Implementing Consistency -- Paxos. Some slides from Michael Freedman

Transcription

1 Implemetig Cosistecy -- Paxos Some slides from Michael Freedma

2 What do cliets see? Distributed stores use replicatio Fault tolerace ad scalability Does replicatio ecessitate icosistecy? Harder to program, cofusig for cliets

3 Problem How to reach cosesus/data cosistecy i distributed system that ca tolerate o-malicious failures? We saw some cosistecy models how to implemet them?

4 Aother perspective Lock is the easiest way to maage cocurrecy Mutex ad semaphore. Read ad write locks. I distributed system: No master for issuig locks. Failures.

5 Recall, cosistecy models

6 Implemetig Liearizability

7 Implemetig Liearizability

8 Ok, what to do? We wat cosistecy ad availability Two optios 1. Master Replica Model All operatios ad orderig happe o a sigle master Replicates to secodary copies 2. Multi-master model Read/write aywhere Replicas order ad replicate cotet before returig

9 Coordiatio protocols

10 Two phase commit (2PC)

11 What about failures? If oe or more acceptors fail: Still esure liearizability if R + W >N+F Read ad write quoroms of acceptors overlap i at least oe o-failed ode Leader fails? Bye bye J: system o loger live Pick a ew leader? How do we agree? Need to make sure that group is kow

12 Cosesus protocol: Requiremets Safety Oe value accepted It was proposed by some ode All N odes agree o the same value Liveess Some proposed value is evetually chose Each ode evetually lears it Fault tolerace If <= F faults i a widow, cosesus reached evetually Liveess ot guarateed: if >F o cosesus

13 Give desired F, what is N? Crash faults eed 2F+1 processes Byzatie faults (malicious) eed 3F+1 processes i.e., some replicas are tryig to itetioally lie to prevet cosesus or chage the value

14 Why is agreemet hard? What if more tha oe ode is leader? What if etwork is partitioed? What if leader crashes i middle? What if ew leader proposes differet values tha those committed? Network is upredictable, delays are ubouded

15 Strawma solutio I Oe ode X desigated as acceptor Each proposer seds its value to X X decides oe value ad aouces it Problem? Failure of acceptor halts decisio Breaks fault-tolerace requiremet!

16 Strawma II Each proposer (leader) proposes to all acceptors (replicas) Acceptor accepts first proposal, rejects rest Acks proposer If leader receives acks from majority, picks that value ad seds it to replicas Problems? Multiple proposals may ot get a majority What if leader dies before chosig value?

17 Paxos! Widely used family of algorithms for asychroous cosesus Due to Leslie Lamport Basic approach Oe or more odes decide to act like a leader Proposes a value, tries to get it accepted Aouces value if accepted

18 Paxos has three phases

19 Example

20 Paxos Properties Paxos is guarateed safe. Cosesus is a stable property: oce reached it is ever violated; the agreed value is ot chaged.

21 Paxos Properties Paxos is ot guarateed live. Cosesus is reached if a large eough subetwork...is o-faulty for a log eough time. Otherwise Paxos might ever termiate.

22 Combiig Paxos ad 2pc Use paxos for view-chage If aybody otices curret master or oe or more replicas uavailable Propose view chage to paxos to establish ew group Forms the ew group for 2pc Use 2PC for actual data Writes go to master for 2pc Reads from ay replica or master No liveess if majority of odes from previous view ureachable What if a ode comes back/jois?

23 Example system Apache zookeeper Used by a large umber of Iteret scale projects Lockig/barriers Leader electio Cosistecy

24 CAP Cojecture System ca have two of: C: Strog cosistecy A: Availability P: Tolerace to etwork partitio 2PC: CA Paxos: CP Evetual cosistecy: AP