the Paxos Commit algorithm

Agenda ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Paxos Commit Algorithm: Overview ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Participants: the resource managers ,[object Object],[object Object],[object Object],Participants: the leader ,[object Object],[object Object],[object Object],[object Object]

Participants: the acceptors ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],p p p p p AC 4 AC 5 AC 1 AC 2 AC 3 Consensus box (MS) RM1 a Ok! RM2 p Ok! RM3 p Ok! a a a a a p p p p p 3 rd instance 1 st instance 2 nd instance Acc 1 Acc 2 Acc 3 Acc 4 Acc 5 aState Paxos

Paxos Commit (base) Not blocked iff F acceptors respond T 1 T 2 (N=5) (F=2) : Writes on log AC0 L AC1 AC2 RM1 RM2 RM3 RM4 RM0 prepare (N-1) x p2a rm 0 v(rm) (N(F+1)-1) x rm 0 v(rm) rm 0 v(rm) p2b acc rm 0 v(rm) rm 0 v(rm) rm 0 v(rm) Opt. F x If ( Global Commit ) then commit p3 else abort p3 x N 0 0 v(0) p2a 1x BeginCommit

Global Commit Condition ,[object Object],p2b acc rm b p

[T 1 ] What if some RMs do not submit their LCV? Leader One majority of acceptors Leader : «Has resource manager j ever proposed you a value?» (Promise not to answer any b L2 <b L1 ) “ accept?” “ promise” “ prepare?” p1a p1b p2a (1) Acceptor i : «Yes, in my last session (ballot) b i with it I accepted its proposal v i » (2) Acceptor i : «No, never» If (at least |MS| acceptors answered) If (for ALL of them case (2) holds) then V=‘a’ [FREE] else V=v(maximum({b i }) [FORCED] Leader : «I am j, I propose V» b L1 >0

[T 2 ] What if the leader fails? ,[object Object],trusted trusted trusted ignored ignored ignored MS b 1 >0 b 2 > b 1 ,[object Object],[object Object],[object Object],b 3 > b 2 b 4 > b 3 T T T L2 L1 trusted

Optimizations for Paxos Commit (1) ,[object Object],[object Object],[object Object],[object Object],RM3 RM0 AC0 p2a BeginCommit L p3 RM1 AC1 p2a RM4 RM2 AC2 p2a RM0 AC0 L RM3 RM4 RM1 AC1 RM2 AC2 Not needed anymore! prepare (N-1) x

Optimizations for Paxos Commit (2) ,[object Object],[object Object],[object Object],p2b RM0 AC0 L RM3 RM4 RM1 AC1 RM2 AC2 RM0 AC0 L RM3 RM4 RM1 AC1 RM2 AC2 p2b p3

Performance ,[object Object],*Not Assuming RMs’ concurrent preparation (slides-like scenario) **Assuming RMs’ concurrent preparation (r.t. constraints needed) N +F +1 N +F +1 N+1 Stable storage writes** 2 2 2 Stable storage write delays** 2FN-2F +3N-3 2NF +3N-1 NF +3N-3 NF+F +3N-1 3N-3 3N-1 Messages* 3 Coloc. 4 4 5 3 4 Message delays* No coloc. Coloc. No coloc. Coloc. No coloc. Faster Paxos Commit Paxos Commit 2PC

Paxos Commit vs. 2PC ,[object Object],T 1 T 2 TM RM1 Other RMs 2PC from Lamport and Gray’s paper 2PC from the slides of the course ,[object Object]

Using a dynamic set of RM ,[object Object],[object Object],[object Object],[object Object],[object Object],AC 4 AC 5 AC 1 AC 2 AC 3 MS RM1 a Ok! RM2 p Ok! RM3 p Ok! Paxos REG RM1;RM2;RM3 Ok! join join join RM1 RM2 RM3 p2b acc rm b p

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