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ElectionAlgorithms

Manydistributedalgorithmsneedoneprocesstoactasaleaderorcoordinator

Doesntmatterwhichprocessdoesthejob,justneedtopickone

Example:pickamasterinBerkeleyclocksynchronizationalgorithm

Electionalgorithms:techniquetopickauniquecoordinator

Assumption:each

process

has

aunique

ID

Goal:findthenoncrashedprocesswiththehighestID

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BullyAlgorithm

Assumptions

EachprocessknowstheIDandaddressofeveryotherprocess

Communicationisreliable

Aprocessinitiatesanelectionifitjustrecoveredfromfailureoritnoticesthatthecoordinatorhasfailed

Threetypesofmessages:Election,OK,Coordinator

Several

processes

can

initiate

an

election

simultaneously

Needconsistentresult

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BullyAlgorithmDetails

AnyprocessPcaninitiateanelection

PsendsElectionmessagestoallprocesswithhigherIDsandawaitsOKmessages IfnoOKmessages,Pbecomescoordinatorandsends

CoordinatormessagestoallprocesseswithlowerIDs

IfitreceivesanOK,itdropsoutandwaitsforanCoordinatormessage

IfaprocessreceivesanElectionmessage ImmediatelysendsCoordinatormessageifitistheprocesswith

highestID

Otherwise,returnsanOKandstartsanelection

Ifaprocess

receives

aCoordinatormessage,

it

treats

senderasthecoordinator

3

Bully

Algorithm

Example

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RingAlgorithm

Processesarearrangedinalogicalring,eachprocessknowsthestructureofthering

Aprocessinitiatesanelectionifitjustrecoveredfromfailure

oritnoticesthatthecoordinatorhasfailed

InitiatorsendsElectionmessagetoclosestdownstreamnode

thatisalive

Electionmessageisforwardedaroundthering

EachprocessaddsitsownIDtotheElectionmessage

WhenElectionmessagecomesback,initiatorpicksnodewith

highestIDandsendsaCoordinatormessagespecifyingthe

winneroftheelection

Coordinatormessageisremovedwhenithascirculatedonce.

Multipleelectionscanbeinprogress5

RingAlgorithm

Example

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ComparisonofBullyandRing

Algorithms

Assumenprocessesandoneelectioninprogress

Bullyalgorithm

Worstcase:initiatorisnodewithlowestID

Triggersn2electionsathigherrankednodes:O(n2)

messages

Bestcase:initiatorisnodewithhighestID

Immediateelection:n1messages

Ringalgorithm

2nmessagesalways

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Electionin

Wireless

Environments

Goal:electthebestleader(e.g.,nodewith

longestbatterylifetime)

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Nodeainitiatesanelection.

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ElectioninWirelessNetworks

9Intheend,sourceanotesthathisthebestleaderandbroadcaststhisinfotoallnodes.

Superpeer Election

Howcanweselectsuperpeers suchthat

Normalnodeshavelowlatencyaccessto

superpeers

Superpeers areevenlydistributedacrossthe

overlaynetwork

Thereisapredefinedfractionofsuperpeers

Eachsuperpeer should

not

need

to

serve

more

thanafixednumberofnormalnodes

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Superpeer ElectioninDHTBased

Systems

Basicidea:ReserveafractionoftheIDspaceforsuperpeers

IfSsuperpeers areneededforasystemthatusesmbitidentifiers,simplyreservethe

leftmostbitsforsuperpeers

WithNnodes,wewillhaveN/2mk superpeers onaverage

Routingtosuperpeer:Letm=8andk=3,sendlookuprequestforkeypto

noderesponsible

for

pAND

11100000

which

is

treatedasthesuperpeer

Sk 2log

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