Paxos算法是大名鼎鼎的Zookeeper中采用的选举Leader的算法，事实上，在涉及到分布式系统的一致性的时候，就只有一种算法，那就是Paxos.

首先来看，Paxos是为了解决什么问题：

Paxos 算法解决的问题是一个分布式系统如何就某个值（决议）达成一致。一个典型的场景是，在一个分布式数据库存储中，如果各节点的初始状态一致，每个节点执行相同的操作序列，那么他们最后能得到一个一致的状态。为保证每个节点执行相同的命令序列，需要在每一条指令上执行一个“一致性算法”以保证每个节点看到的指令一致。一个通用的一致性算法可以应用在许多场景中，是分布式计算中的重要问题。因此从20世纪80年代起对于一致性算法的研究就没有停止过。节点通信存在两种模型：共享内存（Shared memory）和消息传递（Messages passing）。Paxos 算法就是一种基于消息传递模型的一致性算法。

本文的实现参考了LynnCui在知乎上的文章：https://zhuanlan.zhihu.com/p/21438357

有关Paxos的详细介绍及推理过程均可参考上述文章，如果还有不明白，可以查阅相关文章，这一类的文章网上还是比较多的。

关于我的实现，我这里有几点需要说明：

1.关于maxVote（即比本次表决编号小的最大表决编号），如果该编号存在，则应该用改编号对应表决的提案作为下一次的提案，否则，则可以随机指定提案，具体的提案的指定方式依赖于需求逻辑，本程序中采用随机提案;

2.Paxos假设消息的传递过程是不可靠的，这主要是因为实际环境中，网络通信是没有办法保障的。网络延迟以及服务器本身也有可能宕机，这些条件其实都在Paxos的前提假设之中，本程序当中做了一个假设，假设网络通信有50%的概率失败，实际上是我随便指定的；

3.Java本身是一门繁琐的语言，所以难免会有很大冗余代码，这主要是为了保证程序的稳定性，还有一部分是出于习惯。Paxos其实并不是一个复杂的算法，至少基本的不是。程序中所采用的的面向对象的设计也不一定是合理的；

4.本程序依赖了Google的guava包以及common-lang3的包，实际中完全可以去掉。guava主要是在重载hashCode()方法的时候用到，但程序中并没有使用hashCode的地方，实际只是因为在重载equals()方法的时候总是应该重载hashCode()方法。lang3主要做了字符串比较，应对字符串为空的情况。除此之外没有任何依赖。

Java实现参考:

public final class PaxosDemo {private static final HashFunction HASH_FUNCTION = Hashing.murmur3_32();private static final Random RANDOM = new Random();private static final String[] PROPOSALS = {"ProjectA", "ProjectB", "ProjectC"};public static void main(String[] args) {List<Acceptor> acceptors = new ArrayList<Acceptor>();Arrays.asList("A", "B", "C", "D", "E").forEach(name -> acceptors.add(new Acceptor(name)));Proposer.vote(new Proposal(1L, null), acceptors);}private static void printInfo(String subject, String operation, String result) {System.out.println(subject + ":" + operation + "<" + result + ">");}/*** 对于提案的约束，第三条约束要求：* 如果maxVote不存在，那么没有限制，下一次表决可以使用任意提案；* 否则，下一次表决要沿用maxVote的提案** @param currentVoteNumber* @param proposals* @return*/private static Proposal nextProposal(long currentVoteNumber, List<Proposal> proposals) {long voteNumber = currentVoteNumber + 1;if (proposals.isEmpty())return new Proposal(voteNumber, PROPOSALS[RANDOM.nextInt(PROPOSALS.length)]);Collections.sort(proposals);Proposal maxVote = proposals.get(proposals.size() - 1);long maxVoteNumber = maxVote.getVoteNumber();String content = maxVote.getContent();if (maxVoteNumber >= currentVoteNumber)throw new IllegalStateException("illegal state maxVoteNumber");if (content != null)return new Proposal(voteNumber, content);else return new Proposal(voteNumber, PROPOSALS[RANDOM.nextInt(PROPOSALS.length)]);}private static class Proposer {/*** @param proposal* @param acceptors*/public static void vote(Proposal proposal, Collection<Acceptor> acceptors) {int quorum = Math.floorDiv(acceptors.size(), 2) + 1;int count = 0;while (true) {printInfo("VOTE_ROUND", "START", ++count + "");List<Proposal> proposals = new ArrayList<Proposal>();for (Acceptor acceptor : acceptors) {Promise promise = acceptor.onPrepare(proposal);if (promise != null && promise.isAck())proposals.add(promise.getProposal());}if (proposals.size() < quorum) {printInfo("PROPOSER[" + proposal + "]", "VOTE", "NOT PREPARED");proposal = nextProposal(proposal.getVoteNumber(), proposals);continue;}int acceptCount = 0;for (Acceptor acceptor : acceptors) {if (acceptor.onAccept(proposal))acceptCount++;}if (acceptCount < quorum) {printInfo("PROPOSER[" + proposal + "]", "VOTE", "NOT ACCEPTED");proposal = nextProposal(proposal.getVoteNumber(), proposals);continue;}break;}printInfo("PROPOSER[" + proposal + "]", "VOTE", "SUCCESS");}}private static class Acceptor {//上次表决结果private Proposal last = new Proposal();private String name;public Acceptor(String name) {this.name = name;}public Promise onPrepare(Proposal proposal) {//假设这个过程有50%的几率失败if (Math.random() - 0.5 > 0) {printInfo("ACCEPTER_" + name, "PREPARE", "NO RESPONSE");return null;}if (proposal == null)throw new IllegalArgumentException("null proposal");if (proposal.getVoteNumber() > last.getVoteNumber()) {Promise response = new Promise(true, last);last = proposal;printInfo("ACCEPTER_" + name, "PREPARE", "OK");return response;} else {printInfo("ACCEPTER_" + name, "PREPARE", "REJECTED");return new Promise(false, null);}}public boolean onAccept(Proposal proposal) {//假设这个过程有50%的几率失败if (Math.random() - 0.5 > 0) {printInfo("ACCEPTER_" + name, "ACCEPT", "NO RESPONSE");return false;}printInfo("ACCEPTER_" + name, "ACCEPT", "OK");return last.equals(proposal);}}private static class Promise {private final boolean ack;private final Proposal proposal;public Promise(boolean ack, Proposal proposal) {this.ack = ack;this.proposal = proposal;}public boolean isAck() {return ack;}public Proposal getProposal() {return proposal;}}private static class Proposal implements Comparable<Proposal> {private final long voteNumber;private final String content;public Proposal(long voteNumber, String content) {this.voteNumber = voteNumber;this.content = content;}public Proposal() {this(0, null);}public long getVoteNumber() {return voteNumber;}public String getContent() {return content;}@Overridepublic int compareTo(Proposal o) {return Long.compare(voteNumber, o.voteNumber);}@Overridepublic boolean equals(Object obj) {if (obj == null)return false;if (!(obj instanceof Proposal))return false;Proposal proposal = (Proposal) obj;return voteNumber == proposal.voteNumber && StringUtils.equals(content, proposal.content);}@Overridepublic int hashCode() {return HASH_FUNCTION.newHasher().putLong(voteNumber).putString(content, Charsets.UTF_8).hash().asInt();}@Overridepublic String toString() {return new StringBuilder().append(voteNumber).append(':').append(content).toString();}}}

以下是我试着运行了一下程序的结果:

VOTE_ROUND:START<1>
ACCEPTER_A:PREPARE<OK>
ACCEPTER_B:PREPARE<NO RESPONSE>
ACCEPTER_C:PREPARE<OK>
ACCEPTER_D:PREPARE<NO RESPONSE>
ACCEPTER_E:PREPARE<NO RESPONSE>
PROPOSER[1:null]:VOTE<NOT PREPARED>
VOTE_ROUND:START<2>
ACCEPTER_A:PREPARE<OK>
ACCEPTER_B:PREPARE<NO RESPONSE>
ACCEPTER_C:PREPARE<OK>
ACCEPTER_D:PREPARE<NO RESPONSE>
ACCEPTER_E:PREPARE<OK>
ACCEPTER_A:ACCEPT<OK>
ACCEPTER_B:ACCEPT<NO RESPONSE>
ACCEPTER_C:ACCEPT<NO RESPONSE>
ACCEPTER_D:ACCEPT<OK>
ACCEPTER_E:ACCEPT<OK>
PROPOSER[2:ProjectC]:VOTE<NOT ACCEPTED>
VOTE_ROUND:START<3>
ACCEPTER_A:PREPARE<OK>
ACCEPTER_B:PREPARE<OK>
ACCEPTER_C:PREPARE<OK>
ACCEPTER_D:PREPARE<OK>
ACCEPTER_E:PREPARE<OK>
ACCEPTER_A:ACCEPT<OK>
ACCEPTER_B:ACCEPT<NO RESPONSE>
ACCEPTER_C:ACCEPT<NO RESPONSE>
ACCEPTER_D:ACCEPT<NO RESPONSE>
ACCEPTER_E:ACCEPT<NO RESPONSE>
PROPOSER[3:ProjectB]:VOTE<NOT ACCEPTED>
VOTE_ROUND:START<4>
ACCEPTER_A:PREPARE<NO RESPONSE>
ACCEPTER_B:PREPARE<NO RESPONSE>
ACCEPTER_C:PREPARE<OK>
ACCEPTER_D:PREPARE<OK>
ACCEPTER_E:PREPARE<OK>
ACCEPTER_A:ACCEPT<OK>
ACCEPTER_B:ACCEPT<OK>
ACCEPTER_C:ACCEPT<NO RESPONSE>
ACCEPTER_D:ACCEPT<OK>
ACCEPTER_E:ACCEPT<OK>
PROPOSER[4:ProjectC]:VOTE<NOT ACCEPTED>
VOTE_ROUND:START<5>
ACCEPTER_A:PREPARE<OK>
ACCEPTER_B:PREPARE<OK>
ACCEPTER_C:PREPARE<NO RESPONSE>
ACCEPTER_D:PREPARE<NO RESPONSE>
ACCEPTER_E:PREPARE<OK>
ACCEPTER_A:ACCEPT<NO RESPONSE>
ACCEPTER_B:ACCEPT<OK>
ACCEPTER_C:ACCEPT<NO RESPONSE>
ACCEPTER_D:ACCEPT<NO RESPONSE>
ACCEPTER_E:ACCEPT<NO RESPONSE>
PROPOSER[5:ProjectB]:VOTE<NOT ACCEPTED>
VOTE_ROUND:START<6>
ACCEPTER_A:PREPARE<NO RESPONSE>
ACCEPTER_B:PREPARE<OK>
ACCEPTER_C:PREPARE<OK>
ACCEPTER_D:PREPARE<OK>
ACCEPTER_E:PREPARE<NO RESPONSE>
ACCEPTER_A:ACCEPT<NO RESPONSE>
ACCEPTER_B:ACCEPT<NO RESPONSE>
ACCEPTER_C:ACCEPT<NO RESPONSE>
ACCEPTER_D:ACCEPT<NO RESPONSE>
ACCEPTER_E:ACCEPT<NO RESPONSE>
PROPOSER[6:ProjectC]:VOTE<NOT ACCEPTED>
VOTE_ROUND:START<7>
ACCEPTER_A:PREPARE<OK>
ACCEPTER_B:PREPARE<OK>
ACCEPTER_C:PREPARE<OK>
ACCEPTER_D:PREPARE<OK>
ACCEPTER_E:PREPARE<NO RESPONSE>
ACCEPTER_A:ACCEPT<OK>
ACCEPTER_B:ACCEPT<OK>
ACCEPTER_C:ACCEPT<OK>
ACCEPTER_D:ACCEPT<NO RESPONSE>
ACCEPTER_E:ACCEPT<NO RESPONSE>
PROPOSER[7:ProjectB]:VOTE<SUCCESS>

经过7轮表决终于达成一致.