Agreement Problem (Chapter 8) A g r e e e m e n t t P r o t t o c o l l s s

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Need for Agreement Algorithms

Processor failures only with synchronous computation

 Crash fault

Omission fault

Malicious fault

Byzantine fault

Synchronous vs. asynchronous computation

A process knows all the messages it expects (synchronous)

Agreement problem is not solvable in an asynchronous system

 We will solve the agreement problem assuming synchronous computation

Clock synchronization

Atomic commit in DDBS

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Byzantine Agreement Problem

An arbitrarily chosen processor (source processor) will broadcast its initial value to all other processors.

 All non-faulty processors agree on the same value

If the source processor is non-faulty, then the common agreed upon value by all non-faulty processors should be the initial value of the source.

 If the source processor is faulty, then all non-faulty processors can agree on any common value

 It is irrelevant what value faulty processors agree on or whether they agree on a value at all.

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Solution to the Byzantine agreement problem.

 The upper bound on the number of faulty processors.

 In order to reach an agreement on a common value, non-faulty processors need to be free from the influence of faulty processors.

 If faulty processors dominate in number, they can prevent non-faulty processors from reaching a consensus.

 It is impossible to reach a consensus if the number of faulty processors, m, exceeds (n-1)/3.

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Lamport-Shostak-Pease Algorithm ( OM(m) )

 Requires m+1 rounds of message exchanges.

Assumes fully connected network.

 Solves the Byzantine problem for 3m+1 or more processors in the presence of at most m faulty processors.

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The recursive algorithm

 Algorithm OM(0).

 The source processor sends its value to every processor.

Each processor uses the value it receives from the source.

Algorithm OM(m), m>0.

The source processor sends its value to every processor.

 For each i, let v i

be the value processor i receives from the source. Processor i acts as the new source and initiates Algorithm OM(m-1) wherein it sends the value v i

to each of the n-2 other processors.

 For each i and each j (!= i), let v j

be the value processor i received from processor j in the above step using Algorithm OM(m-1). Processor i uses the value majority (v

1

, v

2

, v

3

, … , v n-1

).

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Performance

 O (n m ) message complexity.

 Dolev et al.’s Algorithm

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Synchronization

 Sites maintain physical clocks that are closely synchronized with one another even in presence of Byzantine failures

 Periodic resynchronization

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Assumptions:

All clocks are initially synchronized to approximately the same value

 A nonfaulty process’s clocks runs at approximately the correct rate

 A non-faulty process can read the clock value of another non-faulty process with at most a small error.

Clock synchronization algorithm must satisfy:

 At any time, the value of the clocks of all non-faulty processes must be approximately equal

There is a small bound on the amount by which the clock of a non-faulty process is changed during each resynchronization

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Interactive Convergence Algorithm

 Assumptions:

 Clocks are initially synchronized

Clocks are resynchronized often enough so that two non-faulty clocks never differ by more than

 Each process reads the value of all other processes’ clocks and sets its clock value to the average of these clock values

If a clock value differs from its own clock by more than

, it replaces that value by its own clock value when taking the average

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 This algorithm brings the clocks of non-faulty processes closer

 Assumptions:

 All processes execute the algorithm instantaneously

 The error in reading another process’ clock is zero

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Interactive Consistency Algorithm

 Improves the Interactive Convergence Algorithm by:

 Any two processes will compute approximately the same value for a process clock by taking the median of the clock values rather than the mean

 If a process is non-faulty, then every non-faulty process obtains approximately the correct value for this processes clock

 Therefore, if a majority of processes are non-faulty, the median of all the clock values is either approximately equal to a good clock

’s value or it lies between the values of two good clocks

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Distributed Data Bases

Use of the Byzantine agreement protocol in two-phase commit algorithm

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