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Paxos 算法详解 The basic algorithm of paxos 网络组：胡术、杨诚、杨铮

Paxos 算法详解 The basic algorithm of paxos 网络组：胡术、杨诚、杨铮. 角色介绍. A(rnd,vrnd,vval). P. L. C(crnd,cval). A(rnd,vrnd,vval). L. P. C(crnd,cval). A(rnd,vrnd,vval). L. P. C(crnd,cval). A(rnd,vrnd,vval). L. A(rnd,vrnd,vval). P. P:proposer, 也可以是客户端，提交议案。 C:coordinator, 协调者，

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Paxos 算法详解 The basic algorithm of paxos 网络组：胡术、杨诚、杨铮

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  1. Paxos算法详解 The basic algorithm of paxos 网络组：胡术、杨诚、杨铮

  2. 角色介绍 A(rnd,vrnd,vval) P L C(crnd,cval) A(rnd,vrnd,vval) L P C(crnd,cval) A(rnd,vrnd,vval) L P C(crnd,cval) A(rnd,vrnd,vval) L A(rnd,vrnd,vval) P

  3. P:proposer,也可以是客户端，提交议案。 • C:coordinator,协调者， • A:acceptor,选举者， • L:learner,学习者， • A round include two phases: • (i)phase 1 (ii)phase 2 • crnd:某C开始的rounds中的最高的round number • cval:某C在round crnd中提出的value。 • rnd:某A参加过的rounds中最高的round number • vrnd:某A批准过的rounds中最高的round number • vval:某A在round vrnd中批准的value。 • Quorum:a majority of the acceptors • 所有的消息可能丢失或者延时，但是不会出错。

  4. message • Phase 1a:prepare • Phase 1b:promise • Phase 2a:accept • Phase 2b:accepted

  5. 1、C要提出一个value并获得批准的过程叫做一个round,它两个阶段：phase1和phase2.1、C要提出一个value并获得批准的过程叫做一个round,它两个阶段：phase1和phase2. 2、C开始某个round i,发送phase 1a[i]给所有的A,如果收到某A的reject[j](j>i),重新发送phase 1a[j+x]（为方便举例，令x = 1）,并设置[crnd = j+1,cval = 0]。 3、Pick a value: • 收到来自某quorum的phase 1b[j+1,vrnd,vval]消息回复。令K为来自该quorum消息中所有vrnd的最大值,V为来自该quorum的消息中vrnd = k的消息对应的vval值的集合（当k>0时,fast paxos在文中证明了这样的vval值只有一个）。 If k = 0,C可以任意挑选一个来自P的value作为value_s else，将V中的那个唯一的value作为我们的挑选value_s • 将value_s和round number j+1 作为phase 2a的内容发送给所有的A,并设置cval = value_s,若收到某A的Nack[h],h>j，设置crnd = h+1并重复上述prepare过程。否则，若收到来自某quorum的phase2b消息，则说明对该value_s的选举已经完成。

  6. 任何一个A必须批准它收到的第一个value。 • A在round i的prepare过程(phase 1)收到phase1a: (a)如果i<rnd,且round rnd的C(beginner)不是round i的C’(beginner),即C!=C’，则通知round i的C’拒绝(reject[rnd])它的prepare请求。如果C = C’，则忽略这个prepare请求。(important!) (b)如果i >= rnd,令rnd = i，发送phase 1b消息。 Phase 1b:[ rnd,vrnd,vval] • A在round i的phase 2收到phase 2a[i,vval_i]: (a)如果i<rnd, 即C!=C’，则通知round i的C’拒绝(Nack[rnd])它的accept请求。如果C = C’，则忽略这个accept请求。(important!) (b)如果i>=rnd,令rnd = vrnd = i,vval = vval_i,发送phase2b • 故而在其它的C开始更高round number的过程时,Round有可能被中断， 。

  7. 初始化(round 1) Phase2b Phase2b L receive phase2b from a quorum,then learn value_s A(0,0,null) Phase2a[1,Value_s] C Receive phase1b from a quorum,pick any value proposed by p. Phase1a [ 1] because 1>0 A(1,0,null) P Phase1b[1,0,null] A(1,1,value_s) C(1,value_s) L C(1,null) C(0,null) A(0,0,null) A(1,1,value_s) because 1>0 A(1,0,null) L receive phase2b from a quorum,then learn value_s L P C(0,null) because 1>0 A(1,0,null) A(0,0,null) A(1,1,value_s) L receive phase2b from a quorum,then learn value_s L because 1>0 A(1,0,null) A(1,1,value_s) P C(0,null) A(0,0,null) L L receive phase2b from a quorum,then learn value_s A(1,1,value_s) because 1>0 A(1,0,null) A(0,0,null) P

  8. 正常运行(round 2 as an example) A(2,1,value_s) A(1,1,value_s) P A(2,2,value_s) Phase 2b Phase 2b L C(1,value_s) Reject [1] A(1,1,value_s) Phase 1a[2] A(2,1,value_s) Phase 1b[2,1,value_s] A(2,2,value_s) Phase1a[1] Phase 2a[2,value_s] C(2,null) L P C(1,null) C(0,null) A(1,1,value_s) A(2,1,value_s) A(2,2,value_s) L C(0,null) P A(1,1,value_s) A(2,1,value_s) A(2,2,value_s) L A(1,1,value_s) A(2,1,value_s) P A(2,2,value_s)

  9. 出现活锁的情况

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