zookeeper源码分析之三客户端发送请求流程

　　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：