachlayeris,inprinciple,implementedataseparatemachine.The

highestlayerconsistsofaclientuserinterface,themiddlelayercontainsthe

actualapplication,andthelowestlayerimplementsthedatathatarebeing

used.

3.Q:

Whatisthedifferencebetweenaverticaldistributionandahorizontaldistribution?

A:

Verticaldistributionreferstothedistributionofthedifferentlayersina

multitieredarchitecturesacrossmultiplemachines.Inprinciple,eachlayeris

implementedonadifferentmachine.Horizontaldistributiondealswiththe

distributionofasinglelayeracrossmultiplemachines,suchasdistributinga

singledatabase.

4.Q:

ConsiderachainofprocessesP1,P2,...,Pnimplementingamultitiered

client-serverarchitecture.ProcessPiisclientofprocessPi+1,andPiwill

returnareplytoPi-1onlyafterreceivingareplyfromPi+1.Whatarethemain

problemswiththisorganizationwhentakingalookattherequest-replyperformance

atprocessP1?

A:

Performancecanbeexpectedtobebadforlargen.Theproblemisthat

eachcommunicationbetweentwosuccessivelayersis,inprinciple,between

twodifferentmachines.Consequently,theperformancebetweenP1andP2

mayalsobedeterminedbyn-2request-replyinteractionsbetweentheother

layers.Anotherproblemisthatifonemachineinthechainperformsbadlyor

iseventemporarilyunreachable,thenthiswillimmediatelydegradetheperformance

atthehighestlevel.

5.Q:

Inastructuredoverlaynetwork,messagesareroutedaccordingtothe

topologyoftheoverlay.Whatisanimportantdisadvantageofthisapproach?

A:

Theproblemisthatwearedealingonlywithlogicalpaths.Itmayvery

wellbethecasethattwonodesAandBwhichareneighborsintheoverlay

networkarephysicallyplacedfarapart.Asaconsequence,thelogicallyshort

pathbetweenAandBmayrequireroutingamessagealongaverylongpath

intheunderlyingphysicalnetwork.

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