znode 可以被监控，包括这个目录节点中存储的数据的修改，子节点目录的变化等，一旦变化可以通知设置监控的客户端，这个功能是zookeeper对于应用最重要的特性，通过这个特性可以实现的功能包括配置的集中管理，集群管理，分布式锁等等。
知识准备：
zookeeper定义的状态有：
Unknown (-1),Disconnected (0),NoSyncConnected (1),SyncConnected (3),AuthFailed (4),ConnectedReadOnly (5),SaslAuthenticated(6),Expired (-112);
事件定义的的类型有：None (-1),NodeCreated (1),NodeDeleted (2),NodeDataChanged (3),NodeChildrenChanged (4),DataWatchRemoved (5),ChildWatchRemoved (6);
watcher定义的的类型有Children(1), Data(2), Any(3);
在上一篇
zookeeper源码分析之一客户端
中，我们连接zookeeper时，启动了一个MyWatcher
protected void connectToZK(String newHost) throws InterruptedException, IOException {
if (zk != null && zk.getState().isAlive()) {
zk.close();
}
host = newHost;
boolean readOnly = cl.getOption("readonly") != null;
if (cl.getOption("secure") != null) {
System.setProperty(ZooKeeper.SECURE_CLIENT, "true");
System.out.println("Secure connection is enabled");
}
zk = new ZooKeeper(host,
Integer.parseInt(cl.getOption("timeout")),
new MyWatcher(), readOnly);
}
创建zookeeper示例时，使用到watchManager：
public ZooKeeper(String connectString, int sessionTimeout, Watcher watcher,
boolean canBeReadOnly, HostProvider aHostProvider)
throws IOException {
LOG.info("Initiating client connection, connectString=" + connectString
+ " sessionTimeout=" + sessionTimeout + " watcher=" + watcher); watchManager = defaultWatchManager();
watchManager.defaultWatcher = watcher; ConnectStringParser connectStringParser = new ConnectStringParser(
connectString);
hostProvider = aHostProvider; cnxn = new ClientCnxn(connectStringParser.getChrootPath(),
hostProvider, sessionTimeout, this, watchManager,
getClientCnxnSocket(), canBeReadOnly);
cnxn.start();
}
将传进来的MyWatcher作为默认watcher，存入watchManager，然后通过ClientCnxn包装后，启动线程。
那我们先了解一下ClientCnxn吧，ClientCnxn管理客户端socket的io，它维护了一组可以连接上的server及当需要转换时可以透明的转换到的一组server。
先了解一下如何获取socket的吧：
private static ClientCnxnSocket getClientCnxnSocket() throws IOException {
String clientCnxnSocketName = System
.getProperty(ZOOKEEPER_CLIENT_CNXN_SOCKET);
if (clientCnxnSocketName == null) {
clientCnxnSocketName = ClientCnxnSocketNIO.class.getName();
}
try {
return (ClientCnxnSocket) Class.forName(clientCnxnSocketName)
.newInstance();
} catch (Exception e) {
IOException ioe = new IOException("Couldn't instantiate "
+ clientCnxnSocketName);
ioe.initCause(e);
throw ioe;
}
}
接着启动ClientCnxn的start()方法，在此方法中启动了两个线程：
public void start() {
sendThread.start();
eventThread.start();
}
其中SendThread类为发送的请求队列提供服务，并且产生心跳。它同时也产生ReadThread。
我们看一下SendThread的run方法的主体：
if (!clientCnxnSocket.isConnected()) {
// don't re-establish connection if we are closing
if (closing) {
break;
}
startConnect();
clientCnxnSocket.updateLastSendAndHeard();
}
if (state.isConnected()) {
// determine whether we need to send an AuthFailed event.
if (zooKeeperSaslClient != null) {
boolean sendAuthEvent = false;
if (zooKeeperSaslClient.getSaslState() == ZooKeeperSaslClient.SaslState.INITIAL) {
try {
zooKeeperSaslClient.initialize(ClientCnxn.this);
} catch (SaslException e) {
LOG.error("SASL authentication with Zookeeper Quorum member failed: " + e);
state = States.AUTH_FAILED;
sendAuthEvent = true;
}
}
KeeperState authState = zooKeeperSaslClient.getKeeperState();
if (authState != null) {
if (authState == KeeperState.AuthFailed) {
// An authentication error occurred during authentication with the Zookeeper Server.
state = States.AUTH_FAILED;
sendAuthEvent = true;
} else {
if (authState == KeeperState.SaslAuthenticated) {
sendAuthEvent = true;
}
}
}
if (sendAuthEvent == true) {
eventThread.queueEvent(new WatchedEvent(
Watcher.Event.EventType.None,
authState,null));
}
}
to = readTimeout - clientCnxnSocket.getIdleRecv();
} else {
to = connectTimeout - clientCnxnSocket.getIdleRecv();
}
if (to <= 0) {
String warnInfo;
warnInfo = "Client session timed out, have not heard from server in "
+ clientCnxnSocket.getIdleRecv()
+ "ms"
+ " for sessionid 0x"
+ Long.toHexString(sessionId);
LOG.warn(warnInfo);
throw new SessionTimeoutException(warnInfo);
}
if (state.isConnected()) {
//1000(1 second) is to prevent race condition missing to send the second ping
//also make sure not to send too many pings when readTimeout is small
int timeToNextPing = readTimeout / 2 - clientCnxnSocket.getIdleSend() -
((clientCnxnSocket.getIdleSend() > 1000) ? 1000 : 0);
//send a ping request either time is due or no packet sent out within MAX_SEND_PING_INTERVAL
if (timeToNextPing <= 0 || clientCnxnSocket.getIdleSend() > MAX_SEND_PING_INTERVAL) {
sendPing();
clientCnxnSocket.updateLastSend();
} else {
if (timeToNextPing < to) {
to = timeToNextPing;
}
}
}
// If we are in read-only mode, seek for read/write server
if (state == States.CONNECTEDREADONLY) {
long now = Time.currentElapsedTime();
int idlePingRwServer = (int) (now - lastPingRwServer);
if (idlePingRwServer >= pingRwTimeout) {
lastPingRwServer = now;
idlePingRwServer = 0;
pingRwTimeout =
Math.min(2*pingRwTimeout, maxPingRwTimeout);
pingRwServer();
}
to = Math.min(to, pingRwTimeout - idlePingRwServer);
}
clientCnxnSocket.doTransport(to, pendingQueue, ClientCnxn.this);
ClientCnxnSocketNetty实现了ClientCnxnSocket的抽象方法，它负责连接到server，读取/写入网络流量，并作为网络数据层和更高packet层的中间层。其生命周期如下：
loop:
- try:
- - !isConnected()
- - - connect()
- - doTransport()
- catch:
- - cleanup()
close()
从上述描述中，我们可以看到ClientCnxnSocket的工作流程，先判断是否连接，没有连接则调用connect方法进行连接，有连接则直接使用；然后调用doTransport方法进行通信，若连接过程中出现异常，则调用cleanup()方法；最后关闭连接。故最主要的流程为doTransport()方法：
@Override
void doTransport(int waitTimeOut,
List<Packet> pendingQueue,
ClientCnxn cnxn)
throws IOException, InterruptedException {
try {
if (!firstConnect.await(waitTimeOut, TimeUnit.MILLISECONDS)) {
return;
}
Packet head = null;
if (needSasl.get()) {
if (!waitSasl.tryAcquire(waitTimeOut, TimeUnit.MILLISECONDS)) {
return;
}
} else {
if ((head = outgoingQueue.poll(waitTimeOut, TimeUnit.MILLISECONDS)) == null) {
return;
}
}
// check if being waken up on closing.
if (!sendThread.getZkState().isAlive()) {
// adding back the patck to notify of failure in conLossPacket().
addBack(head);
return;
}
// channel disconnection happened
if (disconnected.get()) {
addBack(head);
throw new EndOfStreamException("channel for sessionid 0x"
+ Long.toHexString(sessionId)
+ " is lost");
}
if (head != null) {
doWrite(pendingQueue, head, cnxn);
}
} finally {
updateNow();
}
}
我们简化一下上面的程序，一个是异常处理addBack(head)，另一个正常流程处理doWrite(pendingQueue, head, cnxn)，我们先抛掉异常，走正常流程看看：
先获取Packet：
Packet head = null;
if (needSasl.get()) {
if (!waitSasl.tryAcquire(waitTimeOut, TimeUnit.MILLISECONDS)) {
return;
}
} else {
if ((head = outgoingQueue.poll(waitTimeOut, TimeUnit.MILLISECONDS)) == null) {
return;
}
}
其中，protected LinkedBlockingDeque<Packet> outgoingQueue是一个链表阻塞队列，保存发出的请求;
然后执行doWrite方法：
/**
* doWrite handles writing the packets from outgoingQueue via network to server.
*/
private void doWrite(List<Packet> pendingQueue, Packet p, ClientCnxn cnxn) {
updateNow();
while (true) {
if (p != WakeupPacket.getInstance()) {
if ((p.requestHeader != null) &&
(p.requestHeader.getType() != ZooDefs.OpCode.ping) &&
(p.requestHeader.getType() != ZooDefs.OpCode.auth)) {
p.requestHeader.setXid(cnxn.getXid());
synchronized (pendingQueue) {
pendingQueue.add(p);
}
}
sendPkt(p);
}
if (outgoingQueue.isEmpty()) {
break;
}
p = outgoingQueue.remove();
}
}
dowrite方法负责将outgoingQueue的报文通过网络写到服务器上。发送报文程序如上红色所示：
private void sendPkt(Packet p) {
// Assuming the packet will be sent out successfully. Because if it fails,
// the channel will close and clean up queues.
p.createBB();
updateLastSend();
sentCount++;
channel.write(ChannelBuffers.wrappedBuffer(p.bb));
}
1. Packet报文的结构如下：
/**
* This class allows us to pass the headers and the relevant records around.
*/
static class Packet {
RequestHeader requestHeader; ReplyHeader replyHeader; Record request; Record response; ByteBuffer bb; /** Client's view of the path (may differ due to chroot) **/
String clientPath;
/** Servers's view of the path (may differ due to chroot) **/
String serverPath; boolean finished; AsyncCallback cb; Object ctx; WatchRegistration watchRegistration; public boolean readOnly; WatchDeregistration watchDeregistration; /** Convenience ctor */
Packet(RequestHeader requestHeader, ReplyHeader replyHeader,
Record request, Record response,
WatchRegistration watchRegistration) {
this(requestHeader, replyHeader, request, response,
watchRegistration, false);
} Packet(RequestHeader requestHeader, ReplyHeader replyHeader,
Record request, Record response,
WatchRegistration watchRegistration, boolean readOnly) { this.requestHeader = requestHeader;
this.replyHeader = replyHeader;
this.request = request;
this.response = response;
this.readOnly = readOnly;
this.watchRegistration = watchRegistration;
} public void createBB() {
try {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
BinaryOutputArchive boa = BinaryOutputArchive.getArchive(baos);
boa.writeInt(-1, "len"); // We'll fill this in later
if (requestHeader != null) {
requestHeader.serialize(boa, "header");
}
if (request instanceof ConnectRequest) {
request.serialize(boa, "connect");
// append "am-I-allowed-to-be-readonly" flag
boa.writeBool(readOnly, "readOnly");
} else if (request != null) {
request.serialize(boa, "request");
}
baos.close();
this.bb = ByteBuffer.wrap(baos.toByteArray());
this.bb.putInt(this.bb.capacity() - 4);
this.bb.rewind();
} catch (IOException e) {
LOG.warn("Ignoring unexpected exception", e);
}
} @Override
public String toString() {
StringBuilder sb = new StringBuilder(); sb.append("clientPath:" + clientPath);
sb.append(" serverPath:" + serverPath);
sb.append(" finished:" + finished); sb.append(" header:: " + requestHeader);
sb.append(" replyHeader:: " + replyHeader);
sb.append(" request:: " + request);
sb.append(" response:: " + response); // jute toString is horrible, remove unnecessary newlines
return sb.toString().replaceAll("\r*\n+", " ");
}
}
从createBB方法中，我们看到在底层实际的网络传输序列化中，zookeeper只会讲requestHeader和request两个属性进行序列化，即只有这两个会被序列化到底层字节数组中去进行网络传输，不会将watchRegistration相关的信息进行网络传输。
2. 更新最后一次发送updateLastSend
void updateLastSend() {
this.lastSend = now;
}
3. 使用nio channel 发送字节缓存到server
channel.write(ChannelBuffers.wrappedBuffer(p.bb));
其中，bb的类型为ByteBuffer，在packet中进行了初始化。
this.bb = ByteBuffer.wrap(baos.toByteArray());
this.bb.putInt(this.bb.capacity() - 4);
this.bb.rewind();
小结：
zookeeper客户端和服务器的连接主要是通过ClientCnxnSocket来实现的，有两个具体的实现类ClientCnxnSocketNetty和ClientCnxnSocketNIO，其工作流程如下：
先判断是否连接，没有连接则调用connect方法进行连接，有连接则进入下一步；
然后调用doTransport方法进行通信，若连接过程中出现异常，则调用cleanup()方法；
最后关闭连接。
上述的发现可以在SendThread的run方法中体现。
另：Zookeeper的特性--》顺序一致性：按照客户端发送请求的顺序更新数据。我们再sendThread里可以看到多次更新时间戳来保证顺序一致性，如下：