zookeeper集群模式（十）zookeeper的lead流程
lead流程
Leader选举出来之后，会创建一个Leader实例，然后调用lead()方法进行lead流程，接下来看一下lead()方法：
void lead() throws IOException, InterruptedException {
self.end_fle = Time.currentElapsedTime();
long electionTimeTaken = self.end_fle - self.start_fle;
self.setElectionTimeTaken(electionTimeTaken);
ServerMetrics.getMetrics().ELECTION_TIME.add(electionTimeTaken);
LOG.info("LEADING - LEADER ELECTION TOOK - {} {}", electionTimeTaken,
QuorumPeer.FLE_TIME_UNIT);
self.start_fle = 0;
self.end_fle = 0;
zk.registerJMX(new LeaderBean(this, zk), self.jmxLocalPeerBean);
try {
self.tick.set(0);
zk.loadData();
leaderStateSummary = new StateSummary(self.getCurrentEpoch(), zk.getLastProcessedZxid());
// Start thread that waits for connection requests from
// new followers.
cnxAcceptor = new LearnerCnxAcceptor();
cnxAcceptor.start();
long epoch = getEpochToPropose(self.getId(), self.getAcceptedEpoch());
zk.setZxid(ZxidUtils.makeZxid(epoch, 0));
synchronized(this){
lastProposed = zk.getZxid();
}
newLeaderProposal.packet = new QuorumPacket(NEWLEADER, zk.getZxid(),
null, null);
if ((newLeaderProposal.packet.getZxid() & 0xffffffffL) != 0) {
LOG.info("NEWLEADER proposal has Zxid of "
+ Long.toHexString(newLeaderProposal.packet.getZxid()));
}
QuorumVerifier lastSeenQV = self.getLastSeenQuorumVerifier();
QuorumVerifier curQV = self.getQuorumVerifier();
if (curQV.getVersion() == 0 && curQV.getVersion() == lastSeenQV.getVersion()) {
// This was added in ZOOKEEPER-1783. The initial config has version 0 (not explicitly
// specified by the user; the lack of version in a config file is interpreted as version=0).
// As soon as a config is established we would like to increase its version so that it
// takes presedence over other initial configs that were not established (such as a config
// of a server trying to join the ensemble, which may be a partial view of the system, not the full config).
// We chose to set the new version to the one of the NEWLEADER message. However, before we can do that
// there must be agreement on the new version, so we can only change the version when sending/receiving UPTODATE,
// not when sending/receiving NEWLEADER. In other words, we can't change curQV here since its the committed quorum verifier,
// and there's still no agreement on the new version that we'd like to use. Instead, we use
// lastSeenQuorumVerifier which is being sent with NEWLEADER message
// so its a good way to let followers know about the new version. (The original reason for sending
// lastSeenQuorumVerifier with NEWLEADER is so that the leader completes any potentially uncommitted reconfigs
// that it finds before starting to propose operations. Here we're reusing the same code path for
// reaching consensus on the new version number.)
// It is important that this is done before the leader executes waitForEpochAck,
// so before LearnerHandlers return from their waitForEpochAck
// hence before they construct the NEWLEADER message containing
// the last-seen-quorumverifier of the leader, which we change below
try {
QuorumVerifier newQV = self.configFromString(curQV.toString());
newQV.setVersion(zk.getZxid());
self.setLastSeenQuorumVerifier(newQV, true);
} catch (Exception e) {
throw new IOException(e);
}
}
newLeaderProposal.addQuorumVerifier(self.getQuorumVerifier());
if (self.getLastSeenQuorumVerifier().getVersion() > self.getQuorumVerifier().getVersion()){
newLeaderProposal.addQuorumVerifier(self.getLastSeenQuorumVerifier());
}
// We have to get at least a majority of servers in sync with
// us. We do this by waiting for the NEWLEADER packet to get
// acknowledged
waitForEpochAck(self.getId(), leaderStateSummary);
self.setCurrentEpoch(epoch);
self.setLeaderAddressAndId(self.getQuorumAddress(), self.getId());
try {
waitForNewLeaderAck(self.getId(), zk.getZxid());
} catch (InterruptedException e) {
shutdown("Waiting for a quorum of followers, only synced with sids: [ "
+ newLeaderProposal.ackSetsToString() + " ]");
HashSet<Long> followerSet = new HashSet<Long>();
for(LearnerHandler f : getLearners()) {
if (self.getQuorumVerifier().getVotingMembers().containsKey(f.getSid())){
followerSet.add(f.getSid());
}
}
boolean initTicksShouldBeIncreased = true;
for (Proposal.QuorumVerifierAcksetPair qvAckset:newLeaderProposal.qvAcksetPairs) {
if (!qvAckset.getQuorumVerifier().containsQuorum(followerSet)) {
initTicksShouldBeIncreased = false;
break;
}
}
if (initTicksShouldBeIncreased) {
LOG.warn("Enough followers present. "+
"Perhaps the initTicks need to be increased.");
}
return;
}
startZkServer();
/**
* WARNING: do not use this for anything other than QA testing
* on a real cluster. Specifically to enable verification that quorum
* can handle the lower 32bit roll-over issue identified in
* ZOOKEEPER-1277. Without this option it would take a very long
* time (on order of a month say) to see the 4 billion writes
* necessary to cause the roll-over to occur.
*
* This field allows you to override the zxid of the server. Typically
* you'll want to set it to something like 0xfffffff0 and then
* start the quorum, run some operations and see the re-election.
*/
String initialZxid = System.getProperty("zookeeper.testingonly.initialZxid");
if (initialZxid != null) {
long zxid = Long.parseLong(initialZxid);
zk.setZxid((zk.getZxid() & 0xffffffff00000000L) | zxid);
}
if (!System.getProperty("zookeeper.leaderServes", "yes").equals("no")) {
self.setZooKeeperServer(zk);
}
self.adminServer.setZooKeeperServer(zk);
// Everything is a go, simply start counting the ticks
// WARNING: I couldn't find any wait statement on a synchronized
// block that would be notified by this notifyAll() call, so
// I commented it out
//synchronized (this) {
// notifyAll();
//}
// We ping twice a tick, so we only update the tick every other
// iteration
boolean tickSkip = true;
// If not null then shutdown this leader
String shutdownMessage = null;
while (true) {
synchronized (this) {
long start = Time.currentElapsedTime();
long cur = start;
long end = start + self.tickTime / 2;
while (cur < end) {
wait(end - cur);
cur = Time.currentElapsedTime();
}
if (!tickSkip) {
self.tick.incrementAndGet();
}
// We use an instance of SyncedLearnerTracker to
// track synced learners to make sure we still have a
// quorum of current (and potentially next pending) view.
SyncedLearnerTracker syncedAckSet = new SyncedLearnerTracker();
syncedAckSet.addQuorumVerifier(self.getQuorumVerifier());
if (self.getLastSeenQuorumVerifier() != null
&& self.getLastSeenQuorumVerifier().getVersion() > self
.getQuorumVerifier().getVersion()) {
syncedAckSet.addQuorumVerifier(self
.getLastSeenQuorumVerifier());
}
syncedAckSet.addAck(self.getId());
for (LearnerHandler f : getLearners()) {
if (f.synced()) {
syncedAckSet.addAck(f.getSid());
}
}
// check leader running status
if (!this.isRunning()) {
// set shutdown flag
shutdownMessage = "Unexpected internal error";
break;
}
if (!tickSkip && !syncedAckSet.hasAllQuorums()) {
// Lost quorum of last committed and/or last proposed
// config, set shutdown flag
shutdownMessage = "Not sufficient followers synced, only synced with sids: [ "
+ syncedAckSet.ackSetsToString() + " ]";
break;
}
tickSkip = !tickSkip;
}
for (LearnerHandler f : getLearners()) {
f.ping();
}
}
if (shutdownMessage != null) {
shutdown(shutdownMessage);
// leader goes in looking state
}
} finally {
zk.unregisterJMX(this);
}
}
void shutdown(String reason) {
LOG.info("Shutting down");
if (isShutdown) {
return;
}
LOG.info("Shutdown called",
new Exception("shutdown Leader! reason: " + reason));
if (cnxAcceptor != null) {
cnxAcceptor.halt();
}
// NIO should not accept conenctions
self.setZooKeeperServer(null);
self.adminServer.setZooKeeperServer(null);
try {
ss.close();
} catch (IOException e) {
LOG.warn("Ignoring unexpected exception during close",e);
}
self.closeAllConnections();
// shutdown the previous zk
if (zk != null) {
zk.shutdown();
}
synchronized (learners) {
for (Iterator<LearnerHandler> it = learners.iterator(); it
.hasNext();) {
LearnerHandler f = it.next();
it.remove();
f.shutdown();
}
}
isShutdown = true;
}
分析如下：
- 1.计算出选举用时并进行性能指标统计
- 2.注册JMX服务
- 3.恢复数据和会话，并创建新的数据快照
- 4.创建Leader和Learner之间通讯的Acceptor线程并启动它
- 5.获取当前集群领导纪元并计算出zxid
- 6.根据当前投票验证器版本判断是否需要更新版本为zxid
- 7.针对NEWLEADER消息创建一个提案并添加相应的投票验证器
- 8.等待initLimit * tickTime时间周期的领导纪元的ack，投票通过后设置currentEpoch并设置Leader的投票地址和serverId
- 9.等待initLimit * tickTime时间周期的NEWLEADER消息的ack，如果没有通过投票并且多数Follower与Leader已经建立了连接，那么就需要延长投票时间；然后会关闭之前创建的Acceptor线程并且清理缓存的LeaderZooKeeperServer，接下来会关闭ServerSocket并关闭所有已经建立的连接，再把LeaderZooKeeperServer停止，然后将LearnerHandler的引用清除并设置停止标志；接下来又将进入Leader选举流程
- 10.如果通过了NEWLEADER的投票，接下来启动LeaderZooKeeperServer服务
- 11.循环的在一个tickTime周期内发送两次PING消息给Follower，并建立一个跟踪Follower是否跟Leader同步的投票统计器，在发送PING消息之前需要判断是否有多数Follower跟Leader同步，如果投票未通过则会退出Leader流程，这里需要注意的是这个投票在一个tickTime周期内只会进行一次而不是两次；接下来会跟第9步时一样shutdown当前LeaderZookeeperServer并注销掉JMX服务
loadData()：恢复数据和会话
public void loadData() throws IOException, InterruptedException {
/*
* When a new leader starts executing Leader#lead, it
* invokes this method. The database, however, has been
* initialized before running leader election so that
* the server could pick its zxid for its initial vote.
* It does it by invoking QuorumPeer#getLastLoggedZxid.
* Consequently, we don't need to initialize it once more
* and avoid the penalty of loading it a second time. Not
* reloading it is particularly important for applications
* that host a large database.
*
* The following if block checks whether the database has
* been initialized or not. Note that this method is
* invoked by at least one other method:
* ZooKeeperServer#startdata.
*
* See ZOOKEEPER-1642 for more detail.
*/
if(zkDb.isInitialized()){
setZxid(zkDb.getDataTreeLastProcessedZxid());
}
else {
setZxid(zkDb.loadDataBase());
}
// Clean up dead sessions
List<Long> deadSessions = new ArrayList<>();
for (Long session : zkDb.getSessions()) {
if (zkDb.getSessionWithTimeOuts().get(session) == null) {
deadSessions.add(session);
}
}
for (long session : deadSessions) {
// XXX: Is lastProcessedZxid really the best thing to use?
killSession(session, zkDb.getDataTreeLastProcessedZxid());
}
// Make a clean snapshot
takeSnapshot();
}
分析如下：
- 1.判断zookeeper数据库ZKDataBase是否已经加载，如果没初始化则进行加载，然后便设置zxid
- 2.清理失效会话
- 3.创建新的数据快照
清理失效会话：killSession(long sessionId, long zxid)
protected void killSession(long sessionId, long zxid) {
zkDb.killSession(sessionId, zxid);
if (LOG.isTraceEnabled()) {
ZooTrace.logTraceMessage(LOG, ZooTrace.SESSION_TRACE_MASK,
"ZooKeeperServer --- killSession: 0x"
+ Long.toHexString(sessionId));
}
if (sessionTracker != null) {
sessionTracker.removeSession(sessionId);
}
}
- DataTree.killSession(long session, long zxid)
void killSession(long session, long zxid) {
// the list is already removed from the ephemerals
// so we do not have to worry about synchronizing on
// the list. This is only called from FinalRequestProcessor
// so there is no need for synchronization. The list is not
// changed here. Only create and delete change the list which
// are again called from FinalRequestProcessor in sequence.
Set<String> list = ephemerals.remove(session);
if (list != null) {
for (String path : list) {
try {
deleteNode(path, zxid);
if (LOG.isDebugEnabled()) {
LOG
.debug("Deleting ephemeral node " + path
+ " for session 0x"
+ Long.toHexString(session));
}
} catch (NoNodeException e) {
LOG.warn("Ignoring NoNodeException for path " + path
+ " while removing ephemeral for dead session 0x"
+ Long.toHexString(session));
}
}
}
}
分析如下：
- 1.循环获取失效的sessionId，然后从临时节点列表中移除相应的数据
- 2.移除之后获得一个已移除的节点路径列表，然后循环删除DataTree中的数据节点
- 3.如果存在会话管理器，那么也会移除会话管理器中的失效会话
创建新的数据快照：takeSnapshot()
public void takeSnapshot() {
takeSnapshot(false);
}
public void takeSnapshot(boolean syncSnap){
long start = Time.currentElapsedTime();
try {
txnLogFactory.save(zkDb.getDataTree(), zkDb.getSessionWithTimeOuts(), syncSnap);
} catch (IOException e) {
LOG.error("Severe unrecoverable error, exiting", e);
// This is a severe error that we cannot recover from,
// so we need to exit
System.exit(ExitCode.TXNLOG_ERROR_TAKING_SNAPSHOT.getValue());
}
long elapsed = Time.currentElapsedTime() - start;
LOG.info("Snapshot taken in " + elapsed + " ms");
ServerMetrics.getMetrics().SNAPSHOT_TIME.add(elapsed);
}
- FileTxnSnapLog.save(DataTree dataTree, ConcurrentHashMap<Long, Integer> sessionsWithTimeouts, boolean syncSnap)
public void save(DataTree dataTree,
ConcurrentHashMap<Long, Integer> sessionsWithTimeouts,
boolean syncSnap)
throws IOException {
long lastZxid = dataTree.lastProcessedZxid;
File snapshotFile = new File(snapDir, Util.makeSnapshotName(lastZxid));
LOG.info("Snapshotting: 0x{} to {}", Long.toHexString(lastZxid),
snapshotFile);
try {
snapLog.serialize(dataTree, sessionsWithTimeouts, snapshotFile, syncSnap);
} catch (IOException e) {
if (snapshotFile.length() == 0) {
/* This may be caused by a full disk. In such a case, the server
* will get stuck in a loop where it tries to write a snapshot
* out to disk, and ends up creating an empty file instead.
* Doing so will eventually result in valid snapshots being
* removed during cleanup. */
if (snapshotFile.delete()) {
LOG.info("Deleted empty snapshot file: " +
snapshotFile.getAbsolutePath());
} else {
LOG.warn("Could not delete empty snapshot file: " +
snapshotFile.getAbsolutePath());
}
} else {
/* Something else went wrong when writing the snapshot out to
* disk. If this snapshot file is invalid, when restarting,
* ZooKeeper will skip it, and find the last known good snapshot
* instead. */
}
throw e;
}
}
- FileSnap. serialize(DataTree dt, Map<Long, Integer> sessions, File snapShot, boolean fsync)
public synchronized void serialize(DataTree dt, Map<Long, Integer> sessions, File snapShot, boolean fsync)
throws IOException {
if (!close) {
try (CheckedOutputStream snapOS = SnapStream.getOutputStream(snapShot)) {
OutputArchive oa = BinaryOutputArchive.getArchive(snapOS);
FileHeader header = new FileHeader(SNAP_MAGIC, VERSION, dbId);
serialize(dt, sessions, oa, header);
SnapStream.sealStream(snapOS, oa);
lastSnapshotInfo = new SnapshotInfo(
Util.getZxidFromName(snapShot.getName(), SNAPSHOT_FILE_PREFIX),
snapShot.lastModified() / 1000);
}
}
}
- FileSnap. serialize(DataTree dt, Map<Long, Integer> sessions, OutputArchive oa, FileHeader header)
protected void serialize(DataTree dt,Map<Long, Integer> sessions,
OutputArchive oa, FileHeader header) throws IOException {
// this is really a programmatic error and not something that can
// happen at runtime
if(header==null)
throw new IllegalStateException(
"Snapshot's not open for writing: uninitialized header");
header.serialize(oa, "fileheader");
SerializeUtils.serializeSnapshot(dt,oa,sessions);
}
- SerializeUtils.serializeSnapshot(DataTree dt,OutputArchive oa,Map<Long, Integer> sessions)
public static void serializeSnapshot(DataTree dt,OutputArchive oa,
Map<Long, Integer> sessions) throws IOException {
HashMap<Long, Integer> sessSnap = new HashMap<Long, Integer>(sessions);
oa.writeInt(sessSnap.size(), "count");
for (Entry<Long, Integer> entry : sessSnap.entrySet()) {
oa.writeLong(entry.getKey().longValue(), "id");
oa.writeInt(entry.getValue().intValue(), "timeout");
}
dt.serialize(oa, "tree");
}
- SnapStream.sealStream(CheckedOutputStream os, OutputArchive oa)
public static void sealStream(CheckedOutputStream os, OutputArchive oa)
throws IOException {
long val = os.getChecksum().getValue();
oa.writeLong(val, "val");
oa.writeString("/", "path");
}
分析如下：
- 1.根据最大事务id-zxid创建一个空的快照文件
- 2.创建文件头FileHeader
- 3.将文件头、会话数量、会话列表、全部数据节点依次写入快照文件中，然后再将checkSum写入快照文件(这里可以跟第三章的反序列化相互印证)
- 4.记录当前进行快照的信息
- 5.进行快照时间的性能指标统计
获取集群领导纪元：getEpochToPropose(self.getId(), self.getAcceptedEpoch())
public long getEpochToPropose(long sid, long lastAcceptedEpoch) throws InterruptedException, IOException {
synchronized(connectingFollowers) {
if (!waitingForNewEpoch) {
return epoch;
}
if (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();
if (sid == self.getId()) {
timeStartWaitForEpoch = start;
}
long cur = start;
long end = start + self.getInitLimit()*self.getTickTime();
while(waitingForNewEpoch && cur < end && !quitWaitForEpoch) {
connectingFollowers.wait(end - cur);
cur = Time.currentElapsedTime();
}
if (waitingForNewEpoch) {
throw new InterruptedException("Timeout while waiting for epoch from quorum");
}
}
return epoch;
}
}
private void quitLeading() {
synchronized(connectingFollowers) {
quitWaitForEpoch = true;
connectingFollowers.notifyAll();
}
ServerMetrics.getMetrics().QUIT_LEADING_DUE_TO_DISLOYAL_VOTER.add(1);
LOG.info("Quit leading due to voter changed mind.");
}
分析如下：
- 1.锁定connectingFollowers列表，值得注意的是在Leader退出领导的方法quitLeading()中也进行了锁定，connectingFollowers对象进入wait时会释放锁，这时候如果退出领导了，会将quitWaitForEpoch标志设置为true并唤醒执行getEpochToPropose()方法的线程，这时候将结束while等待的过程
- 2.如果参数lastAcceptedEpoch大于等于当前缓存的epoch，则将缓存的epoch设置为lastAcceptedEpoch+1
- 3.判断这个serverId是不是投票参与者，如果是的话则将其放入connectingFollowers列表
- 4.判断connectingFollowers列表是否包含传入的serverId并且验证是否通过投票，如果满足条件的话，首先将等待标志waitingForNewEpoch设置为false并且设置acceptedEpoch为当前缓存的epoch，然后唤醒其他执行connectingFollowers对象的wait()方法而陷入休眠状态的线程，这样做将会结束其他线程while等待的过程并且退出当前方法栈
- 5.如果不满足条件的话，将陷入等待，等待周期为initTime * tickTime
waitForEpochAck(long id, StateSummary ss)：
public void waitForEpochAck(long id, StateSummary ss) throws IOException, InterruptedException {
synchronized(electingFollowers) {
if (electionFinished) {
return;
}
if (ss.getCurrentEpoch() != -1) {
if (ss.isMoreRecentThan(leaderStateSummary)) {
throw new IOException("Follower is ahead of the leader, leader summary: "
+ leaderStateSummary.getCurrentEpoch()
+ " (current epoch), "
+ leaderStateSummary.getLastZxid()
+ " (last zxid)");
}
if (ss.getLastZxid() != -1 && 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();
}
if (!electionFinished) {
throw new InterruptedException("Timeout while waiting for epoch to be acked by quorum");
}
}
}
}
- StateSummary.isMoreRecentThan(StateSummary ss)
public boolean isMoreRecentThan(StateSummary ss) {
return (currentEpoch > ss.currentEpoch) || (currentEpoch == ss.currentEpoch && lastZxid > ss.lastZxid);
}
分析如下：
- 1.锁定electingFollowers列表，并判断标志位electionFinished
- 2.判断currentEpoch与zxid是否超出当前Leader，如果超出说明Follower领先于Leader，这时会抛出异常结束lead流程然后重新进入Leader选举流程
- 3.如果这个zookeeper服务的zxid有效并且是投票者，那么将其放入electingFollowers列表
- 4.判断electingFollowers列表是否包含传入的serverId并且验证是否通过投票，如果满足条件的话，首先将结束标志位electionFinished设置为true，然后唤醒其他执行electingFollowers对象的wait()方法而陷入休眠状态的线程，这样做将会结束其他线程while等待的过程并且退出当前方法栈
- 5.如果不满足条件的话，将陷入等待，等待周期为initTime * tickTime
等待NEWLEADER消息的ack：waitForNewLeaderAck(long sid, long zxid)
public void waitForNewLeaderAck(long sid, long zxid)
throws InterruptedException {
synchronized (newLeaderProposal.qvAcksetPairs) {
if (quorumFormed) {
return;
}
long currentZxid = newLeaderProposal.packet.getZxid();
if (zxid != currentZxid) {
LOG.error("NEWLEADER ACK from sid: " + sid
+ " is from a different epoch - current 0x"
+ Long.toHexString(currentZxid) + " receieved 0x"
+ Long.toHexString(zxid));
return;
}
/*
* Note that addAck already checks that the learner
* is a PARTICIPANT.
*/
newLeaderProposal.addAck(sid);
if (newLeaderProposal.hasAllQuorums()) {
quorumFormed = true;
newLeaderProposal.qvAcksetPairs.notifyAll();
} else {
long start = Time.currentElapsedTime();
long cur = start;
long end = start + self.getInitLimit() * self.getTickTime();
while (!quorumFormed && cur < end) {
newLeaderProposal.qvAcksetPairs.wait(end - cur);
cur = Time.currentElapsedTime();
}
if (!quorumFormed) {
throw new InterruptedException(
"Timeout while waiting for NEWLEADER to be acked by quorum");
}
}
}
}
分析如下：
- 1.锁定newLeaderProposal.qvAcksetPairs列表，并判断标志位quorumFormed
- 2.判断其他Follower服务发来的zxid与当前集群中Leader的zxid是否相等，如果不相等说明处于不同纪元则会直接退出当前方法栈，如果是正常同意了NEWLEADER请求的话，Follower服务发来的zxid一定是等于Leader的zxid；如果相等则计入票数
- 3.验证NEWLEADER提案是否通过投票，如果通过投票，首先将标志位quorumFormed设置为true，然后唤醒其他执行newLeaderProposal.qvAcksetPairs对象的wait()方法而陷入休眠状态的线程，这样做将会结束其他线程while等待的过程并且退出当前方法栈
- 4.如果没通过投票，将陷入等待，等待周期为initTime * tickTime
启动LeaderZookeeperServer：startZkServer()
private synchronized void startZkServer() {
// Update lastCommitted and Db's zxid to a value representing the new epoch
lastCommitted = zk.getZxid();
LOG.info("Have quorum of supporters, sids: [ "
+ newLeaderProposal.ackSetsToString()
+ " ]; starting up and setting last processed zxid: 0x{}",
Long.toHexString(zk.getZxid()));
/*
* ZOOKEEPER-1324. the leader sends the new config it must complete
* to others inside a NEWLEADER message (see LearnerHandler where
* the NEWLEADER message is constructed), and once it has enough
* acks we must execute the following code so that it applies the
* config to itself.
*/
QuorumVerifier newQV = self.getLastSeenQuorumVerifier();
Long designatedLeader = getDesignatedLeader(newLeaderProposal, zk.getZxid());
self.processReconfig(newQV, designatedLeader, zk.getZxid(), true);
if (designatedLeader != self.getId()) {
allowedToCommit = false;
}
leaderStartTime = Time.currentElapsedTime();
zk.startup();
/*
* Update the election vote here to ensure that all members of the
* ensemble report the same vote to new servers that start up and
* send leader election notifications to the ensemble.
*
* @see https://issues.apache.org/jira/browse/ZOOKEEPER-1732
*/
self.updateElectionVote(getEpoch());
zk.getZKDatabase().setlastProcessedZxid(zk.getZxid());
}
private long getDesignatedLeader(Proposal reconfigProposal, long zxid) {
//new configuration
Proposal.QuorumVerifierAcksetPair newQVAcksetPair = reconfigProposal.qvAcksetPairs.get(reconfigProposal.qvAcksetPairs.size()-1);
//check if I'm in the new configuration with the same quorum address -
// if so, I'll remain the leader
if (newQVAcksetPair.getQuorumVerifier().getVotingMembers().containsKey(self.getId()) &&
newQVAcksetPair.getQuorumVerifier().getVotingMembers().get(self.getId()).addr.equals(self.getQuorumAddress())){
return self.getId();
}
// start with an initial set of candidates that are voters from new config that
// acknowledged the reconfig op (there must be a quorum). Choose one of them as
// current leader candidate
HashSet<Long> candidates = new HashSet<Long>(newQVAcksetPair.getAckset());
candidates.remove(self.getId()); // if we're here, I shouldn't be the leader
long curCandidate = candidates.iterator().next();
//go over outstanding ops in order, and try to find a candidate that acked the most ops.
//this way it will be the most up-to-date and we'll minimize the number of ops that get dropped
long curZxid = zxid + 1;
Proposal p = outstandingProposals.get(curZxid);
while (p!=null && !candidates.isEmpty()) {
for (Proposal.QuorumVerifierAcksetPair qvAckset: p.qvAcksetPairs){
//reduce the set of candidates to those that acknowledged p
candidates.retainAll(qvAckset.getAckset());
//no candidate acked p, return the best candidate found so far
if (candidates.isEmpty()) return curCandidate;
//update the current candidate, and if it is the only one remaining, return it
curCandidate = candidates.iterator().next();
if (candidates.size() == 1) return curCandidate;
}
curZxid++;
p = outstandingProposals.get(curZxid);
}
return curCandidate;
}
分析如下：
- 1.更新lastCommitted为当前zxid，获取lastSeenQuorumVerifier
- 2.通过newLeaderProposal（即NEWLEADER提案）检查当前Leader是否在具有相同投票地址的新配置中，如果是的话直接返回当前Leader的serverId
- 3.如果不是代表新配置已经移除了当前Leader或者修改了投票地址，这时候会把NEWLEADER提案发送了同意选票（即发送了ack）的serverId放入一个新的列表中并把当前Leader的serverId移除，之后将会从这个列表中选出一个作为Leader的候选server
- 4.依次从提案列表outstandingProposals中获取比zxid更大的提案Proposal，然后尝试找到一个对这些提案相似度最高（即对这些提案全部都发送过ack的server列表中获取一个）的候选server
- 5.执行reconfig流程
- 6.真正启动一个可以对客户端提供服务的zookeeper server