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PAXOS: Consensus in a Distributed System Philipp Fath Supervisor: Dr. Diego Milano Leslie Lamport *1941.

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Presentation on theme: "PAXOS: Consensus in a Distributed System Philipp Fath Supervisor: Dr. Diego Milano Leslie Lamport *1941."— Presentation transcript:

1 PAXOS: Consensus in a Distributed System Philipp Fath Supervisor: Dr. Diego Milano Leslie Lamport *1941

2  Introduction: Consensus at PAXOS Island  non-Byzantine faults  Roles and Behaviors  Example situations  Outlook  Sources Agenda | Slide 2Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25

3 Agenda | Slide 3Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25  Introduction: Consensus at PAXOS Island  non-Byzantine faults  Roles and Behaviors  Example situations  Outlook  Sources

4 | Slide 4Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 Introduction: What is consensus? Origin  Latin: consentire = feel together Definition (Merriam-Webster Britannica)  general agreement: unanimity  the judgment arrived at by most of those concerned  group solidarity in sentiment and belief In a Distributed System  Consistent agreement on a value  Different processes must not give contradictory answers

5 | Slide 5Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 Introduction: PAXOS Island Ancient parliament of the Greek Island of Paxos*  Parliament’s primary task: determine the law (sequence of decrees passed)  Instead of having a secretary recording the law, every legislator records passed decrees in his personal ledger  Problems:> Legislator work on part-time > Communication only by (part-time) messenger possible However: Legislators and Messengers are deeply honest  Requirements: Consistent, not contradicting ledgers *) Note: The ancient parliament never really existet at Paxos, Leslie Lamport invented it totally to popularize his consensus-algorithm named PAXOS.

6 Agenda | Slide 6Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25  Introduction: Consensus at PAXOS Island  non-Byzantine faults  Roles and Behaviors  Example situations  Outlook  Sources

7 | Slide 7Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 non-Byzantine faults Agents operate at arbitrary speed may fail by stopping may restart Messages arbitrary deliver time can be duplicated can be lost  Fail-stop or Crash a process works correct until a certain moment then it crashes  Omission or partial discontinuation Some messages might be lost but all of them are correct  Timing Execution of actions and sending of messages may be early or late  Not assumed in PAXOS: Byzantine errors and inconsistency Incorrect working processes sending corrupted messages

8 Agenda | Slide 8Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25  Introduction: Consensus at PAXOS Island  non-Byzantine faults  Roles and Behaviors  Example situations  Outlook  Sources

9 | Slide 9Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 Consensus-making model: Roles proposerlearneracceptor acceptorsproposerslearners Proposes a value v and assigns a unique proposal number N to it Accepts the proposal if it has not promised to accept a Proposal with higher number N Learns chosen Proposal N with value v Quorum must accept proposal to be chosen

10 | Slide 10Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 Consens-making model: Roles CLIENT leader leader proposersacceptorslearners fault-tolerant memory

11 | Slide 11Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 Roles and behaviors: Basic PAXOS  Phase 1a: Prepare a Proposer selects a proposal number N and sends a prepare-request with number N to a majority of acceptors  Phase 1b: Promise acceptors answer with a promise not to accept any more proposals with a number less than N and answer with the highest-numbered proposal (if any) accepted  Phase 2a: Accept! if the proposer received a answer of a majority of acceptors, it sends an accept-request N with value v to them v is the value of the highest-numbered proposal among the answers, or may be any value if no values were reported  Phase 2b: Accepted acceptors accept proposal if they haven’t given a new promise in meantime

12 Agenda | Slide 12Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25  Introduction: Consensus at PAXOS Island  non-Byzantine faults  Roles and Behaviors  Example situations  Outlook  Sources

13 | Slide 13Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 Situation 1: Basic PAXOS [no faults]

14 | Slide 14Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 Situation 2: Failure of Acceptor

15 | Slide 15Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 Situation 3: Failure of redundand Learner

16 | Slide 16Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 Situation 4: Failure of Proposer

17 | Slide 17Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 Situation 5: Failure Duelling Proposers

18 Agenda | Slide 18Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25  Introduction: Consensus at PAXOS Island  non-Byzantine faults  Roles and Behaviors  Example situations  Outlook  Sources

19 | Slide 19Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 Outlook: PAXOS is a family of algorithms  Multi-PAXOS Several instances of PAXOS produce a continuous stream of agreed values but use the same leader.  Cheap-PAXOS Auxiliary Processors only take part during unstable periods.  Fast-PAXOS Direct Accept!-Message from client: saving one message delay  Generalized PAXOS Aim is not to agree on a single value, but on an increasing set of values.

20 Thank you ! Questions ? Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25| Slide 20

21 | Slide 21Workshop DIS: PAXOS / Consensus in a Distributed SystemPhilipp Fath 2010/10/25 Sources Highlights aus der Informatik, Ingo Wegener, Springer 1996 The part-time parliament, Leslie Lamport, ACM Transactions on Computer Systems 1998 Paxos Made Simple, Leslie Lamport, 2001 Generalized Consensus and Paxos, Leslie Lamport, Microsoft Research 2004 Content Images https://belenus.unirioja.es/~mimarano/Lamport.jpghttps://belenus.unirioja.es/~mimarano/Lamport.jpg,


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