本文详细分析了ZooKeeper的服务器启动过程，重点讲解了FastLeaderElection选举策略的实现，包括选举信息的发送与接收。同时，深入探讨了ZooKeeper的Watcher机制，描述了从客户端注册到触发事件的完整流程。
zookeeper源码分析
（个人学习笔记，如有错误欢迎指正！！！）
服务器启动
首先zookeeper的服务器启动类为 org.apache.zookeeper.server.quorum.QuorumPeerMain该类中包含 main()方法：
public static void main(String[] args) {
QuorumPeerMain main = new QuorumPeerMain();
try {
//从名字上可以分析出，该方法是初始化以及运行
main.initializeAndRun(args);
} catch (IllegalArgumentException e) {
......
} catch (ConfigException e) {
......
} catch (Exception e) {
......
}
LOG.info("Exiting normally");
System.exit(0);
}
protected void initializeAndRun(String[] args)
throws ConfigException, IOException{
QuorumPeerConfig config = new QuorumPeerConfig();
if (args.length == 1) {
//分析传入的参数
config.parse(args[0]);
}
......
if (args.length == 1 && config.servers.size() > 0) {
//根据配置文件，进行相应的运行操作
runFromConfig(config);
} else {
......
ZooKeeperServerMain.main(args);
}
}
public void runFromConfig(QuorumPeerConfig config) throws IOException {
......
try {
//根据配置，对quorumPeer进行相应的信息设置，其中一般quorumPeer对应一个server
ServerCnxnFactory cnxnFactory = ServerCnxnFactory.createFactory();
cnxnFactory.configure(config.getClientPortAddress(),
config.getMaxClientCnxns());
quorumPeer = new QuorumPeer();
quorumPeer.setClientPortAddress(config.getClientPortAddress());
......
//启动server
quorumPeer.start();
quorumPeer.join();
} catch (InterruptedException e) {
// warn, but generally this is ok
LOG.warn("Quorum Peer interrupted", e);
}
}其中，org.apache.zookeeper.server.quorum.QuorumPeer间接继承了 java.lang.Thread类，所以该类可以看作是一个线程，start()方法便是线程的启动方法。
@Override
public synchronized void start() {
loadDataBase();
//启动客户端的监听服务
cnxnFactory.start();
//该方法用来启动leader选举，server在刚启动之后，需要通过leader选举来确定自己在集群中的身份
startLeaderElection();
super.start();
}
synchronized public void startLeaderElection() {
try {
//将自己设置为推荐者，（自己的id，当前最大的事务id，当前的选举轮数）
currentVote = new Vote(myid, getLastLoggedZxid(), getCurrentEpoch());
} catch(IOException e) {
......
}
//遍历所有的server，保存自己的地址信息
for (QuorumServer p : getView().values()) {
if (p.id == myid) {
myQuorumAddr = p.addr;
break;
}
}
if (myQuorumAddr == null) {
......
}
//electionType的信息在前面配置quorumPeer信息时，已经通过config进行了设置，默认值是3，也就 //是说该部分不执行
if (electionType == 0) {
try {
udpSocket = new DatagramSocket(myQuorumAddr.getPort());
responder = new ResponderThread();
responder.start();
} catch (SocketException e) {
throw new RuntimeException(e);
}
}
//根据electionType的值，创建选举算法
this.electionAlg = createElectionAlgorithm(electionType);
}因为electionType的默认值为3，所以默认使用的leader选举策略为 FastLeaderElection。
protected Election createElectionAlgorithm(int electionAlgorithm){
Election le=null;
//TODO: use a factory rather than a switch
switch (electionAlgorithm) {
case 0:
le = new LeaderElection(this);
break;
case 1:
le = new AuthFastLeaderElection(this);
break;
case 2:
le = new AuthFastLeaderElection(this, true);
break;
case 3:
//QuorumCnxManager实例主要负责将信息发送给指定的server和接受来自其他server的信息
//该实例中维护了两种线程SendWorker和RecvWorker，分别用于发送数据和接受数据
qcm = new QuorumCnxManager(this);
QuorumCnxManager.Listener listener = qcm.listener;
if(listener != null){
listener.start();
le = new FastLeaderElection(this, qcm);
} else {
LOG.error("Null listener when initializing cnx manager");
}
break;
default:
assert false;
}
return le;
}创建完leader的选举策略之后，便调用了 super.start()方法，所以便会执行 run()方法：
@Override
public void run() {
setName("QuorumPeer" + "[myid=" + getId() + "]" +
cnxnFactory.getLocalAddress());
LOG.debug("Starting quorum peer");
try {
jmxQuorumBean = new QuorumBean(this);
MBeanRegistry.getInstance().register(jmxQuorumBean, null);
for(QuorumServer s: getView().values()){
ZKMBeanInfo p;
if (getId() == s.id) {
p = jmxLocalPeerBean = new LocalPeerBean(this);
try {
MBeanRegistry.getInstance().register(p, jmxQuorumBean);
} catch (Exception e) {
LOG.warn("Failed to register with JMX", e);
jmxLocalPeerBean = null;
}
} else {
p = new RemotePeerBean(s);
try {
MBeanRegistry.getInstance().register(p, jmxQuorumBean);
} catch (Exception e) {
LOG.warn("Failed to register with JMX", e);
}
}
}
} catch (Exception e) {
LOG.warn("Failed to register with JMX", e);
jmxQuorumBean = null;
}
try {
//主循环，主要是根据自身的身份，作相应的循环操作
while (running) {
switch (getPeerState()) {
case LOOKING:
LOG.info("LOOKING");
if (Boolean.getBoolean("readonlymode.enabled")) {
LOG.info("Attempting to start ReadOnlyZooKeeperServer");
// Create read-only server but don't start it immediately
final ReadOnlyZooKeeperServer roZk = new ReadOnlyZooKeeperServer(
logFactory, this,
new ZooKeeperServer.BasicDataTreeBuilder(),
this.zkDb);
// Instead of starting roZk immediately, wait some grace
// period before we decide we're partitioned.
//
// Thread is used here because otherwise it would require
// changes in each of election strategy classes which is
// unnecessary code coupling.
Thread roZkMgr = new Thread() {
public void run() {
try {
// lower-bound grace period to 2 secs
sleep(Math.max(2000, tickTime));
if (ServerState.LOOKING.equals(getPeerState())) {
roZk.startup();
}
} catch (InterruptedException e) {
LOG.info("Interrupted while attempting to start ReadOnlyZooKeeperServer, not started");
} catch (Exception e) {
LOG.error("FAILED to start ReadOnlyZooKeeperServer", e);
}
}
};
try {
roZkMgr.start();
setBCVote(null);
setCurrentVote(makeLEStrategy().lookForLeader());
} catch (Exception e) {
LOG.warn("Unexpected exception",e);
setPeerState(ServerState.LOOKING);
} finally {
// If the thread is in the the grace period, interrupt
// to come out of waiting.
roZkMgr.interrupt();
roZk.shutdown();
}
} else {
try {
setBCVote(null);
setCurrentVote(makeLEStrategy().lookForLeader());
} catch (Exception e) {
LOG.warn("Unexpected exception", e);
setPeerState(ServerState.LOOKING);
}
}
break;
case OBSERVING:
try {
LOG.info("OBSERVING");
setObserver(makeObserver(logFactory));
observer.observeLeader();
} catch (Exception e) {
LOG.warn("Unexpected exception",e );
} finally {
observer.shutdown();
setObserver(null);
setPeerState(ServerState.LOOKING);
}
break;
case FOLLOWING:
try {
LOG.info("FOLLOWING");
setFollower(makeFollower(logFactory));
follower.followLeader();
} catch (Exception e) {
LOG.warn("Unexpected exception",e);
} finally {
follower.shutdown();
setFollower(null);
setPeerState(ServerState.LOOKING);
}
break;
case LEADING:
LOG.info("LEADING");
try {
setLeader(makeLeader(logFactory));
leader.lead();
setLeader(null);
} catch (Exception e) {
LOG.warn("Unexpected exception",e);
} finally {
if (leader != null) {
leader.shutdown("Forcing shutdown");
setLeader(null);
}
setPeerState(ServerState.LOOKING);
}
break;
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