Antidio Viguria Ann Krueger A Nonblocking Quorum Consensus Protocol for Replicated Data Divyakant...

Antidio ViguriaAnn Krueger

A Nonblocking Quorum Consensus Protocol for Replicated DataDivyakant Agrawal and Arthur J. Bernstein

Paper Presentation:Dependable Distributed Systems

A Nonblocking Quorum Consensus Protocol for Replicated Data

Agenda Introduction Nonblocking Quorum Overview Blocking versus Nonblocking Propagation Mechanism Application in multi-robot systems

Task allocation Fault tolerance for distributed task

allocation algorithms Conclusions


Introduction Motivation for data replication

Fault Tolerance Delays in accessing data

Gifford’s (Blocking) Quorum Protocol qr[x] + qw[x] > n[x] Read-write operations blocked until

current copy is identified Quorum protocol developed for full file

access Adopting for databases inefficient


Nonblocking Quorum Overview

Assume background mechanism to propagate updates to all copies

Optimistic assumption: “Every copy of an object is current with high probability.”

Significantly reduces delay If obsolete copy, transaction can be rolled

back


Nonblocking Quorum Read an object x

1. Send read request to qr[x] nodes2. Continues computation using value in

first reply3. Quorum accumulation in background;

replies of quorum concurrently collected with execution of transaction

4. If obsolete, transaction rolled back


Nonblocking Quorum Write an object x

1. Send write request to qw[x] nodes2. Continues computation3. Replies gather concurrently, used to

calculate version number4. Transaction commit delayed until

required quorum is assembled

21xnwqw (1)


Performance Comparison Assumptions:

Transaction, T, executes without conflicts Delays due to quorum accumulation &

rollback qr[x], qw[x], n[x] are same for each object x

Blocking LatencyLB = k * D

Nonblocking LatencyLNB ≈ (k-1)p*D+D for k*c«D

LNB improvement over LB if p is small


Optimistic Assumption Every copy is current most of the time

cannot be justified in general. Mechanism that propagates updates

made to qw[x] to all copies of x would decrease value of p by increasing the number of current copies of x


Propagation Mechanism Broadcast mechanism to propagate

updates from write quorum to all copies Need not be reliable X messages contain new value of object

Some messages will be received by nodes among X which already current.


Propagation Mechanism General model for spread of information

xnew xnew

xnew

xold xoldxnewxnew


Effect of Propagation p is the probability

that first reply is obsolete

p -> 0 as number of propagation cycles between successive logical accesses increases


Effect of Propagation In order to justify optimistic assumption

that every copy is current most of the time, we must integrate propagation mechanism such that average number of propagation cycles between successive logical accesses is sufficiently large.


Propagation mechanism Log: local copy organized as an ordered

sequence of event records Gossip messages: used to keep copies of the log

up to date Each site maintains a timetable with events and

the timestamps when they occurred A site uses the timetable to decide which

portion of its log it should send to another site and which should be discarded

For a particular event record, a site maintains its local copy if it is not certain that all other sites are aware of this event


Propagation mechanism Run in background and periodically Two properties are guaranteed:

Propagation property: assumes that site failures and networks partitions are not permanent

Causality property For this application:

Event: describes a transaction commit and contains the new version of objects written

Implicit communication: gossip messages Explicit communication: unicast (information

pertinent to the quorum only)


Nonblocking quorum protocol with the propagation mechanism

Atomic transactions: finish with a commit or an abort

Two-phase commit protocol to guarantee the atomicity of transactions that involve multiple sites

Concurrency control protocol only permits recoverable executions The copies of the data object are not modified until a

transaction commits Main difference with the original protocol appears

when a transaction decides to commit Coordinator: site at which a transaction is initiated Participants: other sites that participate in the

execution of the transaction


Two-phase commit protocol Commitment is delayed until t completes its verification

for every object. T explicitly sends a “prepare” message to all participants

If any participant fails, T aborts sending explicitly messages

If not T is committed The coordinator sends a commit message to all

participants using implicit communication. Update coordinator’s copy of the log.

Update database information when a site receives a commit gossip message.

The commit record is discarded from the log when all sites have learned about T’s commitment

Commit messages are sent to every site. More overhead (can be reduced using optimizations) More robust than the original one. Not need a special

termination protocol when failures occur.

1st phase

2nd phase


Application in multi-robot systems

Cooperation Coodination

Time

Space

Task Allocation

Nonblocking quorum protocol can be used for real time applications such as robotics

Use this algorithm to increase dependability in multi-robot systems


Task Allocation Problem:

Given a number of robots and tasks which robot should execute which task in order to minimize a parameter (traveled distance, mission time, etc.)

Illustrative example: 3 robots 3 tasks (go to a certain point) Minimize global distance and time of the

mission


Illustrative example

Robot A

Robot B

Robot C

Task 1Task 2

Task 3


Different approaches Centralized

Exponential computational complexity (NP-hard) Slow response for dynamic environments Single point of failure

Decentralized Tolerate dynamic environments No single point of failure More complex and efficient but not optimal

solutions Most successful approach so far based on the

CNP (Contract Net Protocol)


How does it work? Based on roles:

Director of the auction Bidders

Simplest protocol: Director announces a task

with an associted minimum bid.

Bidders send their bids. The director selects the best

bid. The director allocates the task

to the best bidder. The roles are played

dynamically by the different robots.

Each robot only knows the tasks allocated to him.



Robot A

Robot B

Robot C

Task 1Task 2

Task 3



Robot A

Robot B

Robot C

Task 2



Robot A

Robot B

Robot C

Task 2Task 1



Robot A

Robot B

Robot C

Task 2Task 1

Task 3


What happens when a robot fails?

Types of failures Failure that does not allow the robot to execute the

task (motors stalled, video-cameras broken, etc.) Failure in communications, i.e., the robot is not

able to communicate with the rest of the robots. Robot is dead (for example failure in the main

power supply) Focus on robot deaths and failures in

communication When a robot fails the tasks allocated to him are

lost. Mission failed!!.


Possible solution Use well-know algorithms already tested for dependable

distributed systems Use of quorums in order to replicate the tasks allocated to

each robot. All robots work as clients and servers.

Because data is only read when there is a failure. Number of writings much larger than number of readings. r+w>n (n=number of robots). For this application, in order to minimize the overhead r>>w. Write once, read all

Overhead is very important Use dynamic quorums Just support one crash failure r+w=n+2

Take into account that a team of robots can split up and join frequently.



Robot A

Robot B

Robot C

Task 2Task 1

Task 3



Robot A

Robot B

Robot CTask 2

Task 1

Task 3



Robot A

Robot B

Robot C

Task 2Task 1

Task 3

Mission completed successfully!!


Conclusions Nonblocking quorums better than blocking

quorums (in terms of latency) when you use propagation mechanism.

Propagation mechanism lowers probability that first reply is obsolete.

Algorithms used in distributed systems can be applied in other fields such as robotics

Quorum protocol could be a good solution to make more fault tolerant multi-robot systems

Questions?


Antidio ViguriaAnn Krueger

Thank you for your attention!


Antidio Viguria Ann Krueger A Nonblocking Quorum Consensus Protocol for Replicated Data Divyakant...

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