【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：

cpp 复制代码

class Proposer:
    def propose(value):
        n = generate提案编号()
        send_prepare(n) to all Acceptors
        wait for majority promises
        v = determine_value_to_propose(promises)
        send_accept(n, v) to all Acceptors
        wait for majority accepts
        notify Learners of accepted proposal

class Acceptor:
    def on_prepare(request):
        if request.n > promised_number:
            promised_number = request.n
            send promise with accepted_proposal to Proposer

    def on_accept(request):
        if request.n >= promised_number:
            promised_number = request.n
            accepted_proposal = request
            send accepted_proposal to Learners

class Learner:
    def on_learn(accepted_proposal):
        if enough proposals are accepted:
            chosen_value = accepted_proposal.value
            apply 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

java 复制代码

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

python 复制代码

class Acceptor:
    def __init__(self):
        self.promised_proposal_number = -1
        self.accepted_proposal_number = -1
        self.accepted_value = None

    def prepare(self, proposal_number):
        if proposal_number > self.promised_proposal_number:
            self.promised_proposal_number = proposal_number
            return True
        return False

    def accept(self, proposal_number, value):
        if proposal_number >= self.promised_proposal_number:
            self.accepted_proposal_number = proposal_number
            self.accepted_value = value
            return True
        return False

    def get_accepted_proposal(self):
        return self.accepted_proposal_number, self.accepted_value

class Proposer:
    def __init__(self, acceptors, value):
        self.acceptors = acceptors
        self.proposal_number = 0
        self.value = value

    def propose(self):
        self.proposal_number += 1
        successful_prepares = 0
        for acceptor in self.acceptors:
            if acceptor.prepare(self.proposal_number):
                successful_prepares += 1

        if successful_prepares > len(self.acceptors) // 2:
            successful_accepts = 0
            for acceptor in self.acceptors:
                if acceptor.accept(self.proposal_number, self.value):
                    successful_accepts += 1

            if 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++

cpp 复制代码

#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 复制代码

package main

import (
    "fmt"
)

type Acceptor struct {
    promisedProposalNumber int
    acceptedProposalNumber int
    acceptedValue          string
}

func NewAcceptor() *Acceptor {
    return &Acceptor{
        promisedProposalNumber: -1,
        acceptedProposalNumber: -1,
    }
}

func (a *Acceptor) Prepare(proposalNumber int) bool {
    if proposalNumber > a.promisedProposalNumber {
        a.promisedProposalNumber = proposalNumber
        return true
    }
    return false
}

func (a *Acceptor) Accept(proposalNumber int, value string) bool {
    if proposalNumber >= a.promisedProposalNumber {
        a.acceptedProposalNumber = proposalNumber
        a.acceptedValue = value
        return true
    }
    return false
}

type Proposer struct {
    acceptors      []*Acceptor
    proposalNumber int
    value          string
}

func NewProposer(acceptors []*Acceptor, value string) *Proposer {
    return &Proposer{
        acceptors: acceptors,
        value:     value,
    }
}

func (p *Proposer) Propose() {
    p.proposalNumber++
    successfulPrepares := 0
    for _, acceptor := range p.acceptors {
        if acceptor.Prepare(p.proposalNumber) {
            successfulPrepares++
        }
    }

    if successfulPrepares > len(p.acceptors)/2 {
        successfulAccepts := 0
        for _, 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 复制代码

// 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 owner
        return proposer.propose(resource);
    }

    public boolean unlock(String resource) {
        // Use Paxos to agree on releasing the lock
        return proposer.propose(null);
    }
}