Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Paxos: The Part-Time Parliament CHEN Xinyu

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong 2 Outline  Background  The single-decree protocol  Fault-tolerant distributed system  Conclusion

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong The Parliament  The primary task was to determine the law A sequence of passed decrees  A decree was passed if and only if a majority of legislators voted for the decree 3

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Constraints  The acoustics of the Chamber were poor Communicate only by messenger  Part-time: No one in Paxos was willing to devote his life to the Parliament Legislator  Continually wandered in and out of the parliamentary Chamber No secretary  Each legislator maintained a ledger in which he recorded the numbered sequence of decrees that were passed Messenger  Messages may be delayed, lost, or duplicated 4

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Preconditions  Mutual trust Legislators were willing to pass any decree that was proposed Messengers did not garble messages  When legislators and messengers remained in the Chamber Legislators reacted promptly to any messages Messengers delivered messages in a timely fashion  Resources for each legislator A sturdy ledger  Record the decrees  Write notes to remind himself of the current progress Enough funds to hire as many messengers as he needed Timers 5

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong The Single-Decree Protocol  A decree was chosen through a series of numbered ballots In each ballot, a legislator had the choice only of voting for the decree or not voting  Each ballot was associated with a set of legislators called a quorum A ballot succeeded if and only if every legislator in the quorum voted for the decrees 6

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Requirements  Consistency No two ledgers could contain contradictory information  Progress If a majority of the legislators were in the Chamber, and no one entered or left the Chamber for a sufficiently long period of time, then any decree proposed by a legislator in the Chamber would be passed, and every decree that had been passed would appear in the ledger of every legislator in the Chamber 7

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Achieving Consistency  Each ballot has a unique ballot number  The quorums of any two ballots have at least one legislator in common  For every ballot B, if any legislator in B’s quorum voted in an earlier ballot, then the decree of B equals the decree of the latest of those earlier ballots 8

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong A Sequence of Ballots     2 2     3 3    4 4   5 5   Ballot # DecreeQuorum and voters If a ballot B is successful, then any later ballot is for the same decree as B For every ballot B, if any legislator in B’s quorum voted in an earlier ballot, then the decree of B equals the decree of the latest of those earlier ballots

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Three Roles  Proposer: A legislator who initiated a ballot How to chose a ballot’s number, decree, and quorum? Notes:  pnumber: the largest ballot number that he has proposed  pdecree: the proposed decree for the ballot pnumber  Acceptor: A legislator in the quorum How to decide whether or not to vote? Notes  number: the largest ballot number that he has received  vnumber: the largest ballot number that he has cast  vdecree: the decree voted to accept during the ballot vnumber  Learner: A legislator in Parliament or citizen 10

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Proposer  Ballot number Assign each legislator a unique id l between 0 and N-1  Total N legislators The smallest ballot number s larger than any he has seen such that s mod N = l  Quorum A simple majority A weighted majority  Any set of legislators whose total weight was more than half the total weight of all legislators … 11

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Phase 1: Prepare  Phase 1a: Proposer  Acceptor pnumber = … pdecree = … msg: prepare(pnumber)  Phase 1b: Proposer  Acceptor if (pnumber > number)  number = pnumber  msg: promise(number, vnumber, vdecree) He promised that he would not cast a vote for a decree with ballot number less than number else if (pnumber < number) && different proposers  msg: reject(number) else if (pnumber == number)  ignore 12

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Phase 2: Propose  Phase 2a: Proposer  Acceptor msg:promise(number, vnumber, vdecree)  if (pnumber == number) && majority(number) if(vdecree != null)  pdecree = vdecree with the largest of vnumber (only one such a value) msg: propose(pnumber, pdecree)  Phase 2b: if (pnumber  number) && (vnumber  pnumber)  number = vnumber = pnumber  vdecree = pdecree  Learner  Acceptor msg: vote(vnumber, vdecree) else if (pnumber < number)  Proposer  Acceptor msg: reject(number) 13

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Phase 3: Learn  Phase 3: Learner if majority(vnumber)  Legislator: update his ledger with vdecree  Citizen: informed with vdecree 14

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Example prepare(0) promise(0, - , null) propose(0,  ) vote(0,  ) prepare(pnumber) promise(number, vnumber, vdecree) propose(pnumber, pdecree) vote(vnumber, vdecree) reject(number)

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Example prepare(0) promise(0, - , null) propose(0,  ) prepare(4) promise(4, - , null) propose(9,  ) vote(9,  ) prepare(9) promise(9, 0,  ) citizen vote(0,  ) promise(9, - , null)

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Livelock prepare(0) promise(0, - , null) propose(0,  ) prepare(4) promise(4, - , null) propose(4,  ) vote(4,  ) reject(4) vote(0,  ) prepare(5) promise(5, - , null) promise(5, 0,  ) reject(5) prepare(9)

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong President Selection  The progress condition would be met if only a single proposer, who did not leave the Chamber, was initiating ballots  Having multiple presidents could only impede progress It could not cause inconsistency 18

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Fault-Tolerant Distributed System  A single server: lower availability  Multiple server replicas Legislators  Multiple non-reliable server replicas  Proposer : On behalf of client  Acceptor : Working server replica  Learner: All server replicas Messenger  Non-reliable communication path  Non-Byzantine faults (lost, out of order, duplicated) Decree  User command submitted to server replicas Law (a numbed sequence of passed decrees)  Server replica state  State needs to be consistent among replicas Ledger  Stable storage  Save messages before being sent out 19

Dept. of Computer Science & Engineering, The Chinese University of Hong Kong Conclusion  Paxos: a consensus protocol proposed by Leslie Lamport in 1989 Quorum (Majority) Phase 1 (Prepare): no decree proposed  Used in Google Chubby lock Hadoop Zookeeper (Zab) Scalien Keyspace (key-value NOSQL)  Oracle Berkey DB replication … 20

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