ZooKeeper源码阅读（五）：Leader选举

ZooKeeper中的Leader选举也不是Paxos, 实现相关的类包括FastLeaderElection, LeaderElection, 继承层次如下:

//Election

// +AuthFastLeaderElection

// +FastLeaderElection

// +MockFLE (测试)

// +LeaderElection

// +MockLeaderElection

默认使用的是FastLeaderElection, 不过ZooKeeper其实并不关心Leader选举是如何实现的,只要满足:

• The leader has seen the highest zxid of all the followers.

• A quorum of servers have committed to following the leader.

  如果有follower的zxid比Leader看到的zxid还大,说明是Leader选举结束之后才连上Leader的.这时Leader会发送TRUNC消息,让Follower丢弃.

  
  

  1) FastLeaderElection的实现

  //构造函数

  //两个队列，在Messenger中将会用到

  sendqueue = newLinkedBlockingQueue<ToSend>();

  recvqueue = newLinkedBlockingQueue<Notification>();

  //messenger对象

  //Messenger启动了两个线程：WorkerSender，WorkerReceiver

  this.messenger =newMessenger(manager);

  //WorkerSender消费sendqueue

  ToSendm = sendqueue.poll(3000,TimeUnit.MILLISECONDS);

  process(m);

  //队列中消息类型为ToSend

  //{leader, zxid,elecEpoch, state, sid, peerEpoch, configData}

  //表示的消息为Notification或者对Notification的回复

  //WorkerReceiver接收消息

  //如果不是参与投票的，直接回复自己的投票，放到sendqueue中

  //

  //否则，解析成Notification。自己还是Looking则放到recvqueue中，

  //自己不是Looking，对方是Looking，则回复自己的leader编号

  //lookForLeader开始新的一轮选举

  //更新logicalclock

  synchronized(this){

  logicalclock++;

  updateProposal(getInitId(), getInitLastLoggedZxid(), getPeerEpoch());

  }

  sendNotifications();

  //发送Notification，直到不是LOOKING

  while ((self.getPeerState()== ServerState.LOOKING) &&

  (!stop)){

  /*

  *Remove next notification from queue, times out after 2 times

  * thetermination time

  */

  Notification n=recvqueue.poll(notTimeout, TimeUnit.MILLISECONDS);

  //如果是投票者(VoterFollower)，根据它的状态

  //LOOKING:

  //更新logicclock,更新Proposal，发送Notification

  //放入recvset，然后判断是否达到多数派

  //如果达到，则等待finalizeWait微秒，没有更优的投票则选举结束

  //对方是Following,Leading

  //如果logicalclock相符，检查是否达到多数派

  //放到outofelection的都是LEADING或者FOLLOWING状态的，已经选举完毕

  //都要checkLeader(outofelection,n.leader, n.electionEpoch)检查

  //checkLeader

  /*

  * Ifeveryone else thinks I'm the leader, I must be the leader.

  * Theother two checks are just for the case in which I'm not the

  * leader.If I'm not the leader and I haven't received a message

  * fromleader stating that it is leading, then predicate is false.

  */

  if(leader !=self.getId()){

  if(votes.get(leader)==null) predicate =false;

  else if(votes.get(leader).getState()!= ServerState.LEADING) predicate =false;

  } else if(logicalclock !=electionEpoch) {

  predicate = false;

  }

  logicalclock的增加表示新一轮的选举过程.

  
  

  Leader选举初始投票选自己,收到别人的投票之后判断是否更优,如果是则更新自己的投票,最终zxid最大的follower将收到多数派的投票,等待finalizeWait微秒后仍无更优投票,则转为LEADING状态.

  2) LeaderElection的实现则更简单

  //向所有VotingView中的Server建立socket，发送/接收vote

  //然后countVotes判断截止

  
  

  3) AuthFastLeaderElection则是在FastLeaderElection基础上增加了简单的认证

  Leader选举时，将选择zxid最高的。这也是一个优化，因为不用和其它组员那里找丢失的事务。

  
  

  参考:

  http://zookeeper.apache.org/doc/r3.2.2/zookeeperInternals.html#sc_leaderElection