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大纲

1.单机版的zk服务端的启动过程

(1)预启动阶段

(2)初始化阶段

2.集群版的zk服务端的启动过程

(1)预启动阶段

(2)初始化阶段

(3)Leader选举阶段

(4)Leader和Follower启动阶段

2.集群版的zk服务端的启动过程

(1)预启动阶段

(2)初始化阶段

(3)Leader选举阶段

(4)Leader和Follower启动阶段

什么是集群：

集群是由网络中不同的机器组成的一个系统，集群中的工作是通过集群中调度者服务器来协同完成的。

集群中的调度者服务器：

调度者的工作就是在集群收到客户端请求后，根据集群中机器的使用情况，决定将此次客户端请求交给集群中哪一台服务器或网络节点进行处理。

zk中的集群模式：

zk集群会将服务器分成Leader、Follower、Observer三种角色的服务器；在集群运行期间这三种角色的服务器所负责的工作各不相同。

一.Leader角色服务器(处理事务性请求 + 管理其他服务器)

负责处理事务性请求，以及管理集群中的其他服务器。Leader服务器是集群中工作的分配和调度者。

二.Follower服务器(处理非事务性请求 + 选举Leader服务器)

负责处理非事务性请求，以及选举出Leader服务器。发生Leader选举时，系统会从Follow服务器中，根据过半投票原则选举出一个Follower作为Leader服务器。

三.Observer服务器(处理非事务性请求 + 不参与选举和被选举)

负责处理非事务性请求，不参与Leader服务器的选举，也不会作为候选者被选举为Leader服务器。

zk服务端整体架构图如下：

集群版zk服务端的启动分为四个阶段：

预启动阶段、初始化阶段、Leader选举阶段、Leader和Follower启动阶段

集群版zk服务端的启动流程图如下：

接下来介绍集群版的zk服务端是如何从初始化到对外提供服务的。

(1)预启动阶段

在zk服务端进行初始化之前，首先要对配置文件等信息进行解析和载入，而zk服务端的预启动阶段的主要工作流程如下：

一.QuorumPeerMain启动程序

二.解析zoo.cfg配置文件

三.创建和启动历史文件清理器

四.根据配置判断是集群模式还是单机模式

首先zk服务端会调用QuorumPeerMain类中的main()方法，然后在QuorumPeerMain的initializeAndRun()方法里解析zoo.cfg配置文件。接着继续在initializeAndRun()方法中创建和启动历史文件清理器，以及根据配置文件和启动参数，即args参数和config.isDistributed()方法，来判断zk服务端的启动方式是集群模式还是单机模式。如果配置参数中配置了相关的配置项，并且已经指定了集群模式运行，那么在服务启动时就会调用runFromConfig()方法完成集群模式的初始化。

public class QuorumPeerMain {protected QuorumPeer quorumPeer;...//1.启动程序入口public static void main(String[] args) {QuorumPeerMain main = new QuorumPeerMain();try {//启动程序main.initializeAndRun(args);} catch (IllegalArgumentException e) {...}LOG.info("Exiting normally");System.exit(0);}protected void initializeAndRun(String[] args) {QuorumPeerConfig config = new QuorumPeerConfig();if (args.length == 1) {//2.解析配置文件config.parse(args[0]);}//3.创建和启动历史文件清理器DatadirCleanupManager purgeMgr = new DatadirCleanupManager(config.getDataDir(), config.getDataLogDir(), config.getSnapRetainCount(), config.getPurgeInterval());purgeMgr.start();//4.根据配置判断是集群模式还是单机模式if (args.length == 1 && config.isDistributed()) {//集群模式runFromConfig(config);} else {//单机模式ZooKeeperServerMain.main(args);}}...
}

(2)初始化阶段

一.创建网络连接工厂实例ServerCnxnFactory

二.初始化网络连接工厂实例ServerCnxnFactory

三.创建集群版服务器实例QuorumPeer

四.创建数据持久化工具FileTxnSnapLog并设置到QuorumPeer实例中

五.创建内存数据库ZKDatabase并设置到QuorumPeer实例中

六.初始化集群版服务器实例QuorumPeer

七.恢复集群版服务器实例QuorumPeer本地数据

八.启动网络连接工厂ServerCnxnFactory主线程

public class QuorumPeerMain {protected QuorumPeer quorumPeer;...public void runFromConfig(QuorumPeerConfig config) {...ServerCnxnFactory cnxnFactory = null;if (config.getClientPortAddress() != null) {//1.创建网络连接工厂实例ServerCnxnFactorycnxnFactory = ServerCnxnFactory.createFactory();//2.初始化网络连接工厂实例ServerCnxnFactorycnxnFactory.configure(config.getClientPortAddress(), config.getMaxClientCnxns(), false);}//接下来就是初始化集群版服务器实例QuorumPeer//3.创建集群版服务器实例QuorumPeerquorumPeer = getQuorumPeer();//4.创建zk数据管理器FileTxnSnapLogquorumPeer.setTxnFactory(new FileTxnSnapLog(config.getDataLogDir(), config.getDataDir()));quorumPeer.enableLocalSessions(config.areLocalSessionsEnabled());quorumPeer.enableLocalSessionsUpgrading(config.isLocalSessionsUpgradingEnabled());quorumPeer.setElectionType(config.getElectionAlg());quorumPeer.setMyid(config.getServerId());quorumPeer.setTickTime(config.getTickTime());quorumPeer.setMinSessionTimeout(config.getMinSessionTimeout());quorumPeer.setMaxSessionTimeout(config.getMaxSessionTimeout());quorumPeer.setInitLimit(config.getInitLimit());quorumPeer.setSyncLimit(config.getSyncLimit());quorumPeer.setConfigFileName(config.getConfigFilename());//5.创建并初始化内存数据库ZKDatabasequorumPeer.setZKDatabase(new ZKDatabase(quorumPeer.getTxnFactory()));quorumPeer.setQuorumVerifier(config.getQuorumVerifier(), false);if (config.getLastSeenQuorumVerifier() != null) {quorumPeer.setLastSeenQuorumVerifier(config.getLastSeenQuorumVerifier(), false);}quorumPeer.initConfigInZKDatabase();quorumPeer.setCnxnFactory(cnxnFactory);quorumPeer.setSslQuorum(config.isSslQuorum());quorumPeer.setUsePortUnification(config.shouldUsePortUnification());quorumPeer.setLearnerType(config.getPeerType());quorumPeer.setSyncEnabled(config.getSyncEnabled());quorumPeer.setQuorumListenOnAllIPs(config.getQuorumListenOnAllIPs());if (config.sslQuorumReloadCertFiles) {quorumPeer.getX509Util().enableCertFileReloading();}// sets quorum sasl authentication configurationsquorumPeer.setQuorumSaslEnabled(config.quorumEnableSasl);if (quorumPeer.isQuorumSaslAuthEnabled()) {quorumPeer.setQuorumServerSaslRequired(config.quorumServerRequireSasl);quorumPeer.setQuorumLearnerSaslRequired(config.quorumLearnerRequireSasl);quorumPeer.setQuorumServicePrincipal(config.quorumServicePrincipal);quorumPeer.setQuorumServerLoginContext(config.quorumServerLoginContext);quorumPeer.setQuorumLearnerLoginContext(config.quorumLearnerLoginContext);}quorumPeer.setQuorumCnxnThreadsSize(config.quorumCnxnThreadsSize);quorumPeer.initialize();//6.初始化集群版服务器实例QuorumPeerquorumPeer.start();//join方法会将当前线程挂起，等待QuorumPeer线程结束后再执行当前线程quorumPeer.join();}protected QuorumPeer getQuorumPeer() throws SaslException {return new QuorumPeer();}
}public class QuorumPeer extends ZooKeeperThread implements QuorumStats.Provider {ServerCnxnFactory cnxnFactory;ServerCnxnFactory secureCnxnFactory;...public synchronized void start() {//7.恢复集群版服务器实例QuorumPeer本地数据loadDataBase();//8.启动网络连接工厂ServerCnxnFactory主线程startServerCnxnFactory();adminServer.start();//9.开始Leader选举startLeaderElection();startJvmPauseMonitor();//开启监控JVM停顿的线程//10.启动集群版服务器实例QuorumPeersuper.start();}private void startServerCnxnFactory() {if (cnxnFactory != null) {cnxnFactory.start();}if (secureCnxnFactory != null) {secureCnxnFactory.start();}}...
}public abstract class ServerCnxnFactory {...static public ServerCnxnFactory createFactory() throws IOException {String serverCnxnFactoryName = System.getProperty(ZOOKEEPER_SERVER_CNXN_FACTORY);if (serverCnxnFactoryName == null) {serverCnxnFactoryName = NIOServerCnxnFactory.class.getName();}ServerCnxnFactory serverCnxnFactory = (ServerCnxnFactory) Class.forName(serverCnxnFactoryName).getDeclaredConstructor().newInstance();return serverCnxnFactory;}...
}public class NIOServerCnxnFactory extends ServerCnxnFactory {//最大客户端连接数protected int maxClientCnxns = 60;//处理连接过期的线程private ConnectionExpirerThread expirerThread;//处理客户端建立连接的线程private AcceptThread acceptThread;//处理客户端请求的线程private final Set<SelectorThread> selectorThreads = new HashSet<SelectorThread>();//会话过期相关int sessionlessCnxnTimeout;private ExpiryQueue<NIOServerCnxn> cnxnExpiryQueue;//selector线程数，CPU核数的一半private int numSelectorThreads;//工作线程数private int numWorkerThreads;...public void configure(InetSocketAddress addr, int maxcc, boolean secure) throws IOException {...maxClientCnxns = maxcc;sessionlessCnxnTimeout = Integer.getInteger(ZOOKEEPER_NIO_SESSIONLESS_CNXN_TIMEOUT, 10000);cnxnExpiryQueue = new ExpiryQueue<NIOServerCnxn>(sessionlessCnxnTimeout);//创建一个自动处理过期会话的ConnectionExpirerThread线程expirerThread = new ConnectionExpirerThread();int numCores = Runtime.getRuntime().availableProcessors();numSelectorThreads = Integer.getInteger(ZOOKEEPER_NIO_NUM_SELECTOR_THREADS, Math.max((int) Math.sqrt((float) numCores/2), 1));numWorkerThreads = Integer.getInteger(ZOOKEEPER_NIO_NUM_WORKER_THREADS, 2 * numCores);...//创建一批SelectorThread线程for (int i=0; i<numSelectorThreads; ++i) {selectorThreads.add(new SelectorThread(i));}//打开ServerSocketChannelthis.ss = ServerSocketChannel.open();ss.socket().setReuseAddress(true);//绑定端口，启动NIO服务器ss.socket().bind(addr);ss.configureBlocking(false);//创建一个AcceptThread线程acceptThread = new AcceptThread(ss, addr, selectorThreads);}public void start() {stopped = false;if (workerPool == null) {workerPool = new WorkerService("NIOWorker", numWorkerThreads, false);}for (SelectorThread thread : selectorThreads) {if (thread.getState() == Thread.State.NEW) {thread.start();}}if (acceptThread.getState() == Thread.State.NEW) {acceptThread.start();}if (expirerThread.getState() == Thread.State.NEW) {expirerThread.start();}}
}

一.创建网络连接工厂实例ServerCnxnFactory

在执行QuorumPeerMain的runFromConfig()方法时，首先会通过ServerCnxnFactory的createFactory()方法来创建服务端网络连接工厂。

ServerCnxnFactory的createFactory()方法首先会获取配置值，判断是使用NIO还是使用Netty，然后再通过反射去实例化服务端网络连接工厂。

可以通过配置zookeeper.serverCnxnFactory来指定使用：zk自己实现的NIO还是Netty框架，来构建服务端网络连接工厂。

二.初始化网络连接工厂实例ServerCnxnFactory

在执行QuorumPeerMain的runFromConfig()方法时，创建完服务端网络连接工厂实例ServerCnxnFactory后，就会调用网络连接工厂ServerCnxnFactory的configure()方法来初始化ServerCnxnFactory实例。

这里以NIOServerCnxnFactory的configure()方法为例，该方法主要会启动一个NIO服务器，以及创建三类线程：

一.处理客户端连接的AcceptThread线程

二.处理客户端请求的一批SelectorThread线程

三.处理过期连接的ConnectionExpirerThread线程

初始化完ServerCnxnFactory实例后，虽然此时NIO服务器已对外开放端口，客户端也能访问到2181端口，但此时zk服务端还不能正常处理客户端请求。

三.创建集群版服务器实例QuorumPeer

在执行QuorumPeerMain的runFromConfig()方法时，创建和初始化完网络连接工厂实例ServerCnxnFactory后，接着就会调用QuorumPeerMain的getQuorumPeer()方法创建集群版服务器实例。

ZooKeeperServer是单机版服务端的核心实体类。

QuorumPeer是集群版服务端的核心实体类。

可以将每个QuorumPeer类实例看作是集群中的一台服务器。在zk集群模式中，一个QuorumPeer类实例一般具有3种状态，分别是：

状态一：参与Leader节点的选举

状态二：作为Follower节点同步Leader节点的数据

状态三：作为Leader节点管理集群的Follower节点

在执行QuorumPeerMain的runFromConfig()方法时，创建完QuorumPeer实例后，接着会将集群版服务运行中需要的核心工具类注册到QuorumPeer实例中。这些核心工具类也是单机版服务端运行时需要的，比如：数据持久化类FileTxnSnapLog、NIO工厂类ServerCnxnFactory等。然后还会将配置文件中的服务器地址列表、Leader选举算法、会话超时时间等设置到QuorumPeer实例中。

四.创建数据持久化工具FileTxnSnapLog并设置到QuorumPeer实例中

可以通过FileTxnSnapLog对zk服务器的内存数据进行持久化，具体会将内存数据持久化到配置文件的事务日志文件 + 快照数据文件中。

在执行QuorumPeerMain的runFromConfig()方法时，创建完QuorumPeer实例后，首先会根据zoo.cfg配置文件中的dataDir数据快照目录和dataLogDir事务日志目录，通过"new FileTxnSnapLog()"来创建FileTxnSnapLog类实例，然后设置到QuorumPeer实例中。

五.创建内存数据库ZKDatabase并设置到QuorumPeer实例中

ZKDatabase是zk的内存数据库，主要负责管理zk的所有会话记录以及DataTree和事务日志的存储。

在执行QuorumPeerMain的runFromConfig方法时，创建完QuorumPeer实例以及创建完数据持久化工具FileTxnSnapLog并设置到QuorumPeer后，就会将持久化工具FileTxnSnapLog作为参数去创建ZKDatabase实例，然后设置到QuorumPeer实例。

六.初始化集群版服务器实例QuorumPeer

除了需要将一些核心组件注册到服务器实例QuorumPeer中去，还需要对服务器实例QuorumPeer根据zoo.cfg配置文件设置一些参数，比如服务器地址列表、Leader选举算法、会话超时时间等。其中这些核心组件包括：数据持久化工具FileTxnSnapLog、服务端网络连接工厂ServerCnxnFactory、内存数据库ZKDatabase。

完成初始化QuorumPeer实例并启动QuorumPeer线程后，便会通过QuorumPeer的join()方法将main线程挂起，等待QuorumPeer线程结束后再执行。

七.恢复集群版服务器实例QuorumPeer本地数据

在执行QuorumPeerMain的runFromConfig()方法时，初始化完QuorumPeer实例后，就会调用QuorumPeer的start()方法来启动集群中的服务器。

在QuorumPeer的start()方法中，首先会调用QuorumPeer的loadDataBase()方法来恢复数据。

八.启动网络连接工厂ServerCnxnFactory主线程

在QuorumPeer的start()方法中，在调用完QuorumPeer的loadDataBase()方法来恢复本地数据之后，便会调用QuorumPeer的startServerCnxnFactory()方法来启动网络连接工厂的主线程。

(3)Leader选举阶段

一.初始化Leader选举(初始化当前投票 + 监听选举端口 + 启动选举守护线程)

二.启动QuorumPeer线程检测当前服务器状态

三.进行Leader选举

public class QuorumPeer extends ZooKeeperThread implements QuorumStats.Provider {private volatile boolean running = true;private ServerState state = ServerState.LOOKING;volatile private Vote currentVote;Election electionAlg;...public synchronized void start() {//7.恢复集群版服务器实例QuorumPeer本地数据loadDataBase();//8.启动网络连接工厂ServerCnxnFactory主线程startServerCnxnFactory();adminServer.start();//9.初始化Leader选举(初始化当前投票+监听选举端口+启动选举守护线程)startLeaderElection();startJvmPauseMonitor();//开启监控JVM停顿的线程//10.执行集群版服务器实例QuorumPeer.run()方法super.start();}synchronized public void startLeaderElection() {if (getPeerState() == ServerState.LOOKING) {//zk会根据自身的服务器ID、最新的事务ID和当前的服务器epoch来生成一个初始化的投票currentVote = new Vote(myid, getLastLoggedZxid(), getCurrentEpoch());}...//创建选举算法——FastLeaderElection，每一个QuorumPeer实例都会有一个选举算法实例this.electionAlg = createElectionAlgorithm(electionType);}protected Election createElectionAlgorithm(int electionAlgorithm){Election le=null;QuorumCnxManager qcm = createCnxnManager();QuorumCnxManager.Listener listener = qcm.listener;//启动对Leader选举端口的监听，等待集群中其他服务器创建连接listener.start();//创建选举算法，每一个QuorumPeer实例都会有一个选举算法实例FastLeaderElection fle = new FastLeaderElection(this, qcm);//启动选举算法使用的守护线程，其中发送投票的守护线程会不断将当前的投票发出去fle.start();le = fle;return le;}public QuorumCnxManager createCnxnManager() {return new QuorumCnxManager(this,this.getId(),this.getView(),this.authServer,this.authLearner,this.tickTime * this.syncLimit,this.getQuorumListenOnAllIPs(),this.quorumCnxnThreadsSize,this.isQuorumSaslAuthEnabled());}...public synchronized ServerState getPeerState() {return state;}@Overridepublic void run() {...while (running) {//检测当前服务器状态 + 进行Leader选举switch (getPeerState()) {case LOOKING:LOG.info("LOOKING");...//设置当前的投票setCurrentVote(electionAlg.lookForLeader());                                         break;case OBSERVING:......}}}...
}//创建用来进行选举的服务器，监听端口3888
public class QuorumCnxManager {...public class Listener extends ZooKeeperThread {volatile ServerSocket ss = null;//BIO的ServerSocket...@Overridepublic void run() {int numRetries = 0;InetSocketAddress addr;Socket client = null;Exception exitException = null;while ((!shutdown) && (portBindMaxRetry == 0 || numRetries < portBindMaxRetry)) {ss = new ServerSocket();ss.setReuseAddress(true);if (self.getQuorumListenOnAllIPs()) {int port = self.getElectionAddress().getPort();addr = new InetSocketAddress(port);} else {self.recreateSocketAddresses(self.getId());addr = self.getElectionAddress();}setName(addr.toString());ss.bind(addr);while (!shutdown) {client = ss.accept();setSockOpts(client);LOG.info("Received connection request " + client.getRemoteSocketAddress());if (quorumSaslAuthEnabled) {receiveConnectionAsync(client);} else {receiveConnection(client);}numRetries = 0;}}LOG.info("Leaving listener");if (!shutdown) {LOG.error("As I'm leaving the listener thread, " + "I won't be able to participate in leader " + "election any longer: " + self.getElectionAddress());} else if (ss != null) {try {ss.close();} catch (IOException ie) {LOG.debug("Error closing server socket", ie);}}}...}...
}

在QuorumPeer的start()方法中，调用完loadDataBase()和startServerCnxnFactory()方法，恢复好本地数据以及启动完网络连接工厂ServerCnxnFactory的线程后，便会调用QuorumPeer的startLeaderElection()方法进行Leader选举。

一.初始化Leader选举(初始化当前投票 + 监听选举端口 + 启动选举守护线程)

Leader选举是集群版的zk服务端和单机版的zk服务端的最大不同点。首先QuorumPeer的startLeaderElection()方法会通过"new Vote()"，根据服务器ID、最新的事务ID和当前的服务器epoch，来初始化当前投票。也就是在Leader选举的初始化过程中，集群版zk的每个服务器都会给自己投票。

然后当QuorumPeer的startLeaderElection()方法完成初始化当前投票后，会调用QuorumPeer的createElectionAlgorithm()方法去初始化选举算法，即监听选举端口3888 + 启动选举守护线程。

而在QuorumPeer的createElectionAlgorithm()方法中：首先会创建Leader选举所需的网络IO实例QuorumCnxManager。QuorumCnxManager实例会启动Listener线程对Leader选举端口进行监听，用来等待集群中其他服务器发送的请求。

然后会创建FastLeaderElection选举算法实例，zk默认使用FastLeaderElection选举算法，每一个QuorumPeer实例都会有一个选举算法实例。

接着会启动选举算法FastLeaderElection实例使用的守护线程，其中发送投票的守护线程会不断从发送队列中取出消息进行发送，接收投票的守护线程会不断从接收队列中取出消息进行处理。

二.启动QuorumPeer线程检测当前服务器状态

QuorumPeer也是一个继承了ZooKeeperThread的线程。当QuorumPeer的startLeaderElection()方法完成初始化Leader选举后，便会启动QuorumPeer线程通过while循环不断检查当前服务器状态。

QuorumPeer线程的核心工作就是不断地检测当前服务器的状态并做相应处理。服务器状态会在LOOKING、LEADING、FOLLOWING/OBSERVING之间进行切换。在集群版zk服务端启动时，QuorumPeer的初始状态是LOOKING，所以QuorumPeer线程就会判断此时需要更新当前投票进行Leader选举。

三.进行Leader选举

zk的Leader选举过程，其实就是集群中所有机器相互间进行一系列投票，选举产生最合适的机器成为Leader，其余机器成为Follower或Observer，然后对这些已经确定集群角色的机器通过QuorumPeer线程进行初始化的过程。

Leader选举算法：

就是集群中哪个机器处理的数据越新，就越有可能成为Leader。先通过每个服务器处理过的最大ZXID来比较谁的数据最新，如果每个机器处理的ZXID一致，那么最大SID的服务器就成为Leader。

(4)Leader和Follower启动阶段

一.创建Leader服务端实例和Follower客户端实例

二.Leader启动时会创建LearnerCnxAcceptor监听Learner的连接

三.Learner启动时会发起请求建立和Leader的连接

四.Leader会为请求连接的每个Learner创建一个LearnerHandler

五.Learner建立和Leader的连接后会向Leader发送LearnerInfo进行注册

六.Leader解析LearnerInfo计算出epoch并发送LeaderInfo给Learner

七.Learner收到Leader发送的LeaderInfo后会反馈ackNewEpoch消息

八.Leader收到过半Learner的ackNewEpoch消息后开始进行数据同步

九.过半Learner完成数据同步就启动Leader和Learner绑定的服务器实例

当zk完成Leader选举后，集群中每个服务器基本都已确定自己的角色。zk将集群中的机器分为Leader、Follower、Obervser三种角色，每种角色在集群中起到的作用都各不相同。

Leader角色主要负责处理客户端发送的数据变更等事务性请求，并管理协调集群中Follower角色的服务器。Follower角色则主要处理客户端的获取数据等非事务性请求。Observer角色的服务器的功能和Follower角色的服务器的功能相似，唯一的不同就是不会参与Leader的选举工作。

zk中的这三种角色服务器，在服务启动过程中也有各自的不同，下面分析Leader角色和Follower角色在启动过程中的工作原理，也就是Leader角色和Follower角色启动过程中的交互步骤。

Leader服务端和Follower客户端的启动交互：

一.创建Leader服务端实例和Follower客户端实例

由于QuorumPeer线程会不断检测当前服务器的状态并做相应处理，所以当QuorumPeer线程 + FastLeaderElection守护线程完成Leader选举后，每个zk服务器都会根据节点状态创建相应的角色实例来完成数据同步。

比如zk服务器QuorumPeer实例，如果通过getPeerState()方法发现自己的节点状态为LEADING，那么就会调用QuorumPeer的makeLeader()方法来创建Leader服务端实例。如果通过getPeerState()方法发现自己的节点状态为FOLLOWERING，那么就会调用QuorumPeer的makeFollower()方法来创建Follower客户端实例。

public class QuorumPeer extends ZooKeeperThread implements QuorumStats.Provider {private volatile boolean running = true;public Follower follower;public Leader leader;public Observer observer;...public synchronized ServerState getPeerState(){return state;}@Overridepublic void run() {...while (running) {//检测当前服务器状态 + 进行Leader选举switch (getPeerState()) {case LOOKING:LOG.info("LOOKING");...//设置当前的投票setCurrentVote(electionAlg.lookForLeader());                                        break;case FOLLOWING:LOG.info("FOLLOWING");//1.创建Follower客户端实例setFollower(makeFollower(logFactory));//启动Follower客户端实例，followLeader()方法里面会有个while循环处理心跳等follower.followLeader();...break;case LEADING:LOG.info("LEADING");//1.创建Leader服务端实例setLeader(makeLeader(logFactory));//2.启动Leader服务端实例，lead()方法里面会有一个while循环处理心跳等leader.lead();setLeader(null);...break;...}...}}//Follower客户端实例绑定了FollowerZooKeeperServer服务器实例protected Follower makeFollower(FileTxnSnapLog logFactory) throws IOException {return new Follower(this, new FollowerZooKeeperServer(logFactory, this, this.zkDb));}//Leader服务端实例绑定了LeaderZooKeeperServer服务器实例protected Leader makeLeader(FileTxnSnapLog logFactory) throws IOException, X509Exception {return new Leader(this, new LeaderZooKeeperServer(logFactory, this, this.zkDb));}...
}

二.Leader启动时会创建LearnerCnxAcceptor监听Learner的连接

当QuorumPeer线程通过makeLeader()方法创建好Leader服务端实例后，就会通过调用Leader的lead()方法来启动Leader服务端实例。

首先在Leader的构造方法中，会创建BIO的ServerSocket并监听2888端口。然后在Leader的lead()方法中，会创建Learner接收器LearnerCnxAcceptor。而Leader的lead()方法里会有一个while循环，不断处理与Learner的心跳等。

所有非Leader角色都可称为Learner角色，Follower会继承Learner。LearnerCnxAcceptor接收器则用于接收所有Learner客户端的连接请求。

public class QuorumPeer extends ZooKeeperThread implements QuorumStats.Provider {...//Leader服务端实例绑定了LeaderZooKeeperServer服务器实例protected Leader makeLeader(FileTxnSnapLog logFactory) throws IOException, X509Exception {return new Leader(this, new LeaderZooKeeperServer(logFactory, this, this.zkDb));}
}public class Leader {private final ServerSocket ss;//BIO的ServerSocket//1.创建Leader网络服务端实例Leader(QuorumPeer self,LeaderZooKeeperServer zk) throws IOException {this.self = self;this.proposalStats = new BufferStats();if (self.shouldUsePortUnification() || self.isSslQuorum()) {boolean allowInsecureConnection = self.shouldUsePortUnification();if (self.getQuorumListenOnAllIPs()) {ss = new UnifiedServerSocket(self.getX509Util(), allowInsecureConnection, self.getQuorumAddress().getPort());} else {ss = new UnifiedServerSocket(self.getX509Util(), allowInsecureConnection);}} else {if (self.getQuorumListenOnAllIPs()) {ss = new ServerSocket(self.getQuorumAddress().getPort());} else {ss = new ServerSocket();}}ss.setReuseAddress(true);if (!self.getQuorumListenOnAllIPs()) {ss.bind(self.getQuorumAddress());}this.zk = zk;this.learnerSnapshotThrottler = createLearnerSnapshotThrottler(maxConcurrentSnapshots, maxConcurrentSnapshotTimeout);}//2.启动Leader网络服务端实例void lead() throws IOException, InterruptedException {...//2.Leader网络服务端启动时会创建Learner接收器LearnerCnxAcceptor//Start thread that waits for connection requests from new followers.cnxAcceptor = new LearnerCnxAcceptor();...}...
}

三.Learner启动时会发起请求建立和Leader的连接

当QuorumPeer线程通过makeFollower()方法创建好Follower客户端实例后，就会调用Follower的followLeader()方法来启动Follower客户端实例。在Follower的followLeader()方法中，也就是在Learner客户端实例创建完后，会通过findLeader()方法从Leader选举的投票结果中找到Leader服务端，然后通过connectToLeader()方法来建立和该Leader服务端之间的连接。connectToLeader()方法尝试建立连接时，最多尝试5次，每次睡眠1秒。Follower的followLeader()方法中，会有一个while循环不断处理心跳等消息。

public class QuorumPeer extends ZooKeeperThread implements QuorumStats.Provider {...//Follower客户端实例绑定了FollowerZooKeeperServer服务器实例protected Follower makeFollower(FileTxnSnapLog logFactory) throws IOException {return new Follower(this, new FollowerZooKeeperServer(logFactory, this, this.zkDb));}
}public class Follower extends Learner {final FollowerZooKeeperServer fzk;//1.创建Follower网络客户端实例Follower(QuorumPeer self, FollowerZooKeeperServer zk) {this.self = self;this.zk = zk;this.fzk = zk;}//2.启动Follower网络客户端，也就是启动Learner网络客户端void followLeader() throws InterruptedException {...//3.从Leader选举的投票结果中找到Leader网络服务端QuorumServer leaderServer = findLeader();            //3.建立和Leader网络服务端之间的连接connectToLeader(leaderServer.addr, leaderServer.hostname);...while (this.isRunning()) {//主要处理心跳消息readPacket(qp);processPacket(qp);}}...
}public class Learner {       ...QuorumPeer self;LearnerZooKeeperServer zk;protected BufferedOutputStream bufferedOutput;protected Socket sock;protected InputArchive leaderIs;protected OutputArchive leaderOs;  ...//3.从Leader选举的投票结果中找到Leader服务器protected QuorumServer findLeader() {QuorumServer leaderServer = null;// Find the leader by idVote current = self.getCurrentVote();for (QuorumServer s : self.getView().values()) {if (s.id == current.getId()) {// Ensure we have the leader's correct IP address before attempting to connect.s.recreateSocketAddresses();leaderServer = s;break;}}if (leaderServer == null) {LOG.warn("Couldn't find the leader with id = " + current.getId());}return leaderServer;}//3.建立和Leader服务器之间的连接protected void connectToLeader(InetSocketAddress addr, String hostname) {this.sock = createSocket();  int initLimitTime = self.tickTime * self.initLimit;int remainingInitLimitTime = initLimitTime;long startNanoTime = nanoTime();for (int tries = 0; tries < 5; tries++) {try {remainingInitLimitTime = initLimitTime - (int)((nanoTime() - startNanoTime) / 1000000);if (remainingInitLimitTime <= 0) {LOG.error("initLimit exceeded on retries.");throw new IOException("initLimit exceeded on retries.");}sockConnect(sock, addr, Math.min(self.tickTime * self.syncLimit, remainingInitLimitTime));if (self.isSslQuorum())  {((SSLSocket) sock).startHandshake();}sock.setTcpNoDelay(nodelay);break;} catch (IOException e) {...}Thread.sleep(1000);}self.authLearner.authenticate(sock, hostname);//下面定义的leaderIs和leaderOs是建立在Socket基础上的//所以后续在registerWithLeader方法中可以先后往leaderIs写数据然后等待从leaderOs读数据leaderIs = BinaryInputArchive.getArchive(new BufferedInputStream(sock.getInputStream()));bufferedOutput = new BufferedOutputStream(sock.getOutputStream());leaderOs = BinaryOutputArchive.getArchive(bufferedOutput);}protected void sockConnect(Socket sock, InetSocketAddress addr, int timeout) throws IOException {sock.connect(addr, timeout);}
}

四.Leader会为请求连接的每个Learner创建一个LearnerHandler

LearnerCnxAcceptor也是一个线程。Leader服务端启动时会创建和启动Learner接收器LearnerCnxAcceptor，LearnerCnxAcceptor线程里会通过while循环不断监听Learner发起的连接。

当Leader服务端实例接收到来自Learner的连接请求后，LearnerCnxAcceptor线程就会通过ServerSocket监听到Learner连接请求。此时，LearnerCnxAcceptor就会创建一个LearnerHandler实例。每个LearnerHandler实例都对应了一个Leader与Learner之间的连接，LearnerHandler负责Leader与Learner间几乎所有的消息通信和数据同步。

LearnerHandler也是一个线程，LearnerHandler会通过BIO + while循环来处理和Learner的通信、数据同步和心跳。

public class Leader {private final ServerSocket ss;//BIO的ServerSocket//1.创建Leader服务端实例Leader(QuorumPeer self,LeaderZooKeeperServer zk) throws IOException {this.self = self;this.proposalStats = new BufferStats();if (self.shouldUsePortUnification() || self.isSslQuorum()) {boolean allowInsecureConnection = self.shouldUsePortUnification();if (self.getQuorumListenOnAllIPs()) {ss = new UnifiedServerSocket(self.getX509Util(), allowInsecureConnection, self.getQuorumAddress().getPort());} else {ss = new UnifiedServerSocket(self.getX509Util(), allowInsecureConnection);}} else {if (self.getQuorumListenOnAllIPs()) {ss = new ServerSocket(self.getQuorumAddress().getPort());} else {ss = new ServerSocket();}}ss.setReuseAddress(true);if (!self.getQuorumListenOnAllIPs()) {ss.bind(self.getQuorumAddress());}this.zk = zk;this.learnerSnapshotThrottler = createLearnerSnapshotThrottler(maxConcurrentSnapshots, maxConcurrentSnapshotTimeout);}...void lead() throws IOException, InterruptedException {...//2.Leader服务端启动时会创建Learner接收器LearnerCnxAcceptor//Start thread that waits for connection requests from new followers.cnxAcceptor = new LearnerCnxAcceptor();cnxAcceptor.start();...}class LearnerCnxAcceptor extends ZooKeeperCriticalThread {private volatile boolean stop = false;public LearnerCnxAcceptor() {super("LearnerCnxAcceptor-" + ss.getLocalSocketAddress(), zk.getZooKeeperServerListener());}@Overridepublic void run() {while (!stop) {Socket s = null;boolean error = false;...s = ss.accept();s.setSoTimeout(self.tickTime * self.initLimit);s.setTcpNoDelay(nodelay);BufferedInputStream is = new BufferedInputStream(s.getInputStream());//4.Leader服务端会为每个Learner客户端创建一个LearnerHandlerLearnerHandler fh = new LearnerHandler(s, is, Leader.this);fh.start();...}}public void halt() {stop = true;}}...
}public class LearnerHandler extends ZooKeeperThread {final Leader leader;...@Overridepublic void run() {leader.addLearnerHandler(this);tickOfNextAckDeadline = leader.self.tick.get() + leader.self.initLimit + leader.self.syncLimit;ia = BinaryInputArchive.getArchive(bufferedInput);bufferedOutput = new BufferedOutputStream(sock.getOutputStream());oa = BinaryOutputArchive.getArchive(bufferedOutput);QuorumPacket qp = new QuorumPacket();ia.readRecord(qp, "packet");if (qp.getType() != Leader.FOLLOWERINFO && qp.getType() != Leader.OBSERVERINFO) {LOG.error("First packet " + qp.toString() + " is not FOLLOWERINFO or OBSERVERINFO!");return;}byte learnerInfoData[] = qp.getData();...}...
}

五.Learner建立和Leader的连接后会向Leader发送LearnerInfo进行注册

当Learner通过Learner的connectToLeader()方法和Leader建立起连接后，就会通过Learner的registerWithLeader()方法开始向Leader进行注册。也就是将Learner客户端自己的基本信息LearnerInfo发送给Leader服务端，LearnerInfo中会包括当前服务器的SID和处理事务的最新ZXID。

public class Follower extends Learner {final FollowerZooKeeperServer fzk;//创建Follower客户端实例Follower(QuorumPeer self, FollowerZooKeeperServer zk) {this.self = self;this.zk = zk;this.fzk = zk;}//启动Follower客户端实例，也就是启动Learner客户端实例void followLeader() throws InterruptedException {...//从Leader选举的投票结果中找到Leader服务端QuorumServer leaderServer = findLeader();            try {//3.建立和Leader服务端之间的连接connectToLeader(leaderServer.addr, leaderServer.hostname);//5.向Leader发起注册long newEpochZxid = registerWithLeader(Leader.FOLLOWERINFO);...syncWithLeader(newEpochZxid);                QuorumPacket qp = new QuorumPacket();while (this.isRunning()) {readPacket(qp);processPacket(qp);}} catch (Exception e) {LOG.warn("Exception when following the leader", e);try {sock.close();} catch (IOException e1) {e1.printStackTrace();}pendingRevalidations.clear();}}...
}public class Learner {       ...//Once connected to the leader, perform the handshake protocol to establish a following / observing connection. protected long registerWithLeader(int pktType) throws IOException {long lastLoggedZxid = self.getLastLoggedZxid();QuorumPacket qp = new QuorumPacket();                qp.setType(pktType);qp.setZxid(ZxidUtils.makeZxid(self.getAcceptedEpoch(), 0));//Add sid to payload//5.将Learner客户端自己的基本信息LearnerInfo发送给Leader服务端LearnerInfo li = new LearnerInfo(self.getId(), 0x10000, self.getQuorumVerifier().getVersion());ByteArrayOutputStream bsid = new ByteArrayOutputStream();BinaryOutputArchive boa = BinaryOutputArchive.getArchive(bsid);boa.writeRecord(li, "LearnerInfo");qp.setData(bsid.toByteArray());writePacket(qp, true);...}void writePacket(QuorumPacket pp, boolean flush) throws IOException {synchronized (leaderOs) {if (pp != null) {leaderOs.writeRecord(pp, "packet");}if (flush) {bufferedOutput.flush();}}}...
}

六.Leader解析LearnerInfo计算出epoch并发送LeaderInfo给Learner

由于Leader的LearnerCnxAcceptor在接收到来自Learner的连接请求后，会创建LearnerHandler来处理Leader与Learner的消息通信和数据同步。所以当Learner通过registerWithLeader()方法向Leader发起注册请求后，Leader服务端下对应的LearnerHandler线程就能收到LearnerInfo信息，于是便会根据LearnerInfo信息解析出Learner的SID和ZXID。

首先调用ZxidUtils的getEpochFromZxid()方法，通过将Learner的ZXID右移32位来解析出Learner的epoch。然后调用Leader的getEpochToPropose()方法比较Learner和Leader的epoch，如果Learner的epoch大，则更新Leader的epoch为Learner的epoch + 1。

接着在Leader的getEpochToPropose()方法中，会将Learner的SID添加到HashSet类型的connectingFollowers中。通过Leader的connectingFollowers的wait()方法和notifyAll()方法，便能实现让LearnerHandler进行等待和唤醒。

直到过半Learner已向Leader进行了注册，同时更新了Leader的epoch，之后Leader就可以确定当前集群的epoch了。可见，可以通过Object的wait()方法和notifyAll()方法来实现过半效果：未过半则进行阻塞，过半则进行通知继续后续处理。

当确定好当前集群的epoch后，Leader的每个LearnerHandler，都会发送一个包含该epoch的LeaderInfo消息给对应的Learner，然后再通过Leader的waitForEpochAck()方法等待过半Learner的响应。

public class LearnerHandler extends ZooKeeperThread {final Leader leader;//ZooKeeper server identifier of this learnerprotected long sid = 0;protected final Socket sock;private BinaryInputArchive ia;private BinaryOutputArchive oa;...@Overridepublic void run() {leader.addLearnerHandler(this);tickOfNextAckDeadline = leader.self.tick.get() + leader.self.initLimit + leader.self.syncLimit;//将ia和oa与BIO的Socket进行绑定//以便当Leader通过oa发送LeaderInfo消息给Learner时，可以通过ia读取到Learner的ackNewEpoch响应ia = BinaryInputArchive.getArchive(bufferedInput);bufferedOutput = new BufferedOutputStream(sock.getOutputStream());oa = BinaryOutputArchive.getArchive(bufferedOutput);QuorumPacket qp = new QuorumPacket();ia.readRecord(qp, "packet");byte learnerInfoData[] = qp.getData();...//根据LearnerInfo信息解析出Learner的SIDByteBuffer bbsid = ByteBuffer.wrap(learnerInfoData);if (learnerInfoData.length >= 8) {this.sid = bbsid.getLong();}...//根据Learner的ZXID解析出对应Learner的epochlong lastAcceptedEpoch = ZxidUtils.getEpochFromZxid(qp.getZxid());long zxid = qp.getZxid();//将Learner的epoch和Leader的epoch进行比较//如果Learner的epoch更大，则更新Leader的epoch为Learner的epoch + 1long newEpoch = leader.getEpochToPropose(this.getSid(), lastAcceptedEpoch);long newLeaderZxid = ZxidUtils.makeZxid(newEpoch, 0);...//发送一个包含该epoch的LeaderInfo消息给该LearnerHandler对应的LearnerQuorumPacket newEpochPacket = new QuorumPacket(Leader.LEADERINFO, newLeaderZxid, ver, null);oa.writeRecord(newEpochPacket, "packet");bufferedOutput.flush();//发送包含该epoch的LeaderInfo消息后等待Learner响应//读取Learner返回的ackNewEpoch响应QuorumPacket ackEpochPacket = new QuorumPacket();ia.readRecord(ackEpochPacket, "packet");...//等待过半Learner响应leader.waitForEpochAck(this.getSid(), ss);...}...
}public class Leader {...long epoch = -1;protected final Set<Long> connectingFollowers = new HashSet<Long>();public long getEpochToPropose(long sid, long lastAcceptedEpoch) throws InterruptedException, IOException {synchronized(connectingFollowers) {if (!waitingForNewEpoch) {return epoch;}//将Learner的epoch和Leader的epoch进行比较//如果Learner的epoch更大，则更新Leader的epoch为Learner的epoch + 1if (lastAcceptedEpoch >= epoch) {epoch = lastAcceptedEpoch + 1;}if (isParticipant(sid)) {connectingFollowers.add(sid);}QuorumVerifier verifier = self.getQuorumVerifier();if (connectingFollowers.contains(self.getId()) && verifier.containsQuorum(connectingFollowers)) {waitingForNewEpoch = false;self.setAcceptedEpoch(epoch);connectingFollowers.notifyAll();} else {long start = Time.currentElapsedTime();long cur = start;long end = start + self.getInitLimit()*self.getTickTime();while (waitingForNewEpoch && cur < end) {//通过HashSet类型的connectingFollowers的wait和notifyAll方法，让LearnerHandler就会进行等待connectingFollowers.wait(end - cur);cur = Time.currentElapsedTime();}if (waitingForNewEpoch) {throw new InterruptedException("Timeout while waiting for epoch from quorum");}}return epoch;}}...protected final Set<Long> electingFollowers = new HashSet<Long>();protected boolean electionFinished = false;public void waitForEpochAck(long id, StateSummary ss) throws IOException, InterruptedException {synchronized(electingFollowers) {if (electionFinished) {return;}...           if (isParticipant(id)) {electingFollowers.add(id);}QuorumVerifier verifier = self.getQuorumVerifier();if (electingFollowers.contains(self.getId()) && verifier.containsQuorum(electingFollowers)) {electionFinished = true;electingFollowers.notifyAll();} else {long start = Time.currentElapsedTime();long cur = start;long end = start + self.getInitLimit()*self.getTickTime();while (!electionFinished && cur < end) {electingFollowers.wait(end - cur);cur = Time.currentElapsedTime();}...}}}...
}

七.Learner收到Leader发送的LeaderInfo后会反馈ackNewEpoch消息

Learner通过Learner的writePacket()方法向Leader发送LearnerInfo消息后，会继续通过Learner的readPacket()方法接收Leader返回的LeaderInfo响应。当Learner接收到Leader的LearnerHandler返回的LeaderInfo消息后，就会解析出epoch和ZXID，然后向Leader反馈一个ackNewEpoch响应。

public class Learner {       ...protected Socket sock;protected InputArchive leaderIs;protected OutputArchive leaderOs;  ...//3.建立和Leader服务端之间的连接protected void connectToLeader(InetSocketAddress addr, String hostname) {this.sock = createSocket();  int initLimitTime = self.tickTime * self.initLimit;int remainingInitLimitTime = initLimitTime;long startNanoTime = nanoTime();for (int tries = 0; tries < 5; tries++) {...remainingInitLimitTime = initLimitTime - (int)((nanoTime() - startNanoTime) / 1000000);if (remainingInitLimitTime <= 0) {LOG.error("initLimit exceeded on retries.");throw new IOException("initLimit exceeded on retries.");}sockConnect(sock, addr, Math.min(self.tickTime * self.syncLimit, remainingInitLimitTime));if (self.isSslQuorum())  {((SSLSocket) sock).startHandshake();}sock.setTcpNoDelay(nodelay);break;...Thread.sleep(1000);}self.authLearner.authenticate(sock, hostname);//下面定义的leaderIs和leaderOs是建立在BIO的Socket基础上的//所以后续在registerWithLeader方法中可以先往leaderOs写数据然后阻塞等待从leaderIs读数据leaderIs = BinaryInputArchive.getArchive(new BufferedInputStream(sock.getInputStream()));bufferedOutput = new BufferedOutputStream(sock.getOutputStream());leaderOs = BinaryOutputArchive.getArchive(bufferedOutput);}protected long registerWithLeader(int pktType) throws IOException {long lastLoggedZxid = self.getLastLoggedZxid();QuorumPacket qp = new QuorumPacket();                qp.setType(pktType);qp.setZxid(ZxidUtils.makeZxid(self.getAcceptedEpoch(), 0));//Add sid to payload//5.将Learner客户端自己的基本信息LearnerInfo发送给Leader服务端LearnerInfo li = new LearnerInfo(self.getId(), 0x10000, self.getQuorumVerifier().getVersion());ByteArrayOutputStream bsid = new ByteArrayOutputStream();BinaryOutputArchive boa = BinaryOutputArchive.getArchive(bsid);boa.writeRecord(li, "LearnerInfo");qp.setData(bsid.toByteArray());writePacket(qp, true);//7.接收Leader发送的包含当前集群的epoch的LeaderInfo信息readPacket(qp);        final long newEpoch = ZxidUtils.getEpochFromZxid(qp.getZxid());if (qp.getType() == Leader.LEADERINFO) {leaderProtocolVersion = ByteBuffer.wrap(qp.getData()).getInt();byte epochBytes[] = new byte[4];final ByteBuffer wrappedEpochBytes = ByteBuffer.wrap(epochBytes);if (newEpoch > self.getAcceptedEpoch()) {wrappedEpochBytes.putInt((int)self.getCurrentEpoch());self.setAcceptedEpoch(newEpoch);} else if (newEpoch == self.getAcceptedEpoch()) {wrappedEpochBytes.putInt(-1);} else {throw new IOException("Leaders epoch, " + newEpoch + " is less than accepted epoch, " + self.getAcceptedEpoch());}//7.收到Leader发送的LeaderInfo信息后反馈ackNewEpoch消息给LearnerQuorumPacket ackNewEpoch = new QuorumPacket(Leader.ACKEPOCH, lastLoggedZxid, epochBytes, null);writePacket(ackNewEpoch, true);return ZxidUtils.makeZxid(newEpoch, 0);}...}//write a packet to the leader//@param pp：the proposal packet to be sent to the leadervoid writePacket(QuorumPacket pp, boolean flush) throws IOException {synchronized (leaderOs) {if (pp != null) {leaderOs.writeRecord(pp, "packet");}if (flush) {bufferedOutput.flush();}}}//read a packet from the leader//@param pp：the packet to be instantiatedvoid readPacket(QuorumPacket pp) throws IOException {synchronized (leaderIs) {leaderIs.readRecord(pp, "packet");}long traceMask = ZooTrace.SERVER_PACKET_TRACE_MASK;if (pp.getType() == Leader.PING) {traceMask = ZooTrace.SERVER_PING_TRACE_MASK;}}...
}

八.Leader收到过半Learner的ackNewEpoch消息后开始进行数据同步

Leader每收到一个Learner的连接请求，都会启动一个LearnerHandler处理。每个LearnerHandler线程都会首先通过Leader的getEpochToPropose()方法，阻塞等待过半Learner发送LearnerInfo信息发起对Leader的注册。

当过半Learner已经向Leader进行注册后，每个LearnerHandler线程又继续发送LeaderInfo信息给Learner确认epoch，然后通过Leader的waitForEpochAck()方法，阻塞等待过半Learner返回响应。

当Leader接收到过半Learner向Leader发送的ackNewEpoch响应后，每个LearnerHandler线程便会开始执行与Learner间的数据同步，而Learner会通过Learner的syncWithLeader()方法执行与Leader的数据同步。

public class LearnerHandler extends ZooKeeperThread {final Leader leader;//ZooKeeper server identifier of this learnerprotected long sid = 0;protected final Socket sock;private BinaryInputArchive ia;private BinaryOutputArchive oa;...@Overridepublic void run() {leader.addLearnerHandler(this);tickOfNextAckDeadline = leader.self.tick.get() + leader.self.initLimit + leader.self.syncLimit;//将ia和oa与Socket进行绑定//以便当Leader通过oa发送LeaderInfo消息给Learner时，可以通过ia读取到Learner的ackNewEpoch响应ia = BinaryInputArchive.getArchive(bufferedInput);bufferedOutput = new BufferedOutputStream(sock.getOutputStream());oa = BinaryOutputArchive.getArchive(bufferedOutput);QuorumPacket qp = new QuorumPacket();ia.readRecord(qp, "packet");byte learnerInfoData[] = qp.getData();...//根据LearnerInfo信息解析出Learner的SIDByteBuffer bbsid = ByteBuffer.wrap(learnerInfoData);if (learnerInfoData.length >= 8) {this.sid = bbsid.getLong();}...//根据Learner的ZXID解析出对应Learner的epochlong lastAcceptedEpoch = ZxidUtils.getEpochFromZxid(qp.getZxid());long zxid = qp.getZxid();//将Learner的epoch和Leader的epoch进行比较//如果Learner的epoch更大，则更新Leader的epoch为Learner的epoch + 1long newEpoch = leader.getEpochToPropose(this.getSid(), lastAcceptedEpoch);long newLeaderZxid = ZxidUtils.makeZxid(newEpoch, 0);...//发送一个包含该epoch的LeaderInfo消息给该LearnerHandler对应的LearnerQuorumPacket newEpochPacket = new QuorumPacket(Leader.LEADERINFO, newLeaderZxid, ver, null);oa.writeRecord(newEpochPacket, "packet");bufferedOutput.flush();QuorumPacket ackEpochPacket = new QuorumPacket();//发送包含该epoch的LeaderInfo消息后等待Learner响应//读取Learner返回的ackNewEpoch响应ia.readRecord(ackEpochPacket, "packet");...//等待过半Learner响应leader.waitForEpochAck(this.getSid(), ss);...//下面执行与Learner的数据同步peerLastZxid = ss.getLastZxid();boolean needSnap = syncFollower(peerLastZxid, leader.zk.getZKDatabase(), leader);if (needSnap) {long zxidToSend = leader.zk.getZKDatabase().getDataTreeLastProcessedZxid();oa.writeRecord(new QuorumPacket(Leader.SNAP, zxidToSend, null, null), "packet");bufferedOutput.flush();// Dump data to peerleader.zk.getZKDatabase().serializeSnapshot(oa);oa.writeString("BenWasHere", "signature");bufferedOutput.flush();}LOG.debug("Sending NEWLEADER message to " + sid);if (getVersion() < 0x10000) {QuorumPacket newLeaderQP = new QuorumPacket(Leader.NEWLEADER, newLeaderZxid, null, null);oa.writeRecord(newLeaderQP, "packet");} else {QuorumPacket newLeaderQP = new QuorumPacket(Leader.NEWLEADER, newLeaderZxid, leader.self.getLastSeenQuorumVerifier().toString().getBytes(), null);queuedPackets.add(newLeaderQP);}bufferedOutput.flush();//Start thread that blast packets in the queue to learnerstartSendingPackets();//Have to wait for the first ACK, wait until the leader is ready, and only then we can start processing messages.qp = new QuorumPacket();ia.readRecord(qp, "packet");//阻塞等待过半Learner完成数据同步，接下来就可以启动QuorumPeer服务器实例了leader.waitForNewLeaderAck(getSid(), qp.getZxid());...while (true) {//这里有关于Leader和Learner之间保持心跳的处理}}...
}public class Leader {...protected final Set<Long> electingFollowers = new HashSet<Long>();protected boolean electionFinished = false;public void waitForEpochAck(long id, StateSummary ss) throws IOException, InterruptedException {synchronized(electingFollowers) {if (electionFinished) {return;}...           if (isParticipant(id)) {electingFollowers.add(id);}QuorumVerifier verifier = self.getQuorumVerifier();if (electingFollowers.contains(self.getId()) && verifier.containsQuorum(electingFollowers)) {electionFinished = true;electingFollowers.notifyAll();} else {long start = Time.currentElapsedTime();long cur = start;long end = start + self.getInitLimit()*self.getTickTime();while(!electionFinished && cur < end) {electingFollowers.wait(end - cur);cur = Time.currentElapsedTime();}...}}}...
}

九.过半Learner完成数据同步就启动Leader和Learner绑定的服务器实例

Follower客户端绑定了FollowerZooKeeperServer服务器实例，Leader服务端绑定了LeaderZooKeeperServer服务器实例。

在Leader的lead()方法中，首先创建Learner接收器LearnerCnxAcceptor监听Learner发起的连接请求，然后Leader的lead()方法会阻塞等待过半Learner完成向Leader的注册，接着Leader的lead()方法会阻塞等待过半Learner返回ackNewEpoch响应，接着Leader的lead()方法会阻塞等待过半Learner完成数据同步，然后执行Leader的startZkServer()方法启动Leader绑定的服务器实例，也就是执行LeaderZooKeeperServer的startup()方法启动服务器。

而在Learner进行数据同步的Learner的syncWithLeader()方法中，完成数据同步后同样会启动Learner绑定的服务器实例，也就是执行LearnerZooKeeperServer的startup()方法启动服务器。

Leader和Learner绑定的服务器实例的启动步骤，主要就是执行ZooKeeperServer的startup()方法，即：创建并启动会话管理器 + 初始化服务器的请求处理链。

public class QuorumPeer extends ZooKeeperThread implements QuorumStats.Provider {...//Follower客户端绑定了FollowerZooKeeperServer服务器实例protected Follower makeFollower(FileTxnSnapLog logFactory) throws IOException {return new Follower(this, new FollowerZooKeeperServer(logFactory, this, this.zkDb));}//Leader服务端绑定了LeaderZooKeeperServer服务器实例protected Leader makeLeader(FileTxnSnapLog logFactory) throws IOException, X509Exception {return new Leader(this, new LeaderZooKeeperServer(logFactory, this, this.zkDb));}...
}public class Leader {final LeaderZooKeeperServer zk;...void lead() throws IOException, InterruptedException {...//创建Learner接收器LearnerCnxAcceptor监听Learner发起的连接请求cnxAcceptor = new LearnerCnxAcceptor();cnxAcceptor.start();//阻塞等待过半Learner完成向Leader的注册long epoch = getEpochToPropose(self.getId(), self.getAcceptedEpoch());...//阻塞等待过半Learner返回ackNewEpoch响应waitForEpochAck(self.getId(), leaderStateSummary);...//阻塞等待过半Learner完成数据同步waitForNewLeaderAck(self.getId(), zk.getZxid());...//开始启动Leader绑定的LeaderZooKeeperServer服务器实例startZkServer();...}private synchronized void startZkServer() {...zk.startup();...}...
}public class ZooKeeperServer implements SessionExpirer, ServerStats.Provider {...public synchronized void startup() {startupWithServerState(State.RUNNING);}private void startupWithServerState(State state) {//创建并启动会话管理器if (sessionTracker == null) {createSessionTracker();}startSessionTracker();//初始化服务器的请求处理链setupRequestProcessors();registerJMX();//开启监控JVM停顿的线程startJvmPauseMonitor();setState(state);notifyAll();}...
}