核心枚举
public enum ServerState { LOOKING, FOLLOWING, LEADING, OBSERVING; }
zookeeper服务器状态：刚启动LOOKING，follower是FOLLOWING，leader是LEADING，observer是OBSERVING；
public enum LearnerType { PARTICIPANT, OBSERVER; }
简单来说，zookeeper启动的核心类是QuorumPeerMain，启动之后会加载配置，同时启动QuorumPeer，QuorumPeer会从本地数据目录加载数据，然后开始选举，选举的核心类是FastLeaderElection，选举完成之后会设置状态，根据状态的不同，启动不同的类，比如Leader、Follower、Observer，leader会设置一个新的epoch，同时会等待follower向自己注册，注册完成后follower会向leader同步最新的数据，而leader则定期ping所有的follower，一旦存活的follower不足半数，leader会shutdown，然后触发集群的重新选举。
具体代码以及调用如下：
org.apache.zookeeper.server.quorum.QuorumPeerMain
initializeAndRun
runFromConfig
org.apache.zookeeper.server.quorum.QuorumPeer extends ZooKeeperThread
initialize
start
loadDataBase
org.apache.zookeeper.server.ServerCnxnFactory
start
startLeaderElection
org.apache.zookeeper.server.quorum.QuorumPeer.ResponderThread
start
createElectionAlgorithm
run
org.apache.zookeeper.server.quorum.FastLeaderElection implements Election
lookForLeader
sendNotifications
totalOrderPredicate
termPredicate
org.apache.zookeeper.server.quorum.flexible.QuorumMaj
containsQuorum
QuorumPeer.setPeerState
1 org.apache.zookeeper.server.quorum.Leader extends Learner
lead
org.apache.zookeeper.server.quorum.Leader.LearnerCnxAcceptor extends ZooKeeperThread
run
org.apache.zookeeper.server.quorum.LearnerHandler extends ZooKeeperThread
start
receive Leader.FOLLOWERINFO
epoch++
setZxid
setCurrentEpoch
waitForNewLeaderAck
startZkServer
loop
org.apache.zookeeper.server.quorum.LearnerHandler
ping
org.apache.zookeeper.server.quorum.LearnerHandler.SyncLimitCheck
check
shutdown
Leader.removeLearnerHandler
containsQuorum
shutdown
2 org.apache.zookeeper.server.quorum.Follower extends Learner
followLeader
findLeader
connectToLeader
registerWithLeader
send Leader.FOLLOWERINFO
syncWithLeader
3 org.apache.zookeeper.server.quorum.Observer extends Learner
observeLeader
选举代码解析请见 大数据基础之Zookeeper（3）选举算法