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Web++ A system for fast and reliable web service

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THE ADVANCED COMPUTING SYSTEMS ASSOCIATION

The following paper was originally published in theProceedings of the USENIX Annual Technical ConferenceMonterey, California, USA, June 6-11, 1999

Web++: A System For Fastand Reliable Web Service

Radek Vingralek, Yuri BreitbartBell Laboratories - Lucent TechnologiesMehmet Sayal, Peter ScheuermannNorthwestern University

© 1999 by The USENIX AssociationAll Rights ReservedRights to individual papers remain with the author or the author's employer. Permission is granted for noncommercialreproduction of the work for educational or research purposes. This copyright notice must be included in the reproduced paper.USENIX acknowledges all trademarks herein.For more information about the USENIX Association:Phone: 1 510 528 8649FAX: 1 510 548 5738Email: office@usenix.orgWWW: http://www.usenix.orgWeb++:ASystemForFastandReliableWebService

RadekVingralekYuriBreitbart

InformationScienceResearchCenterBellLaboratories-LucentTechnologies600MountainAvenueMurrayHill,NJ07974rvingral,yuri@research.bell-labs.comCurrentaffiliation:STARLab,InterTrustTechnologies,460Oak-meadParkway,Sunnyvale,CA94086,rvingral@intertrust.com.MehmetSayalPeterScheuermann

NorthwesternUniversityECEDepartment2145SheridanAvenueEvanston,IL60208mehmet,peters@ece.nwu.edu

AbstractWedescribethedesignofasystemforafastandreliableHTTPservicetermedWeb.Webachieveshighre-liabilitybydynamicallyreplicatingWebdataamongmul-tipleWebservers.Webselectsaserverwhichisavail-ableandthatisexpectedtoprovidethefastestresponsetime.Furthermore,Webguaranteesdatadelivery,pro-videdthatatleastoneservercontainingtherequesteddataisavailable.Afterdetectingaserverfailure,Webclientrequestsaresatisfiedtransparentlytotheclientbyanotherserver.WebisbuiltontopofthestandardHTTPprotocolanddoesnotrequireanychangeseitherinexistingWebbrowsers,ortheinstallationofanysoftwareontheclientside.WeimplementaWebprototype;performanceexperimentsindicatethatWebimprovestheclientresponsetimeonaverageby36.6%,andinmanycasesbyasmuchas59%,whencomparedwiththecurrentWebperformance.

1Introduction1.1MotivationThesuccessoftheWebhasproventhevalueofsharingdifferenttypesofdatainanautonomousmanner.ThenumberofWebusers,servers,andtotalInternettraf-fichavebeengrowingexponentiallyinthepast5years[1].ThescaleofWebusageisstressingthecapacityoftheInternetinfrastructureandleadstopoorperfor-manceandlowreliabilityofWebservice.Multisecondresponsetimesfordownloadinga1KBresourcearenotunusual[35].Furthermore,recentstudies[29]indicatethatservermeantimetofailure(MTTF)is15days,thusaclientaccessing10serversmayexperienceafailureevery36.4hours.SuchafailurerateisnotacceptableformanyimportantWebapplicationssuchaselectroniccommerceandonlinestocktrading.Recently,severaltechniqueshavebeenadoptedtoreduceWebresponsetime,improveitsreliability,andbalanceloadamongWebservers.Amongthemostpopularapproachesare:Proxyservercaching.Proxyserversinterceptclientrequestsandcachefrequentlyreferenceddata.Re-questsareinterceptedeitheratanapplicationpro-tocollevel(non-transparentcaches)[20,30]oranetworkprotocollevel(transparentcaches)[13].Cachingimprovestheresponsetimeofsubsequentrequeststhatcanbesatisfieddirectlyfromtheproxycache.ServerclustersAsingledispatcherinterceptsallWebrequestsandredirectsthemtooneoftheserversinthecluster.Therequestsareinterceptedatthenet-workprotocollevel[12,16].SinceaserverclustertypicallyislocatedwithinasingleLAN,theserverselectionismostlybasedonserverloadandavail-abilitywithinthecluster.DNSaliasingAsinglehostnameisassociatedwithmul-tipleIPaddresses.AmodifiedDNSserverselectsoneoftheIPaddressesbasedeitheronround-robinscheduling[26],routingdistance,orTCP/IPproberesponsetime[14].Eachoftheproposedsolutions,however,improvesre-questresponsetime,reliability,orloadbalancingamongservers,butdoesnotaddressalltheseissuestogether.Fur-thermore,manyoftheproposedsolutionsoftenintroduceadditionalproblems.Proxyservercachingimprovesre-questresponsetime,butitintroducespotentialdatain-consistencybetweenthecacheddataandthesamedatastoredattheserver.Non-transparentproxycachescreateasinglepointoffailure.Serverclustersimproverelia-bilityattheserverend,butdonotaddressthereliabilityofthenetworkpathbetweentheclusterdispatcherandaclient.ServerclustersarenotsuitableforbalancingaloadamonggeographicallyreplicatedWebserversbecauseallrequestsmustpassthroughasingledispatcher.Conse-quently,serverclustersonlyimprovetheloadbalanceamongthebackendWebservers.Finally,althoughDNSaliasingimprovesbothrequestresponsetimeandservicereliability,itforcesdataproviderstoreplicatetheentireWebsite.Thisisimpracticalfortworeasons:(1)sincemostWebserversexhibitskewedaccesspattern[32],replicatingtheentireWebservercouldbeanoverkill;(2)insomecasesitisnotpossibleordesirabletoreplicatealldynamicservices.Inaddition,theDNSaliasingim-plementationbecomesproblematicwhenclient-sideDNSagentscacheresultsofDNSqueriesorsubmitrecursiveDNSqueries.1.2PaperPreviewOnewaytoimproveWebperformanceandreliabilityistoreplicatepopularWebresourcesamongdifferentservers.Ifoneoftheserversfails,clientssatisfytheirrequestsfromotherserversthatcontainreplicasofthesamere-source.Clientrequestscanbedirectedtothe“closest”serverthatcontainstherequestedresourceandtherebyimprovetherequestresponsetime.Replicationalsoal-lowsthebalancingofclients’requestsamongdifferentserversandenables“cost-consciousscalability”[9,42]oftheWebservicewherebyasurgeinaserverloadcanbehandledbydynamicallyreplicatinghotdataonadditionalservers.InthispaperwepresentanoverviewofdesignofourWebsystemforreplicationoftheHTTPservice.Un-likeothersimilarsystemsreportedinliterature,WebiscompletelytransparenttothebrowseruserandrequiresnochangestotheexistingWebinfrastructure.Webclientsaredownloadedascryptographicallysignedap-pletstocommerciallyavailablebrowsers.Thereisnoneedforend-userstoinstallaplug-inorclient-sideproxy.Thereisnoneedforanymodificationofthebrowser;theWebappletcanexecuteinbothNetscapeNavigator4.xandMicrosoftExplorer4.xbrowsers.WebserversthatsupportservletscanbedirectlyextendedwithWebservlets.Otherserversareextendedwithaserver-sideproxythatsupportsservlets.Allclient-to-serverandserver-to-servercommunicationiscarriedontopofHTTP1.1.OthersalientfeaturesofWebare:ReliabilityResourcesarereplicatedamongmultipleWebservers.Ifoneoftheserversfails,clientstransparentlyfail-overtoanotherserverthatrepli-catestherequestedresource.Afterafailurerepair,theservertransparentlyreturnstoservicewithoutaffectingclients.Furthermore,Webguaranteesdatadeliveryifatleastoneoftheserversholdingtherequestedresourceisavailable.FastresponsetimeUser’srequestsaredirectedbyWebtotheserverthatisexpectedtoprovidethefastestresponsetimeamongallotheravailableserverswheretheresourceisreplicated.Thisisdonetransparentlytotheuserandtheuserisnotrequiredtoknowwhichserverhasdeliveredtheresource.DynamicreplicationIfthereisahighdemandforare-source,theresourcecanbedynamicallyreplicatedonanotherserverthatislightlyloadedorclosetotheclientsthatfrequentlyrequesttheresource.Furthermore,whendemandforaresourcedrops,someserversmaydroptheresourcecopy.Addi-tionalserversmayberecruitedfromapoolofunder-utilizedserverstohelpsustainaloadpeak.Light-WeightClientsTheclientappletsmaintainverylittlestateinformation.Infact,theonlyinformationthattheymaintainistheHTTPlatencyofthevariousservers.Thisallowsoursystemtoberunonmanyhardwareconfigurationswithlimitedresources.

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