ZooKeeper（三）：请求处理链路的创建过程解析
我们知道，zk就是一个个处理链组成的。
但是，这些处理链是在什么创建的呢？
ZooKeeper 中有三种角色的服务节点存在: Leader, Follower, Observer .
而每个服务节点的承担的任务是不一样的，所以处理任务的逻辑是不一样的。而在ZK中，则是巧妙的通过责任链模式将各自节点的处理能力建立起来的。
而这个创建时机是在什么时候呢？服务一启动的时候？还是每个请求进来的时候？
其实ZK服务节点的处理链路是在角色被确定下来了之后，才创建的！我们一起看一下过程！
一、选举线程会一直工作
QuorumPeer 线程会一直循环检查当前节点的状态，当当前节点的决定确认之后，其处理链自然就定了！
所以，我们需要先来看一下他是如何处理当前角色的先！
// org.apache.zookeeper.server.quorum.QuorumPeer#run
@Override
public void run() {
updateThreadName(); // 注册jmx 度量信息
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 {
RemotePeerBean rBean = new RemotePeerBean(this, s);
try {
MBeanRegistry.getInstance().register(rBean, jmxQuorumBean);
jmxRemotePeerBean.put(s.id, rBean);
} 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 {
/*
* Main loop
*/
// 死循环，一直检测当前节点的状态，当确认角色后，进行正式工作循环
// 当状态再次变更时，会抛出相应异常，从而进行重新选举操作
while (running) {
switch (getPeerState()) {
// LOOKING 将触发选举，很关键，但我们后续再来解析这块东西
case LOOKING:
LOG.info("LOOKING");
ServerMetrics.getMetrics().LOOKING_COUNT.add(1); 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, 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();
reconfigFlagClear();
if (shuttingDownLE) {
shuttingDownLE = false;
startLeaderElection();
}
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 {
reconfigFlagClear();
if (shuttingDownLE) {
shuttingDownLE = false;
startLeaderElection();
}
setCurrentVote(makeLEStrategy().lookForLeader());
} catch (Exception e) {
LOG.warn("Unexpected exception", e);
setPeerState(ServerState.LOOKING);
}
}
break;
// OBSERVING 节点只会接收来自 Leader 的数据
case OBSERVING:
try {
LOG.info("OBSERVING");
setObserver(makeObserver(logFactory));
observer.observeLeader();
} catch (Exception e) {
LOG.warn("Unexpected exception", e);
} finally {
observer.shutdown();
setObserver(null);
updateServerState(); // Add delay jitter before we switch to LOOKING
// state to reduce the load of ObserverMaster
if (isRunning()) {
Observer.waitForObserverElectionDelay();
}
}
break;
// FOLLOWING 节点将参与数据的决策
case FOLLOWING:
try {
LOG.info("FOLLOWING");
setFollower(makeFollower(logFactory));
follower.followLeader();
} catch (Exception e) {
LOG.warn("Unexpected exception", e);
} finally {
follower.shutdown();
setFollower(null);
updateServerState();
}
break;
// LEADING 负责数据的写入，以及维护各Follower的数据同步等
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);
}
updateServerState();
}
break;
}
}
} finally {
// 最终退出，可能是异常退出，也可能是主动关闭了（running=false）,总之做好记录、清理工作
LOG.warn("QuorumPeer main thread exited");
MBeanRegistry instance = MBeanRegistry.getInstance();
instance.unregister(jmxQuorumBean);
instance.unregister(jmxLocalPeerBean); for (RemotePeerBean remotePeerBean : jmxRemotePeerBean.values()) {
instance.unregister(remotePeerBean);
} jmxQuorumBean = null;
jmxLocalPeerBean = null;
jmxRemotePeerBean = null;
}
}
总体来说就是，始终会有一个后台线程，一直去检查当前节点的状态，然后根据状态去分派任务。
当任务分派下去之后，没有变化就不要返回了，即一直处理自己任务即可。只要不发生异常，它会一直保持下去。
总共有四个状态: LOOKING, OBSERVING, FOLLOWING, LEADING, 也字如其义，做各自的工作。
二、Follower 的链路创建过程
// 如下: 1. 创建 Follower 实例; 2. 调用 followLeader() 执行业务逻辑;
case FOLLOWING:
try {
LOG.info("FOLLOWING");
// 先创建 Follower 实例
setFollower(makeFollower(logFactory));
// 然后调用 followLeader() 进行处理
// 当然，它一般会进行循环处理任务，而不是处理完一次后就退出
follower.followLeader();
} catch (Exception e) {
LOG.warn("Unexpected exception", e);
} finally {
follower.shutdown();
setFollower(null);
updateServerState();
}
break;
// org.apache.zookeeper.server.quorum.QuorumPeer#makeFollower
protected Follower makeFollower(FileTxnSnapLog logFactory) throws IOException {
// FollowerZooKeeperServer 是个关键的实例
// 将上下文传递到 Follower 中，备用
return new Follower(this, new FollowerZooKeeperServer(logFactory, this, this.zkDb));
}
// org.apache.zookeeper.server.quorum.FollowerZooKeeperServer#FollowerZooKeeperServer
FollowerZooKeeperServer(FileTxnSnapLog logFactory, QuorumPeer self, ZKDatabase zkDb) throws IOException {
super(logFactory, self.tickTime, self.minSessionTimeout, self.maxSessionTimeout, self.clientPortListenBacklog, zkDb, self);
// 使用 ConcurrentLinkedQueue 队列进行消息传递
this.pendingSyncs = new ConcurrentLinkedQueue<Request>();
} // 创建好 follower 实例后，就调用其主要方法 followLeader()
// 这里承担了所有的 follower 的动作指令
/**
* the main method called by the follower to follow the leader
*
* @throws InterruptedException
*/
void followLeader() throws 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("FOLLOWING - LEADER ELECTION TOOK - {} {}", electionTimeTaken, QuorumPeer.FLE_TIME_UNIT);
self.start_fle = 0;
self.end_fle = 0;
fzk.registerJMX(new FollowerBean(this, zk), self.jmxLocalPeerBean); long connectionTime = 0;
boolean completedSync = false; // 先做一些准备工作
try {
self.setZabState(QuorumPeer.ZabState.DISCOVERY);
QuorumServer leaderServer = findLeader();
try {
connectToLeader(leaderServer.addr, leaderServer.hostname);
connectionTime = System.currentTimeMillis();
long newEpochZxid = registerWithLeader(Leader.FOLLOWERINFO);
if (self.isReconfigStateChange()) {
throw new Exception("learned about role change");
}
//check to see if the leader zxid is lower than ours
//this should never happen but is just a safety check
long newEpoch = ZxidUtils.getEpochFromZxid(newEpochZxid);
if (newEpoch < self.getAcceptedEpoch()) {
LOG.error("Proposed leader epoch "
+ ZxidUtils.zxidToString(newEpochZxid)
+ " is less than our accepted epoch "
+ ZxidUtils.zxidToString(self.getAcceptedEpoch()));
throw new IOException("Error: Epoch of leader is lower");
}
long startTime = Time.currentElapsedTime();
try {
self.setLeaderAddressAndId(leaderServer.addr, leaderServer.getId());
self.setZabState(QuorumPeer.ZabState.SYNCHRONIZATION);
// 与 Leader 建立同步，即发起 Socket 连接
syncWithLeader(newEpochZxid);
self.setZabState(QuorumPeer.ZabState.BROADCAST);
completedSync = true;
} finally {
long syncTime = Time.currentElapsedTime() - startTime;
ServerMetrics.getMetrics().FOLLOWER_SYNC_TIME.add(syncTime);
}
// 如果设置了 observeMasterPort() 则开启相应处理线程
if (self.getObserverMasterPort() > 0) {
LOG.info("Starting ObserverMaster"); om = new ObserverMaster(self, fzk, self.getObserverMasterPort());
om.start();
} else {
om = null;
}
// create a reusable packet to reduce gc impact
QuorumPacket qp = new QuorumPacket();
// 会在此处一直循环等待 leader 节点的信息，进行任务处理，直到发生异常退出
while (this.isRunning()) {
readPacket(qp);
processPacket(qp);
}
} catch (Exception e) {
LOG.warn("Exception when following the leader", e);
closeSocket(); // clear pending revalidations
pendingRevalidations.clear();
}
} finally {
if (om != null) {
om.stop();
}
zk.unregisterJMX(this); if (connectionTime != 0) {
long connectionDuration = System.currentTimeMillis() - connectionTime;
LOG.info(
"Disconnected from leader (with address: {}). Was connected for {}ms. Sync state: {}",
leaderAddr,
connectionDuration,
completedSync);
messageTracker.dumpToLog(leaderAddr.toString());
}
}
} // org.apache.zookeeper.server.quorum.Learner#syncWithLeader
/**
* Finally, synchronize our history with the Leader (if Follower)
* or the LearnerMaster (if Observer).
* @param newLeaderZxid
* @throws IOException
* @throws InterruptedException
*/
protected void syncWithLeader(long newLeaderZxid) throws Exception {
QuorumPacket ack = new QuorumPacket(Leader.ACK, 0, null, null);
QuorumPacket qp = new QuorumPacket();
long newEpoch = ZxidUtils.getEpochFromZxid(newLeaderZxid); QuorumVerifier newLeaderQV = null; // In the DIFF case we don't need to do a snapshot because the transactions will sync on top of any existing snapshot
// For SNAP and TRUNC the snapshot is needed to save that history
boolean snapshotNeeded = true;
boolean syncSnapshot = false;
readPacket(qp);
Deque<Long> packetsCommitted = new ArrayDeque<>();
Deque<PacketInFlight> packetsNotCommitted = new ArrayDeque<>();
synchronized (zk) {
if (qp.getType() == Leader.DIFF) {
LOG.info("Getting a diff from the leader 0x{}", Long.toHexString(qp.getZxid()));
self.setSyncMode(QuorumPeer.SyncMode.DIFF);
snapshotNeeded = false;
} else if (qp.getType() == Leader.SNAP) {
self.setSyncMode(QuorumPeer.SyncMode.SNAP);
LOG.info("Getting a snapshot from leader 0x{}", Long.toHexString(qp.getZxid()));
// The leader is going to dump the database
// db is clear as part of deserializeSnapshot()
zk.getZKDatabase().deserializeSnapshot(leaderIs);
// ZOOKEEPER-2819: overwrite config node content extracted
// from leader snapshot with local config, to avoid potential
// inconsistency of config node content during rolling restart.
if (!QuorumPeerConfig.isReconfigEnabled()) {
LOG.debug("Reset config node content from local config after deserialization of snapshot.");
zk.getZKDatabase().initConfigInZKDatabase(self.getQuorumVerifier());
}
String signature = leaderIs.readString("signature");
if (!signature.equals("BenWasHere")) {
LOG.error("Missing signature. Got {}", signature);
throw new IOException("Missing signature");
}
zk.getZKDatabase().setlastProcessedZxid(qp.getZxid()); // immediately persist the latest snapshot when there is txn log gap
syncSnapshot = true;
} else if (qp.getType() == Leader.TRUNC) {
//we need to truncate the log to the lastzxid of the leader
self.setSyncMode(QuorumPeer.SyncMode.TRUNC);
LOG.warn("Truncating log to get in sync with the leader 0x{}", Long.toHexString(qp.getZxid()));
boolean truncated = zk.getZKDatabase().truncateLog(qp.getZxid());
if (!truncated) {
// not able to truncate the log
LOG.error("Not able to truncate the log 0x{}", Long.toHexString(qp.getZxid()));
System.exit(ExitCode.QUORUM_PACKET_ERROR.getValue());
}
zk.getZKDatabase().setlastProcessedZxid(qp.getZxid()); } else {
LOG.error("Got unexpected packet from leader: {}, exiting ... ", LearnerHandler.packetToString(qp));
System.exit(ExitCode.QUORUM_PACKET_ERROR.getValue()); }
zk.getZKDatabase().initConfigInZKDatabase(self.getQuorumVerifier());
zk.createSessionTracker(); long lastQueued = 0; // in Zab V1.0 (ZK 3.4+) we might take a snapshot when we get the NEWLEADER message, but in pre V1.0
// we take the snapshot on the UPDATE message, since Zab V1.0 also gets the UPDATE (after the NEWLEADER)
// we need to make sure that we don't take the snapshot twice.
boolean isPreZAB1_0 = true;
//If we are not going to take the snapshot be sure the transactions are not applied in memory
// but written out to the transaction log
boolean writeToTxnLog = !snapshotNeeded;
// we are now going to start getting transactions to apply followed by an UPTODATE
outerLoop:
while (self.isRunning()) {
readPacket(qp);
switch (qp.getType()) {
case Leader.PROPOSAL:
PacketInFlight pif = new PacketInFlight();
pif.hdr = new TxnHeader();
pif.rec = SerializeUtils.deserializeTxn(qp.getData(), pif.hdr);
if (pif.hdr.getZxid() != lastQueued + 1) {
LOG.warn(
"Got zxid 0x{} expected 0x{}",
Long.toHexString(pif.hdr.getZxid()),
Long.toHexString(lastQueued + 1));
}
lastQueued = pif.hdr.getZxid(); if (pif.hdr.getType() == OpCode.reconfig) {
SetDataTxn setDataTxn = (SetDataTxn) pif.rec;
QuorumVerifier qv = self.configFromString(new String(setDataTxn.getData()));
self.setLastSeenQuorumVerifier(qv, true);
} packetsNotCommitted.add(pif);
break;
case Leader.COMMIT:
case Leader.COMMITANDACTIVATE:
pif = packetsNotCommitted.peekFirst();
if (pif.hdr.getZxid() == qp.getZxid() && qp.getType() == Leader.COMMITANDACTIVATE) {
QuorumVerifier qv = self.configFromString(new String(((SetDataTxn) pif.rec).getData()));
boolean majorChange = self.processReconfig(
qv,
ByteBuffer.wrap(qp.getData()).getLong(), qp.getZxid(),
true);
if (majorChange) {
throw new Exception("changes proposed in reconfig");
}
}
if (!writeToTxnLog) {
if (pif.hdr.getZxid() != qp.getZxid()) {
LOG.warn(
"Committing 0x{}, but next proposal is 0x{}",
Long.toHexString(qp.getZxid()),
Long.toHexString(pif.hdr.getZxid()));
} else {
zk.processTxn(pif.hdr, pif.rec);
packetsNotCommitted.remove();
}
} else {
packetsCommitted.add(qp.getZxid());
}
break;
case Leader.INFORM:
case Leader.INFORMANDACTIVATE:
PacketInFlight packet = new PacketInFlight();
packet.hdr = new TxnHeader(); if (qp.getType() == Leader.INFORMANDACTIVATE) {
ByteBuffer buffer = ByteBuffer.wrap(qp.getData());
long suggestedLeaderId = buffer.getLong();
byte[] remainingdata = new byte[buffer.remaining()];
buffer.get(remainingdata);
packet.rec = SerializeUtils.deserializeTxn(remainingdata, packet.hdr);
QuorumVerifier qv = self.configFromString(new String(((SetDataTxn) packet.rec).getData()));
boolean majorChange = self.processReconfig(qv, suggestedLeaderId, qp.getZxid(), true);
if (majorChange) {
throw new Exception("changes proposed in reconfig");
}
} else {
packet.rec = SerializeUtils.deserializeTxn(qp.getData(), packet.hdr);
// Log warning message if txn comes out-of-order
if (packet.hdr.getZxid() != lastQueued + 1) {
LOG.warn(
"Got zxid 0x{} expected 0x{}",
Long.toHexString(packet.hdr.getZxid()),
Long.toHexString(lastQueued + 1));
}
lastQueued = packet.hdr.getZxid();
}
if (!writeToTxnLog) {
// Apply to db directly if we haven't taken the snapshot
zk.processTxn(packet.hdr, packet.rec);
} else {
packetsNotCommitted.add(packet);
packetsCommitted.add(qp.getZxid());
} break;
// 无需更新数据时，直接退出 while
case Leader.UPTODATE:
LOG.info("Learner received UPTODATE message");
if (newLeaderQV != null) {
boolean majorChange = self.processReconfig(newLeaderQV, null, null, true);
if (majorChange) {
throw new Exception("changes proposed in reconfig");
}
}
if (isPreZAB1_0) {
zk.takeSnapshot(syncSnapshot);
self.setCurrentEpoch(newEpoch);
}
self.setZooKeeperServer(zk);
self.adminServer.setZooKeeperServer(zk);
break outerLoop;
case Leader.NEWLEADER: // Getting NEWLEADER here instead of in discovery
// means this is Zab 1.0
LOG.info("Learner received NEWLEADER message");
if (qp.getData() != null && qp.getData().length > 1) {
try {
QuorumVerifier qv = self.configFromString(new String(qp.getData()));
self.setLastSeenQuorumVerifier(qv, true);
newLeaderQV = qv;
} catch (Exception e) {
e.printStackTrace();
}
} if (snapshotNeeded) {
zk.takeSnapshot(syncSnapshot);
} self.setCurrentEpoch(newEpoch);
writeToTxnLog = true; //Anything after this needs to go to the transaction log, not applied directly in memory
isPreZAB1_0 = false;
writePacket(new QuorumPacket(Leader.ACK, newLeaderZxid, null, null), true);
break;
}
}
}
ack.setZxid(ZxidUtils.makeZxid(newEpoch, 0));
writePacket(ack, true);
sock.setSoTimeout(self.tickTime * self.syncLimit);
self.setSyncMode(QuorumPeer.SyncMode.NONE);
// 将 zk 拉起来
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(newEpoch); // We need to log the stuff that came in between the snapshot and the uptodate
if (zk instanceof FollowerZooKeeperServer) {
FollowerZooKeeperServer fzk = (FollowerZooKeeperServer) zk;
for (PacketInFlight p : packetsNotCommitted) {
fzk.logRequest(p.hdr, p.rec);
}
for (Long zxid : packetsCommitted) {
fzk.commit(zxid);
}
} else if (zk instanceof ObserverZooKeeperServer) {
// Similar to follower, we need to log requests between the snapshot
// and UPTODATE
ObserverZooKeeperServer ozk = (ObserverZooKeeperServer) zk;
for (PacketInFlight p : packetsNotCommitted) {
Long zxid = packetsCommitted.peekFirst();
if (p.hdr.getZxid() != zxid) {
// log warning message if there is no matching commit
// old leader send outstanding proposal to observer
LOG.warn(
"Committing 0x{}, but next proposal is 0x{}",
Long.toHexString(zxid),
Long.toHexString(p.hdr.getZxid()));
continue;
}
packetsCommitted.remove();
Request request = new Request(null, p.hdr.getClientId(), p.hdr.getCxid(), p.hdr.getType(), null, null);
request.setTxn(p.rec);
request.setHdr(p.hdr);
ozk.commitRequest(request);
}
} else {
// New server type need to handle in-flight packets
throw new UnsupportedOperationException("Unknown server type");
}
}
// org.apache.zookeeper.server.ZooKeeperServer#startup
public synchronized void startup() {
if (sessionTracker == null) {
createSessionTracker();
}
startSessionTracker();
// 此实现由子类实现,创建处理链
setupRequestProcessors(); startRequestThrottler(); registerJMX(); startJvmPauseMonitor(); registerMetrics(); setState(State.RUNNING); requestPathMetricsCollector.start(); localSessionEnabled = sessionTracker.isLocalSessionsEnabled();
notifyAll();
} // org.apache.zookeeper.server.quorum.FollowerZooKeeperServer#setupRequestProcessors
@Override
protected void setupRequestProcessors() {
//最后一个处理器是 FinalRequestProcessor
RequestProcessor finalProcessor = new FinalRequestProcessor(this);
// 倒数第二个处理器是 CommitProcessor
commitProcessor = new CommitProcessor(finalProcessor, Long.toString(getServerId()), true, getZooKeeperServerListener());
commitProcessor.start();
// 倒数第三个处理器是 FollowerRequestProcessor
// 所以，最终的调用链是 FollowerRequestProcessor -> CommitProcessor -> FinalRequestProcessor
firstProcessor = new FollowerRequestProcessor(this, commitProcessor);
((FollowerRequestProcessor) firstProcessor).start();
// 另外还有一个 SyncRequestProcessor 单独启动
syncProcessor = new SyncRequestProcessor(this, new SendAckRequestProcessor(getFollower()));
syncProcessor.start();
}
三、Leader 的处理链的建立
// LEADING 时，创建 Leader 实例， 调用 lead() 处理，仍然会存在内部处理，抛出异常后进行重新获取状态判定
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);
}
updateServerState();
}
break;
}
// org.apache.zookeeper.server.quorum.QuorumPeer#makeLeader
protected Leader makeLeader(FileTxnSnapLog logFactory) throws IOException, X509Exception {
// LeaderZooKeeperServer 负责进行通信类，数据类操作
// Leader 本身则只进行调度服务
return new Leader(this, new LeaderZooKeeperServer(logFactory, this, this.zkDb));
} // org.apache.zookeeper.server.quorum.Leader#lead
/**
* This method is main function that is called to lead
*
* @throws IOException
* @throws InterruptedException
*/
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; // 环境初始化
zk.registerJMX(new LeaderBean(this, zk), self.jmxLocalPeerBean); try {
self.setZabState(QuorumPeer.ZabState.DISCOVERY);
self.tick.set(0);
zk.loadData(); leaderStateSummary = new StateSummary(self.getCurrentEpoch(), zk.getLastProcessedZxid()); // Start thread that waits for connection requests from
// new followers.
// 开启服务端接收数据线程， sock 端口已在构造函数中初始化好了
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());
self.setZabState(QuorumPeer.ZabState.SYNCHRONIZATION); 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;
} // 开启服务器，进入链路创建
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.setZabState(QuorumPeer.ZabState.BROADCAST);
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);
}
} // org.apache.zookeeper.server.quorum.Leader#startZkServer
/**
* Start up Leader ZooKeeper server and initialize zxid to the new epoch
*/
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: [{}]; starting up and setting last processed zxid: 0x{}",
newLeaderProposal.ackSetsToString(),
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();
// 仍然调用 zk 进行服务创建
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());
} // org.apache.zookeeper.server.quorum.LeaderZooKeeperServer#startup
@Override
public synchronized void startup() {
super.startup();
if (containerManager != null) {
containerManager.start();
}
}
// org.apache.zookeeper.server.ZooKeeperServer#startup
// 父类框架
public synchronized void startup() {
if (sessionTracker == null) {
createSessionTracker();
}
startSessionTracker();
setupRequestProcessors(); startRequestThrottler(); registerJMX(); startJvmPauseMonitor(); registerMetrics(); setState(State.RUNNING); requestPathMetricsCollector.start(); localSessionEnabled = sessionTracker.isLocalSessionsEnabled();
notifyAll();
} // org.apache.zookeeper.server.quorum.LeaderZooKeeperServer#setupRequestProcessors
@Override
protected void setupRequestProcessors() {
// 最后一个处理器是 FinalRequestProcessor
RequestProcessor finalProcessor = new FinalRequestProcessor(this);
// 倒数第二个处理器是 ToBeAppliedRequestProcessor
RequestProcessor toBeAppliedProcessor = new Leader.ToBeAppliedRequestProcessor(finalProcessor, getLeader());
// 倒数第三个处理器是 CommitProcessor
commitProcessor = new CommitProcessor(toBeAppliedProcessor, Long.toString(getServerId()), false, getZooKeeperServerListener());
commitProcessor.start();
// 倒数第四个处理器是 ProposalRequestProcessor
ProposalRequestProcessor proposalProcessor = new ProposalRequestProcessor(this, commitProcessor);
proposalProcessor.initialize();
// 第二个处理器是 PrepRequestProcessor
prepRequestProcessor = new PrepRequestProcessor(this, proposalProcessor);
prepRequestProcessor.start();
// 第一个处理器是 LeaderRequestProcessor
firstProcessor = new LeaderRequestProcessor(this, prepRequestProcessor); // 所以最终的链路是 LeaderRequestProcessor -> PrepRequestProcessor -> ProposalRequestProcessor -> CommitProcessor -> ToBeAppliedRequestProcessor -> FinalRequestProcessor
setupContainerManager();
}
private synchronized void setupContainerManager() {
containerManager = new ContainerManager(
getZKDatabase(),
prepRequestProcessor,
Integer.getInteger("znode.container.checkIntervalMs", (int) TimeUnit.MINUTES.toMillis(1)),
Integer.getInteger("znode.container.maxPerMinute", 10000));
}
四、Observer 的链路创建过程
// 创建 Observer, 调用 observeLeader() 方法
case OBSERVING:
try {
LOG.info("OBSERVING");
setObserver(makeObserver(logFactory));
observer.observeLeader();
} catch (Exception e) {
LOG.warn("Unexpected exception", e);
} finally {
observer.shutdown();
setObserver(null);
updateServerState(); // Add delay jitter before we switch to LOOKING
// state to reduce the load of ObserverMaster
if (isRunning()) {
Observer.waitForObserverElectionDelay();
}
}
break; // org.apache.zookeeper.server.quorum.Observer#observeLeader
/**
* the main method called by the observer to observe the leader
* @throws Exception
*/
void observeLeader() throws Exception {
zk.registerJMX(new ObserverBean(this, zk), self.jmxLocalPeerBean);
long connectTime = 0;
boolean completedSync = false;
try {
self.setZabState(QuorumPeer.ZabState.DISCOVERY);
QuorumServer master = findLearnerMaster();
try {
connectToLeader(master.addr, master.hostname);
connectTime = System.currentTimeMillis();
long newLeaderZxid = registerWithLeader(Leader.OBSERVERINFO);
if (self.isReconfigStateChange()) {
throw new Exception("learned about role change");
} self.setLeaderAddressAndId(master.addr, master.getId());
self.setZabState(QuorumPeer.ZabState.SYNCHRONIZATION);
// 与 Leader 同步时，建立处理链路，调用 父类 Learner 的方法
syncWithLeader(newLeaderZxid);
self.setZabState(QuorumPeer.ZabState.BROADCAST);
completedSync = true;
QuorumPacket qp = new QuorumPacket();
// 一直循环处理
while (this.isRunning() && nextLearnerMaster.get() == null) {
readPacket(qp);
processPacket(qp);
}
} catch (Exception e) {
LOG.warn("Exception when observing the leader", e);
closeSocket(); // clear pending revalidations
pendingRevalidations.clear();
}
} finally {
currentLearnerMaster = null;
zk.unregisterJMX(this);
if (connectTime != 0) {
long connectionDuration = System.currentTimeMillis() - connectTime; LOG.info(
"Disconnected from leader (with address: {}). Was connected for {}ms. Sync state: {}",
leaderAddr,
connectionDuration,
completedSync);
messageTracker.dumpToLog(leaderAddr.toString());
}
}
} // org.apache.zookeeper.server.quorum.Learner#syncWithLeader
/**
* Finally, synchronize our history with the Leader (if Follower)
* or the LearnerMaster (if Observer).
* @param newLeaderZxid
* @throws IOException
* @throws InterruptedException
*/
protected void syncWithLeader(long newLeaderZxid) throws Exception {
QuorumPacket ack = new QuorumPacket(Leader.ACK, 0, null, null);
QuorumPacket qp = new QuorumPacket();
long newEpoch = ZxidUtils.getEpochFromZxid(newLeaderZxid); QuorumVerifier newLeaderQV = null; // In the DIFF case we don't need to do a snapshot because the transactions will sync on top of any existing snapshot
// For SNAP and TRUNC the snapshot is needed to save that history
boolean snapshotNeeded = true;
boolean syncSnapshot = false;
readPacket(qp);
Deque<Long> packetsCommitted = new ArrayDeque<>();
Deque<PacketInFlight> packetsNotCommitted = new ArrayDeque<>();
synchronized (zk) {
if (qp.getType() == Leader.DIFF) {
LOG.info("Getting a diff from the leader 0x{}", Long.toHexString(qp.getZxid()));
self.setSyncMode(QuorumPeer.SyncMode.DIFF);
snapshotNeeded = false;
} else if (qp.getType() == Leader.SNAP) {
self.setSyncMode(QuorumPeer.SyncMode.SNAP);
LOG.info("Getting a snapshot from leader 0x{}", Long.toHexString(qp.getZxid()));
// The leader is going to dump the database
// db is clear as part of deserializeSnapshot()
zk.getZKDatabase().deserializeSnapshot(leaderIs);
// ZOOKEEPER-2819: overwrite config node content extracted
// from leader snapshot with local config, to avoid potential
// inconsistency of config node content during rolling restart.
if (!QuorumPeerConfig.isReconfigEnabled()) {
LOG.debug("Reset config node content from local config after deserialization of snapshot.");
zk.getZKDatabase().initConfigInZKDatabase(self.getQuorumVerifier());
}
String signature = leaderIs.readString("signature");
if (!signature.equals("BenWasHere")) {
LOG.error("Missing signature. Got {}", signature);
throw new IOException("Missing signature");
}
zk.getZKDatabase().setlastProcessedZxid(qp.getZxid()); // immediately persist the latest snapshot when there is txn log gap
syncSnapshot = true;
} else if (qp.getType() == Leader.TRUNC) {
//we need to truncate the log to the lastzxid of the leader
self.setSyncMode(QuorumPeer.SyncMode.TRUNC);
LOG.warn("Truncating log to get in sync with the leader 0x{}", Long.toHexString(qp.getZxid()));
boolean truncated = zk.getZKDatabase().truncateLog(qp.getZxid());
if (!truncated) {
// not able to truncate the log
LOG.error("Not able to truncate the log 0x{}", Long.toHexString(qp.getZxid()));
System.exit(ExitCode.QUORUM_PACKET_ERROR.getValue());
}
zk.getZKDatabase().setlastProcessedZxid(qp.getZxid()); } else {
LOG.error("Got unexpected packet from leader: {}, exiting ... ", LearnerHandler.packetToString(qp));
System.exit(ExitCode.QUORUM_PACKET_ERROR.getValue()); }
zk.getZKDatabase().initConfigInZKDatabase(self.getQuorumVerifier());
zk.createSessionTracker(); long lastQueued = 0; // in Zab V1.0 (ZK 3.4+) we might take a snapshot when we get the NEWLEADER message, but in pre V1.0
// we take the snapshot on the UPDATE message, since Zab V1.0 also gets the UPDATE (after the NEWLEADER)
// we need to make sure that we don't take the snapshot twice.
boolean isPreZAB1_0 = true;
//If we are not going to take the snapshot be sure the transactions are not applied in memory
// but written out to the transaction log
boolean writeToTxnLog = !snapshotNeeded;
// we are now going to start getting transactions to apply followed by an UPTODATE
outerLoop:
while (self.isRunning()) {
readPacket(qp);
switch (qp.getType()) {
case Leader.PROPOSAL:
PacketInFlight pif = new PacketInFlight();
pif.hdr = new TxnHeader();
pif.rec = SerializeUtils.deserializeTxn(qp.getData(), pif.hdr);
if (pif.hdr.getZxid() != lastQueued + 1) {
LOG.warn(
"Got zxid 0x{} expected 0x{}",
Long.toHexString(pif.hdr.getZxid()),
Long.toHexString(lastQueued + 1));
}
lastQueued = pif.hdr.getZxid(); if (pif.hdr.getType() == OpCode.reconfig) {
SetDataTxn setDataTxn = (SetDataTxn) pif.rec;
QuorumVerifier qv = self.configFromString(new String(setDataTxn.getData()));
self.setLastSeenQuorumVerifier(qv, true);
} packetsNotCommitted.add(pif);
break;
case Leader.COMMIT:
case Leader.COMMITANDACTIVATE:
pif = packetsNotCommitted.peekFirst();
if (pif.hdr.getZxid() == qp.getZxid() && qp.getType() == Leader.COMMITANDACTIVATE) {
QuorumVerifier qv = self.configFromString(new String(((SetDataTxn) pif.rec).getData()));
boolean majorChange = self.processReconfig(
qv,
ByteBuffer.wrap(qp.getData()).getLong(), qp.getZxid(),
true);
if (majorChange) {
throw new Exception("changes proposed in reconfig");
}
}
if (!writeToTxnLog) {
if (pif.hdr.getZxid() != qp.getZxid()) {
LOG.warn(
"Committing 0x{}, but next proposal is 0x{}",
Long.toHexString(qp.getZxid()),
Long.toHexString(pif.hdr.getZxid()));
} else {
zk.processTxn(pif.hdr, pif.rec);
packetsNotCommitted.remove();
}
} else {
packetsCommitted.add(qp.getZxid());
}
break;
case Leader.INFORM:
case Leader.INFORMANDACTIVATE:
PacketInFlight packet = new PacketInFlight();
packet.hdr = new TxnHeader(); if (qp.getType() == Leader.INFORMANDACTIVATE) {
ByteBuffer buffer = ByteBuffer.wrap(qp.getData());
long suggestedLeaderId = buffer.getLong();
byte[] remainingdata = new byte[buffer.remaining()];
buffer.get(remainingdata);
packet.rec = SerializeUtils.deserializeTxn(remainingdata, packet.hdr);
QuorumVerifier qv = self.configFromString(new String(((SetDataTxn) packet.rec).getData()));
boolean majorChange = self.processReconfig(qv, suggestedLeaderId, qp.getZxid(), true);
if (majorChange) {
throw new Exception("changes proposed in reconfig");
}
} else {
packet.rec = SerializeUtils.deserializeTxn(qp.getData(), packet.hdr);
// Log warning message if txn comes out-of-order
if (packet.hdr.getZxid() != lastQueued + 1) {
LOG.warn(
"Got zxid 0x{} expected 0x{}",
Long.toHexString(packet.hdr.getZxid()),
Long.toHexString(lastQueued + 1));
}
lastQueued = packet.hdr.getZxid();
}
if (!writeToTxnLog) {
// Apply to db directly if we haven't taken the snapshot
zk.processTxn(packet.hdr, packet.rec);
} else {
packetsNotCommitted.add(packet);
packetsCommitted.add(qp.getZxid());
} break;
case Leader.UPTODATE:
LOG.info("Learner received UPTODATE message");
if (newLeaderQV != null) {
boolean majorChange = self.processReconfig(newLeaderQV, null, null, true);
if (majorChange) {
throw new Exception("changes proposed in reconfig");
}
}
if (isPreZAB1_0) {
zk.takeSnapshot(syncSnapshot);
self.setCurrentEpoch(newEpoch);
}
self.setZooKeeperServer(zk);
self.adminServer.setZooKeeperServer(zk);
break outerLoop;
case Leader.NEWLEADER: // Getting NEWLEADER here instead of in discovery
// means this is Zab 1.0
LOG.info("Learner received NEWLEADER message");
if (qp.getData() != null && qp.getData().length > 1) {
try {
QuorumVerifier qv = self.configFromString(new String(qp.getData()));
self.setLastSeenQuorumVerifier(qv, true);
newLeaderQV = qv;
} catch (Exception e) {
e.printStackTrace();
}
} if (snapshotNeeded) {
zk.takeSnapshot(syncSnapshot);
} self.setCurrentEpoch(newEpoch);
writeToTxnLog = true; //Anything after this needs to go to the transaction log, not applied directly in memory
isPreZAB1_0 = false;
writePacket(new QuorumPacket(Leader.ACK, newLeaderZxid, null, null), true);
break;
}
}
}
ack.setZxid(ZxidUtils.makeZxid(newEpoch, 0));
writePacket(ack, true);
sock.setSoTimeout(self.tickTime * self.syncLimit);
self.setSyncMode(QuorumPeer.SyncMode.NONE);
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(newEpoch); // We need to log the stuff that came in between the snapshot and the uptodate
if (zk instanceof FollowerZooKeeperServer) {
FollowerZooKeeperServer fzk = (FollowerZooKeeperServer) zk;
for (PacketInFlight p : packetsNotCommitted) {
fzk.logRequest(p.hdr, p.rec);
}
for (Long zxid : packetsCommitted) {
fzk.commit(zxid);
}
} else if (zk instanceof ObserverZooKeeperServer) {
// Similar to follower, we need to log requests between the snapshot
// and UPTODATE
ObserverZooKeeperServer ozk = (ObserverZooKeeperServer) zk;
for (PacketInFlight p : packetsNotCommitted) {
Long zxid = packetsCommitted.peekFirst();
if (p.hdr.getZxid() != zxid) {
// log warning message if there is no matching commit
// old leader send outstanding proposal to observer
LOG.warn(
"Committing 0x{}, but next proposal is 0x{}",
Long.toHexString(zxid),
Long.toHexString(p.hdr.getZxid()));
continue;
}
packetsCommitted.remove();
Request request = new Request(null, p.hdr.getClientId(), p.hdr.getCxid(), p.hdr.getType(), null, null);
request.setTxn(p.rec);
request.setHdr(p.hdr);
ozk.commitRequest(request);
}
} else {
// New server type need to handle in-flight packets
throw new UnsupportedOperationException("Unknown server type");
}
} // org.apache.zookeeper.server.ZooKeeperServer#startup
public synchronized void startup() {
if (sessionTracker == null) {
createSessionTracker();
}
startSessionTracker();
setupRequestProcessors(); startRequestThrottler(); registerJMX(); startJvmPauseMonitor(); registerMetrics(); setState(State.RUNNING); requestPathMetricsCollector.start(); localSessionEnabled = sessionTracker.isLocalSessionsEnabled();
notifyAll();
} // org.apache.zookeeper.server.quorum.ObserverZooKeeperServer#setupRequestProcessors
/**
* Set up the request processors for an Observer:
* firstProcesor->commitProcessor->finalProcessor
*/
@Override
protected void setupRequestProcessors() {
// We might consider changing the processor behaviour of
// Observers to, for example, remove the disk sync requirements.
// Currently, they behave almost exactly the same as followers.
RequestProcessor finalProcessor = new FinalRequestProcessor(this);
commitProcessor = new CommitProcessor(finalProcessor, Long.toString(getServerId()), true, getZooKeeperServerListener());
commitProcessor.start();
// 最终的处理链路是 ObserverRequestProcessor -> CommitProcessor -> FinalRequestProcessor
firstProcessor = new ObserverRequestProcessor(this, commitProcessor);
((ObserverRequestProcessor) firstProcessor).start(); /*
* Observer should write to disk, so that the it won't request
* too old txn from the leader which may lead to getting an entire
* snapshot.
*
* However, this may degrade performance as it has to write to disk
* and do periodic snapshot which may double the memory requirements
*/
if (syncRequestProcessorEnabled) {
syncProcessor = new SyncRequestProcessor(this, null);
syncProcessor.start();
}
}
四、各个处理器的功能简要介绍
AckRequestProcessor: 将前一阶段的请求作为ACK转发给Leader。
CommitProcessor: 将到来的请求与本地提交的请求进行匹配，这是因为改变系统状态的本地请求的返回结果是到来的请求。
FinalRequestProcessor: 通常是请求处理链的最后一个处理器。
FollowerRequestProcessor: 将修改了系统状态的请求转发给Leader。
ObserverRequestProcessor: 同FollowerRequestProcessor一样，将修改了系统状态的请求转发给Leader。
PrepRequestProcessor: 通常是请求处理链的第一个处理器。
ProposalRequestProcessor: 将请求转发给AckRequestProcessor和SyncRequestProcessor。
ReadOnlyRequestProcessor: 是ReadOnlyZooKeeperServer请求处理链的第一个处理器，将只读请求传递给下个处理器，抛弃改变状态的请求。
SendAckRequestProcessor: 发送ACK请求的处理器。
SyncRequestProcessor: 发送Sync请求的处理器。
ToBeAppliedRequestProcessor: 维护toBeApplied列表，下个处理器必须是FinalRequestProcessor并且FinalRequestProcessor必须同步处理请求。
在这个创建过程，我们可以看到 几个设计模式的身影: 责任链模式、模板方法模式、状态模式。 以及如何及时处理来自来自服务端的请求。
看完各个处理器的整体架构之后，后续我再从各个细节看问题，相信会更容易理解全局。
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