基于python的Paxos算法实现

理解一个算法最快，最深刻的做法，我觉着可能是自己手动实现，虽然项目中不用自己实现，有已经封装好的算法库，供我们调用，我觉着还是有必要自己亲自实践一下。

这里首先说明一下，python这种动态语言，对不熟悉的人可能看着比较别扭，不像java那样参数类型是固定的，所以看着会有些蛋疼。这里环境用的是python2.7。

class Message:
  # command
  MSG_ACCEPTOR_AGREE = 0 # 追随者约定
  MSG_ACCEPTOR_ACCEPT = 1 # 追随者接受
  MSG_ACCEPTOR_REJECT = 2 # 追随者拒绝-网络不通
  MSG_ACCEPTOR_UNACCEPT = 3 # 追随者网络通-不同意
  MSG_ACCEPT = 4 # 接受
  MSG_PROPOSE = 5 # 提议
  MSG_EXT_PROPOSE = 6 # 额外提议
  MSG_HEARTBEAT = 7 # 心跳，每隔一段时间同步消息
  def __init__(self, command=None):
    self.command = command
  # 把收到的消息原原路返回，作为应答消息
  def copyAsReply(self, message):
    # 提议ID #当前的ID #发给谁 #谁发的
    self.proposalID, self.instanceID, self.to, self.source = message.proposalID, message.instanceID, message.source, message.to
    self.value = message.value # 发的信息

然后是利用socket,线程和队列实现的消息处理器：

# 基于socket传递消息，封装网络传递消息
import threading
import pickle
import socket
import queue
class MessagePump(threading.Thread):
  # 收取消息线程
  class MPHelper(threading.Thread):
    #
    def __init__(self, owner):
      self.owner = owner
      threading.Thread.__init__(self)
    def run(self):
      while not self.owner.abort: # 只要所有者线程没有结束,一直接受消息
        try:
          (bytes, addr) = self.owner.socket.recvfrom(2048) # 收取消息
          msg = pickle.loads(bytes) # 读取二进制数据转化为消息
          msg.source = addr[1]
          self.owner.queue.put(msg) # 队列存入消息
        except Exception as e:
          pass

  def __init__(self, owner, port, timeout=2):
    threading.Thread.__init__(self)
    self.owner = owner
    self.abort = False
    self.timeout = 2
    self.port = port
    self.socket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) # UDP通信
    self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF, 200000) # 通信参数
    self.socket.bind(("localhost", port)) # 通信地址，ip，端口
    self.socket.settimeout(timeout) # 超时设置
    self.queue = queue.Queue() # 队列
    self.helper = MessagePump.MPHelper(self) # 接收消息

  # 运行主线程
  def run(self):
    self.helper.start() # 开启收消息的线程
    while not self.abort:
      message = self.waitForMessage() # 阻塞等待
      self.owner.recvMessage(message) # 收取消息

  # 等待消息
  def waitForMessage(self):
    try:
      msg = self.queue.get(True, 3) # 抓取数据，最多等待3s
      return msg
    except:
      return None

  # 发送消息
  def sendMessage(self, message):
    bytes = pickle.dumps(message) # 转化为二进制
    address = ("localhost", message.to) # 地址ip,端口(ip,port)
    self.socket.sendto(bytes, address)
    return True
  #是否停止收取消息
  def doAbort(self):
    self.abort = True

再来一个消息处理器，模拟消息的传递，延迟，丢包，其实这个类没什么卵用，这个是为模拟测试准备的

from MessagePump import MessagePump
import random
class AdversarialMessagePump(MessagePump): # 类的继承
  # 对抗消息传输，延迟消息并任意顺序传递,模拟网络的延迟，消息传送并不是顺序
  def __init__(self, owner, port, timeout=2):
    MessagePump.__init__(self, owner, port, timeout) # 初始化父类
    self.messages = set() # 集合避免重复

  def waitForMessage(self):
    try:
      msg = self.queue.get(True, 0.1) # 从队列抓取数据
      self.messages.add(msg) # 添加消息
    except Exception as e: # 处理异常
      pass
      # print(e)
    if len(self.messages) > 0 and random.random() < 0.95: # Arbitrary!
      msg = random.choice(list(self.messages)) # 随机抓取消息发送
      self.messages.remove(msg) # 删除消息
    else:
      msg = None
    return msg

再来一个是记录类

# InstanceRecord本地记录类，主要记录追随者、领导者最高编号的协议
from PaxosLeaderProtocol import PaxosLeaderProtocol
class InstanceRecord:
  def __init__(self):
    self.protocols = {}
    self.highestID = (-1, -1) # (port,count)
    self.value = None

  def addProtocol(self, protocol):
    self.protocols[protocol.proposalID] = protocol
    #
    if protocol.proposalID[1] > self.highestID[1] or (
        protocol.proposalID[1] == self.highestID[1] and protocol.proposalID[0] > self.highestID[0]):
      self.highestID = protocol.proposalID # 取得编号最大的协议

  def getProtocol(self, protocolID):
    return self.protocols[protocolID]

  def cleanProtocols(self):
    keys = self.protocols.keys()
    for k in keys:
      protocol = self.protocols[k]
      if protocol.state == PaxosLeaderProtocol.STATE_ACCEPTED:
        print("删除协议")
        del self.protocols[k]

下面就是Acceptor的实现：

# 追随者
from MessagePump import MessagePump
from Message import Message
from InstanceRecord import InstanceRecord
from PaxosAcceptorProtocol import PaxosAcceptorProtocol
class PaxosAcceptor:
  def __init__(self, port, leaders):
    self.port = port
    self.leaders = leaders
    self.instances = {} # 接口列表
    self.msgPump = MessagePump(self, self.port) # 消息传递器
    self.failed = False

  # 开始消息传送
  def start(self):
    self.msgPump.start()

  # 停止
  def stop(self):
    self.msgPump.doAbort()

  # 失败
  def fail(self):
    self.failed = True

  def recover(self):
    self.failed = False

  # 发送消息
  def sendMessage(self, message):
    self.msgPump.sendMessage(message)

  # 收消息,只收取为提议的消息
  def recvMessage(self, message):
    if message == None:
      return
    if self.failed: # 失败状态不收取消息
      return

    if message.command == Message.MSG_PROPOSE: # 判断消息是否为提议
      if message.instanceID not in self.instances:
        record = InstanceRecord() # 记录器
        self.instances[message.instanceID] = record
      protocol = PaxosAcceptorProtocol(self) # 创建协议
      protocol.recvProposal(message) # 收取消息
      self.instances[message.instanceID].addProtocol(protocol)
    else:
      self.instances[message.instanceID].getProtocol(message.proposalID).doTransition(message)

  # 通知客户端,
  def notifyClient(self, protocol, message):
    if protocol.state == PaxosAcceptorProtocol.STATE_PROPOSAL_ACCEPTED: # 提议被接受，通知
      self.instances[protocol.instanceID].value = message.value # 储存信息
      print(u"协议被客户端接受 %s" % message.value)

  # 获取最高同意的建议
  def getHighestAgreedProposal(self, instance):
    return self.instances[instance].highestID # （port,count）

  # 获取接口数据
  def getInstanceValue(self, instance):
    return self.instances[instance].value

那再看下AcceptorProtocol的实现：

from Message import Message
class PaxosAcceptorProtocol(object):
  # State variables
  STATE_UNDEFINED = -1 # 协议没有定义的情况0
  STATE_PROPOSAL_RECEIVED = 0 # 收到消息
  STATE_PROPOSAL_REJECTED = 1 # 拒绝链接
  STATE_PROPOSAL_AGREED = 2 # 同意链接
  STATE_PROPOSAL_ACCEPTED = 3 # 同意请求
  STATE_PROPOSAL_UNACCEPTED = 4 # 拒绝请求

  def __init__(self, client):
    self.client = client
    self.state = PaxosAcceptorProtocol.STATE_UNDEFINED

  # 收取,只处理协议类型的消息
  def recvProposal(self, message):

    if message.command == Message.MSG_PROPOSE: # 协议
      self.proposalID = message.proposalID
      self.instanceID = message.instanceID
      (port, count) = self.client.getHighestAgreedProposal(message.instanceID) # 端口，协议内容的最高编号
      # 检测编号处理消息协议
      # 判断协议是否最高
      if count < self.proposalID[1] or (count == self.proposalID[1] and port < self.proposalID[0]):
        self.state = PaxosAcceptorProtocol.STATE_PROPOSAL_AGREED # 协议同意
        print("同意协议:%s, %s " % (message.instanceID, message.value))
        value = self.client.getInstanceValue(message.instanceID)
        msg = Message(Message.MSG_ACCEPTOR_AGREE) # 同意协议
        msg.copyAsReply(message)
        msg.value = value
        msg.sequence = (port, count)
        self.client.sendMessage(msg) # 发送消息
      else: # 不再接受比最高协议小的提议
        self.state = PaxosAcceptorProtocol.STATE_PROPOSAL_REJECTED
      return self.proposalID
    else:
      # 错误重试
      pass
  # 过度
  def doTransition(self, message): # 如果当前协议状态是接受连接，消息类型是接受
    if self.state == PaxosAcceptorProtocol.STATE_PROPOSAL_AGREED and message.command == Message.MSG_ACCEPT:
      self.state = PaxosAcceptorProtocol.STATE_PROPOSAL_ACCEPTED # 接收协议
      msg = Message(Message.MSG_ACCEPTOR_ACCEPT) # 创造消息
      msg.copyAsReply(message) # 拷贝并回复
      for l in self.client.leaders:
        msg.to = l
        self.client.sendMessage(msg) # 给领导发送消息
      self.notifyClient(message) # 通知自己
      return True
    raise Exception("并非预期的状态和命令")

  # 通知 自己客户端
  def notifyClient(self, message):
    self.client.notifyClient(self, message)

接着看下Leader和LeaderProtocol实现：

# 领导者
import threading
import Queue
import time
from Message import Message
from MessagePump import MessagePump
from InstanceRecord import InstanceRecord
from PaxosLeaderProtocol import PaxosLeaderProtocol
class PaxosLeader:
  # 定时监听
  class HeartbeatListener(threading.Thread):
    def __init__(self, leader):
      self.leader = leader
      self.queue = Queue.Queue() # 消息队列
      self.abort = False
      threading.Thread.__init__(self)

    def newHB(self, message):
      self.queue.put(message)

    def doAbort(self):
      self.abort = True

    def run(self): # 读取消息
      elapsed = 0
      while not self.abort:
        s = time.time()
        try:
          hb = self.queue.get(True, 2)
          # 设定规则，谁的端口号比较高，谁就是领导
          if hb.source > self.leader.port:
            self.leader.setPrimary(False)
        except:
          self.leader.setPrimary(True)

  # 定时发送
  class HeartbeatSender(threading.Thread):
    def __init__(self, leader):
      threading.Thread.__init__(self)
      self.leader = leader
      self.abort = False
    def doAbort(self):
      self.abort = True
    def run(self):
      while not self.abort:
        time.sleep(1)
        if self.leader.isPrimary:
          msg = Message(Message.MSG_HEARTBEAT)
          msg.source = self.leader.port
          for leader in self.leader.leaders:
            msg.to = leader
            self.leader.sendMessage(msg)

  def __init__(self, port, leaders=None, acceptors=None):
    self.port = port
    if leaders == None:
      self.leaders = []
    else:
      self.leaders = leaders
    if acceptors == None:
      self.acceptors = []
    else:
      self.acceptors = acceptors
    self.group = self.leaders + self.acceptors # 集合合并
    self.isPrimary = False # 自身是不是领导
    self.proposalCount = 0
    self.msgPump = MessagePump(self, port) # 消息传送器
    self.instances = {}
    self.hbListener = PaxosLeader.HeartbeatListener(self) # 监听
    self.hbSender = PaxosLeader.HeartbeatSender(self) # 发送心跳
    self.highestInstance = -1 # 协议状态
    self.stoped = True # 是否正在运行
    self.lasttime = time.time() # 最后一次时间

  def sendMessage(self, message):
    self.msgPump.sendMessage(message)

  def start(self):
    self.hbSender.start()
    self.hbListener.start()
    self.msgPump.start()
    self.stoped = False

  def stop(self):
    self.hbSender.doAbort()
    self.hbListener.doAbort()
    self.msgPump.doAbort()
    self.stoped = True

  def setPrimary(self, primary): # 设置领导者
    if self.isPrimary != primary:
      # Only print if something's changed
      if primary:
        print(u"我是leader%s" % self.port)
      else:
        print(u"我不是leader%s" % self.port)
    self.isPrimary = primary

  # 获取所有的领导下面的追随者
  def getGroup(self):
    return self.group

  def getLeaders(self):
    return self.leaders

  def getAcceptors(self):
    return self.acceptors

  # 必须获得1/2以上的人支持
  def getQuorumSize(self):
    return (len(self.getAcceptors()) / 2) + 1

  def getInstanceValue(self, instanceID):
    if instanceID in self.instances:
      return self.instances[instanceID].value
    return None

  def getHistory(self): # 历史记录
    return [self.getInstanceValue(i) for i in range(1, self.highestInstance + 1)]

  # 抓取同意的数量
  def getNumAccpted(self):
    return len([v for v in self.getHistory() if v != None])

  # 抓取空白时间处理下事务
  def findAndFillGaps(self):
    for i in range(1, self.highestInstance):
      if self.getInstanceValue(i) == None:
        print("填充空白", i)
        self.newProposal(0, i)
    self.lasttime = time.time()

  # 采集无用信息
  def garbageCollect(self):
    for i in self.instances:
      self.instances[i].cleanProtocols()

  # 通知领导
  def recvMessage(self, message):
    if self.stoped:
      return
    if message == None:
      if self.isPrimary and time.time() - self.lasttime > 15.0:
        self.findAndFillGaps()
        self.garbageCollect()
      return
    #处理心跳信息
    if message.command == Message.MSG_HEARTBEAT:
      self.hbListener.newHB(message)
      return True
    #处理额外的提议
    if message.command == Message.MSG_EXT_PROPOSE:
      print("额外的协议", self.port, self.highestInstance)
      if self.isPrimary:
        self.newProposal(message.value)
      return True

    if self.isPrimary and message.command != Message.MSG_ACCEPTOR_ACCEPT:
      self.instances[message.instanceID].getProtocol(message.proposalID).doTransition(message)

    if message.command == Message.MSG_ACCEPTOR_ACCEPT:
      if message.instanceID not in self.instances:
        self.instances[message.instanceID] = InstanceRecord()
      record = self.instances[message.instanceID]
      if message.proposalID not in record.protocols:#创建协议
        protocol = PaxosLeaderProtocol(self)
        protocol.state = PaxosLeaderProtocol.STATE_AGREED
        protocol.proposalID = message.proposalID
        protocol.instanceID = message.instanceID
        protocol.value = message.value
        record.addProtocol(protocol)
      else:
        protocol = record.getProtocol(message.proposalID)

      protocol.doTransition(message)

    return True
  # 新建提议
  def newProposal(self, value, instance=None):
    protocol = PaxosLeaderProtocol(self)
    if instance == None: # 创建协议标号
      self.highestInstance += 1
      instanceID = self.highestInstance
    else:
      instanceID = instance
    self.proposalCount += 1
    id = (self.port, self.proposalCount)
    if instanceID in self.instances:
      record = self.instances[instanceID]
    else:
      record = InstanceRecord()
      self.instances[instanceID] = record
    protocol.propose(value, id, instanceID)
    record.addProtocol(protocol)

  def notifyLeader(self, protocol, message):
    if protocol.state == PaxosLeaderProtocol.STATE_ACCEPTED:
      print("协议接口%s被%s接受" % (message.instanceID, message.value))
      self.instances[message.instanceID].accepted = True
      self.instances[message.instanceID].value = message.value
      self.highestInstance = max(message.instanceID, self.highestInstance)
      return
    if protocol.state == PaxosLeaderProtocol.STATE_REJECTED: # 重新尝试
      self.proposalCount = max(self.proposalCount, message.highestPID[1])
      self.newProposal(message.value)
      return True
    if protocol.state == PaxosLeaderProtocol.STATE_UNACCEPTED:
      pass

LeaderProtocol实现：

from Message import Message
class PaxosLeaderProtocol(object):
  STATE_UNDEFINED = -1 # 协议没有定义的情况0
  STATE_PROPOSED = 0 # 协议消息
  STATE_REJECTED = 1 # 拒绝链接
  STATE_AGREED = 2 # 同意链接
  STATE_ACCEPTED = 3 # 同意请求
  STATE_UNACCEPTED = 4 # 拒绝请求
  def __init__(self, leader):
    self.leader = leader
    self.state = PaxosLeaderProtocol.STATE_UNDEFINED
    self.proposalID = (-1, -1)
    self.agreecount, self.acceptcount = (0, 0)
    self.rejectcount, self.unacceptcount = (0, 0)
    self.instanceID = -1
    self.highestseen = (0, 0)
  # 提议
  def propose(self, value, pID, instanceID):
    self.proposalID = pID
    self.value = value
    self.instanceID = instanceID
    message = Message(Message.MSG_PROPOSE)
    message.proposalID = pID
    message.instanceID = instanceID
    message.value = value
    for server in self.leader.getAcceptors():
      message.to = server
      self.leader.sendMessage(message)
    self.state = PaxosLeaderProtocol.STATE_PROPOSED

    return self.proposalID

  # 過度
  def doTransition(self, message):
    # 根據狀態運行協議
    if self.state == PaxosLeaderProtocol.STATE_PROPOSED:
      if message.command == Message.MSG_ACCEPTOR_AGREE:
        self.agreecount += 1
        if self.agreecount >= self.leader.getQuorumSize(): # 选举
          print(u"达成协议的法定人数，最后的价值回答是:%s" % message.value)
          if message.value != None:
            if message.sequence[0] > self.highestseen[0] or (
                message.sequence[0] == self.highestseen[0] and message.sequence[1] > self.highestseen[
              1]):
              self.value = message.value
              self.highestseen = message.sequence

            self.state = PaxosLeaderProtocol.STATE_AGREED # 同意更新
            # 发送同意消息
            msg = Message(Message.MSG_ACCEPT)
            msg.copyAsReply(message)
            msg.value = self.value
            msg.leaderID = msg.to
            for server in self.leader.getAcceptors():
              msg.to = server
              self.leader.sendMessage(msg)
            self.leader.notifyLeader(self, message)
          return True

        if message.command == Message.MSG_ACCEPTOR_REJECT:
          self.rejectcount += 1
          if self.rejectcount >= self.leader.getQuorumSize():
            self.state = PaxosLeaderProtocol.STATE_REJECTED
            self.leader.notifyLeader(self, message)
          return True

    if self.state == PaxosLeaderProtocol.STATE_AGREED:
      if message.command == Message.MSG_ACCEPTOR_ACCEPT: # 同意协议
        self.acceptcount += 1
        if self.acceptcount >= self.leader.getQuorumSize():
          self.state = PaxosLeaderProtocol.STATE_ACCEPTED # 接受
          self.leader.notifyLeader(self, message)
      if message.command == Message.MSG_ACCEPTOR_UNACCEPT:
        self.unacceptcount += 1
        if self.unacceptcount >= self.leader.getQuorumSize():
          self.state = PaxosLeaderProtocol.STATE_UNACCEPTED
          self.leader.notifyLeader(self, message)

测试模块：

import socket, pickle, time
from Message import Message
from PaxosAcceptor import PaxosAcceptor
from PaxosLeader import PaxosLeader

if __name__ == "__main__":
  # 设定5个客户端
  numclients = 5
  clients = [PaxosAcceptor(port, [54321, 54322]) for port in range(64320, 64320 + numclients)]
  # 两个领导者
  leader1 = PaxosLeader(54321, [54322], [c.port for c in clients])
  leader2 = PaxosLeader(54322, [54321], [c.port for c in clients])

  # 开启领导者与追随者
  leader1.start()
  leader1.setPrimary(True)
  leader2.setPrimary(True)
  leader2.start()
  for c in clients:
    c.start()

  # 破坏,客户端不链接
  clients[0].fail()
  clients[1].fail()

  # 通信
  s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) # udp协议
  start = time.time()
  for i in range(1000):
    m = Message(Message.MSG_EXT_PROPOSE) # 消息
    m.value = 0 + i # 消息参数
    m.to = 54322 # 设置传递的端口
    bytes = pickle.dumps(m) # 提取的二进制数据
    s.sendto(bytes, ("localhost", m.to)) # 发送消息

  while leader2.getNumAccpted() < 999:
    print("休眠的这一秒 %d " % leader2.getNumAccpted())
    time.sleep(1)
  print(u"休眠10秒")
  time.sleep(10)
  print(u"停止leaders")
  leader1.stop()
  leader2.stop()
  print(u"停止客户端")
  for c in clients:
    c.stop()
  print(u"leader1历史纪录")
  print(leader1.getHistory())
  print(u"leader2历史纪录")
  print(leader2.getHistory())
  end = time.time()
  print(u"一共用了%f秒" % (end - start))

代码确实比较长，看起来有些困难，最好还是在pycharm上看这个逻辑，可以快速定位参数指向，如果有不对的地方欢迎指正

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