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Voting ProtocolIn Distributed system

By: Vasundhara GhoseB.Tech (CSE)

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Topic covered : Need of voting protocol What is voting protocol? Types of voting protocol System model for static voting protocol Static voting protocol algorithm Vote assign ment policy Demo time Moving towards dynamic voting protocol conclusion

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Why we need Voting protocols? Problem of service-denialNon-blocking protocols can’t

tolerateMultiple site failuresCommunication failuresNetwork partitioning failures

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What is VOTING PROTOCOL? SET OF RULES FOR FAILURE RECOVERY

IN D.S. Assignment of no. of votes of Each

replica If(majority of votes collected)

{ Replica is accessed by process}

More fault tolerant than a commit protocol

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Types of Voting protocols:

Types of voting

protocol

Static protocol

Dynamic protocol

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What is static protocol?Proposed by Gifford.System model of S.V.P. :-

Replica of files lies on different sites.

Sites wants to access file

(LOCK EXISTS}

Site has to unlock it

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System model for S.V.P. (coun..) Lock generating rules(while

assessing a file):One writer and No readersMultiple readers and no writers

Each site has lock manager. Lock manger >> LOCK related

operations Version number for every file

8System model for static voting protocol:

Version number stores In Stable storage

After every successful updating

Updates its Version number

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If read or WRITE quorum needed

Site must collects certain number of votes

permission granted ,IF U HAVE Required VOTES

Basic !dea of Static Voting protocol:Assignment of votes to each replica Votes are collected by requesting process

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Voting protocol algo: When process of site i requests for read/write Step 1: LOCK_REQUEST Step 2: Vote_REQUEST(WHEN lock is granted)Step 3: reception of VOTE REQUEST at site j

If(lock request granted){

return( VN , NO. of votes assigned to replica );

} to site i

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Voting protocol algo(cout..):

Weather I have

quorum or not ??

Site i

Okay let me decide on basis of rule of algorithm

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If(Request is for Read)Vr=ΣVk

P= sets of sites which have replied

Vk= No of votes assigns/given by site k

If (Vr>= r )

{

Successful in obtaining read quorum

}

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If(Request is for WRITE)VW=ΣVk

P= sets of sites which have replied

Vk= No of votes assigns/given by site k Q can be defined as

M=max{VNj : j€P}Q={j€P : VNj=M}

If (VW>= W )

{

Successful in obtaining WRITE quorum(w)

}

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If ( Site’s request is unsuccessful)

{ RELEASE_LOCK

to all sites and its lock manger

}

If ( site’s request gets Quorum)

{ if(Weather it is

latest copy or not)

yes-> next step

No> obtain current copy

}

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If ( request was for read)

{ Read the latest

current copy available locally

}

If ( Reqest for write)

{ Update local copy

Update Version number

Send all updates and version

number to all sites Q

}

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Last steps of SVP RELEASE_LOCK REQUEST to all manager

and all sites in PAfter receiving > updating in local copy by

sitesReleasing of locks (after receiving

RELEASE_LOCK)

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Vote assignment policy:

V= total no. of votes assigned to all the replica

R= read quorum ; w=write quorum

r + w > v w > v/2

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Condition to be satisfied by r and w:None of old copies are updated.W = sum of subsets of replicaR intersection w != nullW value should high and high for

disallowing simultaneous updating

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Demo time:

1

3

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Votes=175 msecs

Votes=1750 msecs

Votes=2750 msecs

Votes=1100 msecs

Case 1: r=1 and w=5Case 2: r=w=3Case 3: site 3 is more reliable

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So this type of voting scheme is known as static voting scheme,

because voting assign policy does not bother with state of

system..

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Limitation of SVP:

1

3

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Votes=175 msecs

Votes=1750 msecs

Votes=1750 msecs

Votes=2100 msecs

Case 4: Site 3 fails /unreachable,simultanously with any site unavailable

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We face:System failure /non-working state.

Solution >> Dynamic voting protocol

DVP solves problem by adapting nature

It adapt number of /set of sites that can form quorum

Prevention from communication failure

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Approaches of DVP• Adaptive set of

sites• To allow access to

data for updating when state of system change.

Majority based

approach

• The number of votes changes assigned to sites

• To allow access to data for updating when state of system change

Dynamic vote re-assignm

ent

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Conclusion:Dynamic voting protocol ensures

more availability than static voting protocol.

Static voting protocol is non-adaptive in nature, that's why its hard to survive for that.

Selection time dependent on system characterstics in dynamic voting protocol.

25THANK

YOU Voti

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