本文通过实战方式介绍了Paxos算法的实现细节，包括消息处理、Acceptor与Leader的实现等，并提供了一个测试实例。
如果对Paxos算法逻辑理论不清楚的，请参考Paxos算法原理与推导这篇文章，这里不再赘述。
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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