【算法】分布式共识Paxos

一、引言

        在分布式系统中，一致性是至关重要的一个问题。Paxos算法是由莱斯利·兰伯特（Leslie Lamport）在1990年提出的一种解决分布式系统中一致性问题的算法。

二、算法原理

        Paxos算法的目标是让一个分布式系统中的多个节点就某个值达成一致。算法通过多个阶段的消息传递来确保一致性：

        准备阶段（Prepare）：提议者（Proposer）选择一个提案编号n，并向接受者（Acceptor）发送准备请求。

        承诺阶段（Promise）：接受者收到准备请求后，如果提案编号n大于它之前承诺过的任何提案编号，则承诺不再接受编号小于n的提案，并将其之前接受的最高编号的提案作为响应发送给提议者。

        接受阶段（Accept）：提议者收到足够多的承诺后，发送接受请求给接受者，请求它们接受提案[n, v]，其中v是提议者选择的值。

        学习阶段（Learn）：一旦接受者接受了某个提案，它会通知学习者（Learner）该提案已被接受。

三、数据结构

Paxos算法主要涉及以下数据结构：

        提案（Proposal）：由提案编号和提议的值组成。

        承诺（Promise）：包含接受者承诺不再接受编号小于n的提案，以及它之前接受的最高编号的提案。

四、使用场景

Paxos算法适用于以下场景：

        分布式数据库中的日志复制。

        分布式系统中的状态机复制。

        分布式锁服务。

五、算法实现

以下是Paxos算法的伪代码实现：

class Proposer:def propose(value):n = generate提案编号()send_prepare(n) to all Acceptorswait for majority promisesv = determine_value_to_propose(promises)send_accept(n, v) to all Acceptorswait for majority acceptsnotify Learners of accepted proposalclass Acceptor:def on_prepare(request):if request.n > promised_number:promised_number = request.nsend promise with accepted_proposal to Proposerdef on_accept(request):if request.n >= promised_number:promised_number = request.naccepted_proposal = requestsend accepted_proposal to Learnersclass Learner:def on_learn(accepted_proposal):if enough proposals are accepted:chosen_value = accepted_proposal.valueapply chosen_value to state machine

六、其他同类算法对比

• Raft算法：相比Paxos更易于理解和实现，提供了更明确的领导选举机制。
• Zab算法：Zookeeper中使用的算法，结合了Paxos的一些元素，并针对特定场景进行了优化。

七、多语言实现

以下是Paxos算法的简化版实现：

Java

import java.util.HashMap;
import java.util.Map;class Acceptor {private int promisedProposalNumber = -1;private int acceptedProposalNumber = -1;private String acceptedValue = null;public synchronized boolean prepare(int proposalNumber) {if (proposalNumber > promisedProposalNumber) {promisedProposalNumber = proposalNumber;return true;}return false;}public synchronized boolean accept(int proposalNumber, String value) {if (proposalNumber >= promisedProposalNumber) {acceptedProposalNumber = proposalNumber;acceptedValue = value;return true;}return false;}public synchronized int getAcceptedProposalNumber() {return acceptedProposalNumber;}public synchronized String getAcceptedValue() {return acceptedValue;}
}class Proposer {private final Map<Integer, Acceptor> acceptors;private int proposalNumber = 0;private String value;public Proposer(Map<Integer, Acceptor> acceptors, String value) {this.acceptors = acceptors;this.value = value;}public void propose() {proposalNumber++;int successfulPrepares = 0;for (Acceptor acceptor : acceptors.values()) {if (acceptor.prepare(proposalNumber)) {successfulPrepares++;}}if (successfulPrepares > acceptors.size() / 2) {int successfulAccepts = 0;for (Acceptor acceptor : acceptors.values()) {if (acceptor.accept(proposalNumber, value)) {successfulAccepts++;}}if (successfulAccepts > acceptors.size() / 2) {System.out.println("Proposal accepted: " + value);} else {System.out.println("Proposal rejected");}} else {System.out.println("Prepare rejected");}}
}public class PaxosExample {public static void main(String[] args) {Map<Integer, Acceptor> acceptors = new HashMap<>();for (int i = 0; i < 5; i++) {acceptors.put(i, new Acceptor());}Proposer proposer1 = new Proposer(acceptors, "Value 1");proposer1.propose();}
}

Python

class Acceptor:def __init__(self):self.promised_proposal_number = -1self.accepted_proposal_number = -1self.accepted_value = Nonedef prepare(self, proposal_number):if proposal_number > self.promised_proposal_number:self.promised_proposal_number = proposal_numberreturn Truereturn Falsedef accept(self, proposal_number, value):if proposal_number >= self.promised_proposal_number:self.accepted_proposal_number = proposal_numberself.accepted_value = valuereturn Truereturn Falsedef get_accepted_proposal(self):return self.accepted_proposal_number, self.accepted_valueclass Proposer:def __init__(self, acceptors, value):self.acceptors = acceptorsself.proposal_number = 0self.value = valuedef propose(self):self.proposal_number += 1successful_prepares = 0for acceptor in self.acceptors:if acceptor.prepare(self.proposal_number):successful_prepares += 1if successful_prepares > len(self.acceptors) // 2:successful_accepts = 0for acceptor in self.acceptors:if acceptor.accept(self.proposal_number, self.value):successful_accepts += 1if successful_accepts > len(self.acceptors) // 2:print(f"Proposal accepted: {self.value}")else:print("Proposal rejected")else:print("Prepare rejected")if __name__ == "__main__":acceptors = [Acceptor() for _ in range(5)]proposer = Proposer(acceptors, "Value 1")proposer.propose()

C++

#include <iostream>
#include <vector>
#include <memory>class Acceptor {
public:Acceptor() : promisedProposalNumber(-1), acceptedProposalNumber(-1) {}bool prepare(int proposalNumber) {if (proposalNumber > promisedProposalNumber) {promisedProposalNumber = proposalNumber;return true;}return false;}bool accept(int proposalNumber, const std::string &value) {if (proposalNumber >= promisedProposalNumber) {acceptedProposalNumber = proposalNumber;acceptedValue = value;return true;}return false;}std::pair<int, std::string> getAcceptedProposal() {return {acceptedProposalNumber, acceptedValue};}private:int promisedProposalNumber;int acceptedProposalNumber;std::string acceptedValue;
};class Proposer {
public:Proposer(std::vector<std::shared_ptr<Acceptor>> &acceptors, const std::string &value): acceptors(acceptors), proposalNumber(0), value(value) {}void propose() {proposalNumber++;int successfulPrepares = 0;for (auto &acceptor : acceptors) {if (acceptor->prepare(proposalNumber)) {successfulPrepares++;}}if (successfulPrepares > acceptors.size() / 2) {int successfulAccepts = 0;for (auto &acceptor : acceptors) {if (acceptor->accept(proposalNumber, value)) {successfulAccepts++;}}if (successfulAccepts > acceptors.size() / 2) {std::cout << "Proposal accepted: " << value << std::endl;} else {std::cout << "Proposal rejected" << std::endl;}} else {std::cout << "Prepare rejected" << std::endl;}}private:std::vector<std::shared_ptr<Acceptor>> &acceptors;int proposalNumber;std::string value;
};int main() {std::vector<std::shared_ptr<Acceptor>> acceptors;for (int i = 0; i < 5; ++i) {acceptors.push_back(std::make_shared<Acceptor>());}Proposer proposer(acceptors, "Value 1");proposer.propose();return 0;
}

Go

package mainimport ("fmt"
)type Acceptor struct {promisedProposalNumber intacceptedProposalNumber intacceptedValue          string
}func NewAcceptor() *Acceptor {return &Acceptor{promisedProposalNumber: -1,acceptedProposalNumber: -1,}
}func (a *Acceptor) Prepare(proposalNumber int) bool {if proposalNumber > a.promisedProposalNumber {a.promisedProposalNumber = proposalNumberreturn true}return false
}func (a *Acceptor) Accept(proposalNumber int, value string) bool {if proposalNumber >= a.promisedProposalNumber {a.acceptedProposalNumber = proposalNumbera.acceptedValue = valuereturn true}return false
}type Proposer struct {acceptors      []*AcceptorproposalNumber intvalue          string
}func NewProposer(acceptors []*Acceptor, value string) *Proposer {return &Proposer{acceptors: acceptors,value:     value,}
}func (p *Proposer) Propose() {p.proposalNumber++successfulPrepares := 0for _, acceptor := range p.acceptors {if acceptor.Prepare(p.proposalNumber) {successfulPrepares++}}if successfulPrepares > len(p.acceptors)/2 {successfulAccepts := 0for _, acceptor := range p.acceptors {if acceptor.Accept(p.proposalNumber, p.value) {successfulAccepts++}}if successfulAccepts > len(p.acceptors)/2 {fmt.Println("Proposal accepted:", p.value)} else {fmt.Println("Proposal rejected")}} else {fmt.Println("Prepare rejected")}
}func main() {acceptors := make([]*Acceptor, 5)for i := range acceptors {acceptors[i] = NewAcceptor()}proposer := NewProposer(acceptors, "Value 1")proposer.Propose()
}

八、实际服务应用场景代码框架

        以下是一个使用Paxos算法实现分布式锁服务的代码框架：

// Java - Distributed Lock Service with Paxos
public class DistributedLockService {private final Proposer proposer;private final Acceptor acceptor;private final Learner learner;public DistributedLockService() {this.proposer = new Proposer();this.acceptor = new Acceptor();this.learner = new Learner();}public boolean lock(String resource) {// Use Paxos to agree on the lock ownerreturn proposer.propose(resource);}public boolean unlock(String resource) {// Use Paxos to agree on releasing the lockreturn proposer.propose(null);}
}