Paxos Made Simple

1. Paxos Made Simple

4. Consensus

5. Let’s get pizza! Let’s get teriyaki! Let’s get burgers!

6. See you at burgers. Oh, the humanity! See you at burgers.

7. Consensus

8. Consensus

10. The Most Important Idea In Distributed Systems

11. Consensus

12. 3,X 3,X 3,X 3,X 3,X 3,X 3,X 3,X 3,X 3,X 3,X 1,X 3,X 1,X 1,X 3,X 3,X 3,X 3,X 3,X 4,X 3,X 4,X 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 4,X

13. 3,X 3,X 3,X 3,X 3,X 3,X 3,X 3,X 3,X 3,X 3,X 1,X 3,X 1,X 1,X 3,X 3,X 3,X 3,X 3,X 4,X 3,X 4,X 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 2,Y 4,X !decided decided(X)

14. Prepare 1? {⊥, ⊥} Propose 1,X? Propose 1,X? Prepare 2? Propose 2,Y? Prepare 3? Propose 3,Y? Propose 3,Y? P1 Prepare 3! Accept 3,Y! Accept 3,Y! {⊥, ⊥} P2 {2:Y, 1:X, 2:Y} P3 Prepare 1! Accept 1,X! Accept 1,X! Accept 2,Y! Prepare 2! Decide Y A1 2,Y 3,Y A2 1,X 1,X 3,Y A3 2,Y L

15. Ballots: motivation

16. Ballots

17. How to maintain the invariant

18. P1a. An acceptor can accept a proposal numbered n iff it has not responded to a prepare request having a number greater than n. ==> P2c. For any v and n, if a proposal with value v and number n is issued, then there is a set S consisting of a majority of acceptors such that either (a) no acceptor in S has accepted any proposal numbered less than n, or (b) v is the value of the highest-ballot-numbered proposal among all proposals numbered less than n accepted by the acceptors in S . ==> P2b. If a proposal with value v is chosen, then every higher-numbered pro- posal issued by any proposer has value v . ==> P2a. If a proposal with value v is chosen, then every higher-numbered pro- posal accepted by any acceptor has value v . ==> P2. If a proposal with value v is chosen, then every higher-numbered pro- posal that is chosen has value v .

20. Alice: transfer $5 Bob Alice: deposit $20 Bob: withdraw $10 Alice: balance $80 Bob: denied NSF Alice: transfer OK

21. Alice: transfer $5 Bob Alice: deposit $20 Bob: withdraw $10 Alice: balance $80 Bob: denied NSF Alice: transfer OK

22. Bob: withdraw $10 proposals Alice: deposit $20 Alice: transfer $5 Alice: transfer $5 Decide Decide Decide [ , , , ...] Alice: transfer $5 Bob Alice: deposit $20 Bob: withdraw $10 decided inputs

23. Multipaxos Decide each slot in the input sequence separately Can execute through the prefix of decided slots

24. Prepare doesn’tdepend on values Prepare whole range with one message Leader 1 allocates, prepares these slots Leader 1 commits some ops ...and then dies. Leader 2 allocates, prepares these slots Leader 2 starts committing ops … but they can’t execute until these holes are filled! Leader 2 backfills holes with decided values or no-ops

25. Determinisma practical challenge

26. Determinism