Watcher的基本流程
ZooKeeper 的 Watcher 机制，总的来说可以分为三个过程：客户端注册 Watcher、服务器处理 Watcher 和客户端回调 Watcher
客户端注册watcher有3种方式，getData、exists、getChildren；以如下代码为例来分析整个触发机制的原理
| ZooKeeper zookeeper=new ZooKeeper(“192.168.11.152:2181”,4000,new Watcher(){ public void processor(WatchedEvent event){ System.out.println(“event.type”); } })； zookeeper.create(“/mic”,”0”.getByte(),ZooDefs.Ids. OPEN_ACL_UNSAFE,CreateModel. PERSISTENT); //创建节点 zookeeper.exists(“/mic”,true); //注册监听 zookeeper.setData(“/mic”, “1”.getByte(),-1) ; //修改节点的值触发监听 |
ZooKeeper API的初始化过程
| ZooKeeper zookeeper=new ZooKeeper(“192.168.11.152:2181”,4000,new Watcher(){ public void processor(WatchedEvent event){ System.out.println(“event.type”); } })； |
在创建一个 ZooKeeper 客户端对象实例时，我们通过new Watcher()向构造方法中传入一个默认的 Watcher, 这个 Watcher 将作为整个 ZooKeeper会话期间的默认 Watcher，会一直被保存在客户端 ZKWatchManager 的 defaultWatcher 中;代码如下
| public ZooKeeper(String connectString, int sessionTimeout, Watcher watcher, boolean canBeReadOnly, HostProvider aHostProvider, ZKClientConfig clientConfig) throws IOException { LOG.info("Initiating client connection, connectString=" + connectString + " sessionTimeout=" + sessionTimeout + " watcher=" + watcher); if (clientConfig == null) { clientConfig = new ZKClientConfig(); } this.clientConfig = clientConfig; watchManager = defaultWatchManager(); watchManager.defaultWatcher = watcher; --在这里将watcher设置到ZKWatchManager ConnectStringParser connectStringParser = new ConnectStringParser( connectString); hostProvider = aHostProvider; --初始化了ClientCnxn，并且调用cnxn.start()方法 cnxn = new ClientCnxn(connectStringParser.getChrootPath(), hostProvider, sessionTimeout, this, watchManager, getClientCnxnSocket(), canBeReadOnly); cnxn.start(); } |
ClientCnxn:是Zookeeper客户端和Zookeeper服务器端进行通信和事件通知处理的主要类，它内部包含两个类，
1. SendThread ：负责客户端和服务器端的数据通信, 也包括事件信息的传输
2. EventThread : 主要在客户端回调注册的Watchers进行通知处理
ClientCnxn初始化
| public ClientCnxn(String chrootPath, HostProvider hostProvider, int sessionTimeout, ZooKeeper zooKeeper, ClientWatchManager watcher, ClientCnxnSocket clientCnxnSocket, long sessionId, byte[] sessionPasswd, boolean canBeReadOnly) { this.zooKeeper = zooKeeper; this.watcher = watcher; this.sessionId = sessionId; this.sessionPasswd = sessionPasswd; this.sessionTimeout = sessionTimeout; this.hostProvider = hostProvider; this.chrootPath = chrootPath; connectTimeout = sessionTimeout / hostProvider.size(); readTimeout = sessionTimeout * 2 / 3; readOnly = canBeReadOnly; sendThread = new SendThread(clientCnxnSocket); --初始化sendThread eventThread = new EventThread(); --初始化eventThread this.clientConfig=zooKeeper.getClientConfig(); } public void start() { --启动两个线程 sendThread.start(); eventThread.start(); } |
客户端通过exists注册监听
| zookeeper.exists(“/mic”,true); //注册监听 |
通过exists方法来注册监听，代码如下
| public Stat exists(final String path, Watcher watcher) throws KeeperException, InterruptedException { final String clientPath = path; PathUtils.validatePath(clientPath); // the watch contains the un-chroot path WatchRegistration wcb = null; if (watcher != null) { wcb = new ExistsWatchRegistration(watcher, clientPath); //构建ExistWatchRegistration } final String serverPath = prependChroot(clientPath); RequestHeader h = new RequestHeader(); h.setType(ZooDefs.OpCode.exists); //设置操作类型为exists ExistsRequest request = new ExistsRequest(); // 构造ExistsRequest request.setPath(serverPath); request.setWatch(watcher != null); //是否注册监听 SetDataResponse response = new SetDataResponse(); //设置服务端响应的接收类 //将封装的RequestHeader、ExistsRequest、SetDataResponse、WatchRegistration添加到发送队列 ReplyHeader r = cnxn.submitRequest(h, request, response, wcb); if (r.getErr() != 0) { if (r.getErr() == KeeperException.Code.NONODE.intValue()) { return null; } throw KeeperException.create(KeeperException.Code.get(r.getErr()), clientPath); } //返回exists得到的结果（Stat信息） return response.getStat().getCzxid() == -1 ? null : response.getStat(); } |
cnxn.submitRequest
| public ReplyHeader submitRequest(RequestHeader h, Record request, Record response, WatchRegistration watchRegistration, WatchDeregistration watchDeregistration) throws InterruptedException { ReplyHeader r = new ReplyHeader(); //将消息添加到队列,并构造一个Packet传输对象 Packet packet = queuePacket(h, r, request, response, null, null, null, null, watchRegistration, watchDeregistration); synchronized (packet) { while (!packet.finished) { //在数据包没有处理完成之前，一直阻塞 packet.wait(); } } return r; } |
| public Packet queuePacket(RequestHeader h, ReplyHeader r, Record request, Record response, AsyncCallback cb, String clientPath, String serverPath, Object ctx, WatchRegistration watchRegistration, WatchDeregistration watchDeregistration) { //将相关传输对象转化成Packet Packet packet = null; packet = new Packet(h, r, request, response, watchRegistration); packet.cb = cb; packet.ctx = ctx; packet.clientPath = clientPath; packet.serverPath = serverPath; packet.watchDeregistration = watchDeregistration;
synchronized (state) { if (!state.isAlive() || closing) { conLossPacket(packet); } else { if (h.getType() == OpCode.closeSession) { closing = true; } outgoingQueue.add(packet); //添加到outgoingQueue } } sendThread.getClientCnxnSocket().packetAdded();//此处是多路复用机制，唤醒Selector，告诉他有数据包添加过来了 return packet; } |
在 ZooKeeper 中，Packet 是一个最小的通信协议单元，即数据包。Pakcet 用于进行客户端与服务端之间的网络传输，任何需要传输的对象都需要包装成一个 Packet 对象。在 ClientCnxn 中 WatchRegistration 也会被封装到 Pakcet 中，然后由 SendThread 线程调用queuePacket方法把 Packet 放入发送队列中等待客户端发送，这又是一个异步过程，分布式系统采用异步通信是一个非常常见的手段
SendThread的发送过程
在初始化连接的时候，zookeeper初始化了两个线程并且启动了。接下来我们来分析SendThread的发送过程，因为是一个线程，所以启动的时候会调用SendThread.run方法
| public void run() { clientCnxnSocket.introduce(this, sessionId, outgoingQueue); clientCnxnSocket.updateNow(); clientCnxnSocket.updateLastSendAndHeard(); int to; long lastPingRwServer = Time.currentElapsedTime(); final int MAX_SEND_PING_INTERVAL = 10000; //10 seconds while (state.isAlive()) { try { if (!clientCnxnSocket.isConnected()) {// 如果没有连接：发起连接 // don't re-establish connection if we are closing if (closing) { break; } startConnect(); //发起连接 clientCnxnSocket.updateLastSendAndHeard(); } if (state.isConnected()) { //如果是连接状态，则处理sasl的认证授权 // 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(); } //to,表示客户端距离timeout还剩多少时间，准备发起ping连接 if (to <= 0) {//表示已经超时了。 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()) { //计算下一次ping请求的时间 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(); //发送ping请求 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); } 调用clientCnxnSocket，发起传输 其中 pendingQueue是一个用来存放已经发送、等待回应的Packet队列， clientCnxnSocket默认使用ClientCnxnSocketNIO（ps：还记得在哪里初始化吗？在实例化zookeeper的时候） clientCnxnSocket.doTransport(to, pendingQueue, ClientCnxn.this); } catch (Throwable e) { if (closing) { if (LOG.isDebugEnabled()) { // closing so this is expected LOG.debug("An exception was thrown while closing send thread for session 0x" + Long.toHexString(getSessionId()) + " : " + e.getMessage()); } break; } else { // this is ugly, you have a better way speak up if (e instanceof SessionExpiredException) { LOG.info(e.getMessage() + ", closing socket connection"); } else if (e instanceof SessionTimeoutException) { LOG.info(e.getMessage() + RETRY_CONN_MSG); } else if (e instanceof EndOfStreamException) { LOG.info(e.getMessage() + RETRY_CONN_MSG); } else if (e instanceof RWServerFoundException) { LOG.info(e.getMessage()); } else { LOG.warn( "Session 0x" + Long.toHexString(getSessionId()) + " for server " + clientCnxnSocket.getRemoteSocketAddress() + ", unexpected error" + RETRY_CONN_MSG, e); } // At this point, there might still be new packets appended to outgoingQueue. // they will be handled in next connection or cleared up if closed. cleanup(); if (state.isAlive()) { eventThread.queueEvent(new WatchedEvent( Event.EventType.None, Event.KeeperState.Disconnected, null)); } clientCnxnSocket.updateNow(); clientCnxnSocket.updateLastSendAndHeard(); } } } synchronized (state) { // When it comes to this point, it guarantees that later queued // packet to outgoingQueue will be notified of death. cleanup(); } clientCnxnSocket.close(); if (state.isAlive()) { eventThread.queueEvent(new WatchedEvent(Event.EventType.None, Event.KeeperState.Disconnected, null)); } ZooTrace.logTraceMessage(LOG, ZooTrace.getTextTraceLevel(), "SendThread exited loop for session: 0x" + Long.toHexString(getSessionId())); } |
client 和 server的网络交互
| 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 { //判断outgoingQueue是否存在待发送的数据包，不存在则直接返回 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()) { //异常流程，channel关闭了，讲当前的packet添加到addBack中 addBack(head); throw new EndOfStreamException("channel for sessionid 0x" + Long.toHexString(sessionId) + " is lost"); } if (head != null) { //如果当前存在需要发送的数据包，则调用doWrite方法，pendingQueue表示处于已经发送过等待响应的packet队列 doWrite(pendingQueue, head, cnxn); } } finally { updateNow(); } } |
DoWrite方法
| private void doWrite(List<Packet> pendingQueue, Packet p, ClientCnxn cnxn) { updateNow(); while (true) { if (p != WakeupPacket.getInstance()) { if ((p.requestHeader != null) && //判断请求头以及判断当前请求类型不是ping或者auth操作 (p.requestHeader.getType() != ZooDefs.OpCode.ping) && (p.requestHeader.getType() != ZooDefs.OpCode.auth)) { p.requestHeader.setXid(cnxn.getXid()); //设置xid，这个xid用来区分请求类型 synchronized (pendingQueue) { pendingQueue.add(p); //将当前的packet添加到pendingQueue队列中 } } sendPkt(p); //将数据包发送出去 } if (outgoingQueue.isEmpty()) { break; } p = outgoingQueue.remove(); } } |
sendPkt
| 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(); //序列化请求数据 updateLastSend(); //更新最后一次发送updateLastSend sentCount++; //更新发送次数 channel.write(ChannelBuffers.wrappedBuffer(p.bb)); //通过nio channel发送字节缓存到服务端 } |
createBB
| 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"); //序列化header头(requestHeader) } 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"); //序列化request(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); } } |
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。
总结
用户调用exists注册监听以后，会做几个事情
- 讲请求数据封装为packet，添加到outgoingQueue
- SendThread这个线程会执行数据发送操作，主要是将outgoingQueue队列中的数据发送到服务端
- 通过clientCnxnSocket.doTransport(to, pendingQueue, ClientCnxn.this); 其中ClientCnxnSocket只zookeeper客户端和服务端的连接通信的封装，有两个具体的实现类ClientCnxnSocketNetty和ClientCnxnSocketNIO;具体使用哪一个类来实现发送，是在初始化过程是在实例化Zookeeper的时候设置的，代码如下
| cnxn = new ClientCnxn(connectStringParser.getChrootPath(), hostProvider, sessionTimeout, this, watchManager, getClientCnxnSocket(), canBeReadOnly); |
private ClientCnxnSocket getClientCnxnSocket() throws IOException { String clientCnxnSocketName = getClientConfig().getProperty( ZKClientConfig.ZOOKEEPER_CLIENT_CNXN_SOCKET); if (clientCnxnSocketName == null) { clientCnxnSocketName = ClientCnxnSocketNIO.class.getName(); } try { Constructor<?> clientCxnConstructor = Class.forName(clientCnxnSocketName).getDeclaredConstructor(ZKClientConfig.class); ClientCnxnSocket clientCxnSocket = (ClientCnxnSocket) clientCxnConstructor.newInstance(getClientConfig()); return clientCxnSocket; } catch (Exception e) { IOException ioe = new IOException("Couldn't instantiate " + clientCnxnSocketName); ioe.initCause(e); throw ioe; } } |
4.基于第3步，最终会在ClientCnxnSocketNetty方法中执行sendPkt将请求的数据包发送到服务端
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