zookeeper源碼分析之二客戶端啓動

ZooKeeper Client Library提供了豐富直觀的API供用戶程序使用，下面是一些經常使用的API：

• create(path, data, flags): 建立一個ZNode, path是其路徑，data是要存儲在該ZNode上的數據，flags經常使用的有: PERSISTEN, PERSISTENT_SEQUENTAIL, EPHEMERAL, EPHEMERAL_SEQUENTAIL
• delete(path, version): 刪除一個ZNode，能夠經過version刪除指定的版本, 若是version是-1的話，表示刪除全部的版本
• exists(path, watch): 判斷指定ZNode是否存在，並設置是否Watch這個ZNode。這裏若是要設置Watcher的話，Watcher是在建立ZooKeeper實例時指定的，若是要設置特定的Watcher的話，能夠調用另外一個重載版本的exists(path, watcher)。如下幾個帶watch參數的API也都相似
• getData(path, watch): 讀取指定ZNode上的數據，並設置是否watch這個ZNode
• setData(path, watch): 更新指定ZNode的數據，並設置是否Watch這個ZNode
• getChildren(path, watch): 獲取指定ZNode的全部子ZNode的名字，並設置是否Watch這個ZNode
• sync(path): 把全部在sync以前的更新操做都進行同步，達到每一個請求都在半數以上的ZooKeeper Server上生效。path參數目前沒有用
• setAcl(path, acl): 設置指定ZNode的Acl信息
• getAcl(path): 獲取指定ZNode的Acl信息

具體是如何其做用的呢？

客戶端鏈接到服務器

啓動客戶端的腳本zookeeper/bin/zkCli.sh

# use POSTIX interface, symlink is followed automatically
ZOOBIN="${BASH_SOURCE-$0}"
ZOOBIN="$(dirname "${ZOOBIN}")"
ZOOBINDIR="$(cd "${ZOOBIN}"; pwd)"

if [ -e "$ZOOBIN/../libexec/zkEnv.sh" ]; then
  . "$ZOOBINDIR"/../libexec/zkEnv.sh
else
  . "$ZOOBINDIR"/zkEnv.sh
fi

ZOO_LOG_FILE=zookeeper-$USER-cli-$HOSTNAME.log

"$JAVA" "-Dzookeeper.log.dir=${ZOO_LOG_DIR}" "-Dzookeeper.root.logger=${ZOO_LOG4J_PROP}" "-Dzookeeper.log.file=${ZOO_LOG_FILE}" \
     -cp "$CLASSPATH" $CLIENT_JVMFLAGS $JVMFLAGS \
     org.apache.zookeeper.ZooKeeperMain "$@"

從上述腳本能夠看出：

鏈接到zookeeper服務器時，須要啓動org.apache.zookeeper.ZooKeeperMain類，其入口main方法以下：

    public static void main(String args[])
        throws KeeperException, IOException, InterruptedException
    {
        ZooKeeperMain main = new ZooKeeperMain(args);
        main.run();
    }

　　void run() throws KeeperException, IOException, InterruptedException {
        if (cl.getCommand() == null) {
            System.out.println("Welcome to ZooKeeper!");

            boolean jlinemissing = false;
            // only use jline if it's in the classpath
            try {
                Class<?> consoleC = Class.forName("jline.console.ConsoleReader");
                Class<?> completorC =
                    Class.forName("org.apache.zookeeper.JLineZNodeCompleter");

                System.out.println("JLine support is enabled");

                Object console =
                    consoleC.getConstructor().newInstance();

                Object completor =
                    completorC.getConstructor(ZooKeeper.class).newInstance(zk);
                Method addCompletor = consoleC.getMethod("addCompleter",
                        Class.forName("jline.console.completer.Completer"));
                addCompletor.invoke(console, completor);

                String line;
                Method readLine = consoleC.getMethod("readLine", String.class);
                while ((line = (String)readLine.invoke(console, getPrompt())) != null) {
                    executeLine(line);
                }
            } catch (ClassNotFoundException e) {
                LOG.debug("Unable to start jline", e);
                jlinemissing = true;
            } catch (NoSuchMethodException e) {
                LOG.debug("Unable to start jline", e);
                jlinemissing = true;
            } catch (InvocationTargetException e) {
                LOG.debug("Unable to start jline", e);
                jlinemissing = true;
            } catch (IllegalAccessException e) {
                LOG.debug("Unable to start jline", e);
                jlinemissing = true;
            } catch (InstantiationException e) {
                LOG.debug("Unable to start jline", e);
                jlinemissing = true;
            }

            if (jlinemissing) {
                System.out.println("JLine support is disabled");
                BufferedReader br =
                    new BufferedReader(new InputStreamReader(System.in));

                String line;
                while ((line = br.readLine()) != null) {
                    executeLine(line);
                }
            }
        } else {
            // Command line args non-null.  Run what was passed.
 processCmd(cl);
        }
    }

其中，主要方法爲processCmd(cl)：

protected boolean processZKCmd(MyCommandOptions co)
        throws KeeperException, IOException, InterruptedException
    {
        String[] args = co.getArgArray();
        String cmd = co.getCommand();
        if (args.length < 1) {
            usage();
            return false;
        }

        if (!commandMap.containsKey(cmd)) {
            usage();
            return false;
        }
        
        boolean watch = false;
        LOG.debug("Processing " + cmd);
        
        try {
        if (cmd.equals("quit")) {
            zk.close();
            System.exit(0);
        } else if (cmd.equals("redo") && args.length >= 2) {
            Integer i = Integer.decode(args[1]);
            if (commandCount <= i){ // don't allow redoing this redo
                System.out.println("Command index out of range");
                return false;
            }
            cl.parseCommand(history.get(i));
            if (cl.getCommand().equals( "redo" )){
                System.out.println("No redoing redos");
                return false;
            }
            history.put(commandCount, history.get(i));
            processCmd( cl);
        } else if (cmd.equals("history")) {
            for (int i=commandCount - 10;i<=commandCount;++i) {
                if (i < 0) continue;
                System.out.println(i + " - " + history.get(i));
            }
        } else if (cmd.equals("printwatches")) {
            if (args.length == 1) {
                System.out.println("printwatches is " + (printWatches ? "on" : "off"));
            } else {
                printWatches = args[1].equals("on");
            }
        } else if (cmd.equals("connect")) {
            if (args.length >=2) {
                connectToZK(args[1]);
            } else {
                connectToZK(host);                
            }
        } 

咱們以connect命令來看看鏈接的過程。

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);
    }

建立客戶端：

/**
     * To create a ZooKeeper client object, the application needs to pass a
     * connection string containing a comma separated list of host:port pairs,
     * each corresponding to a ZooKeeper server.
     * <p>
     * Session establishment is asynchronous. This constructor will initiate
     * connection to the server and return immediately - potentially (usually)
     * before the session is fully established. The watcher argument specifies
     * the watcher that will be notified of any changes in state. This
     * notification can come at any point before or after the constructor call
     * has returned.
     * <p>
     * The instantiated ZooKeeper client object will pick an arbitrary server
     * from the connectString and attempt to connect to it. If establishment of
     * the connection fails, another server in the connect string will be tried
     * (the order is non-deterministic, as we random shuffle the list), until a
     * connection is established. The client will continue attempts until the
     * session is explicitly closed.
     * <p>
     * Added in 3.2.0: An optional "chroot" suffix may also be appended to the
     * connection string. This will run the client commands while interpreting
     * all paths relative to this root (similar to the unix chroot command).
     * <p>
     *
     * @param connectString
     *            comma separated host:port pairs, each corresponding to a zk
     *            server. e.g. "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002" If
     *            the optional chroot suffix is used the example would look
     *            like: "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002/app/a"
     *            where the client would be rooted at "/app/a" and all paths
     *            would be relative to this root - ie getting/setting/etc...
     *            "/foo/bar" would result in operations being run on
     *            "/app/a/foo/bar" (from the server perspective).
     * @param sessionTimeout
     *            session timeout in milliseconds
     * @param watcher
     *            a watcher object which will be notified of state changes, may
     *            also be notified for node events
     * @param canBeReadOnly
     *            (added in 3.4) whether the created client is allowed to go to
     *            read-only mode in case of partitioning. Read-only mode
     *            basically means that if the client can't find any majority
     *            servers but there's partitioned server it could reach, it
     *            connects to one in read-only mode, i.e. read requests are
     *            allowed while write requests are not. It continues seeking for
     *            majority in the background.
     *
     * @throws IOException
     *             in cases of network failure
     * @throws IllegalArgumentException
     *             if an invalid chroot path is specified
     */
    public ZooKeeper(String connectString, int sessionTimeout, Watcher watcher,
            boolean canBeReadOnly) throws IOException {
        this(connectString, sessionTimeout, watcher, canBeReadOnly,
                createDefaultHostProvider(connectString));
    }

調用建立客戶端方法：

/**
     * To create a ZooKeeper client object, the application needs to pass a
     * connection string containing a comma separated list of host:port pairs,
     * each corresponding to a ZooKeeper server.
     * <p>
     * Session establishment is asynchronous. This constructor will initiate
     * connection to the server and return immediately - potentially (usually)
     * before the session is fully established. The watcher argument specifies
     * the watcher that will be notified of any changes in state. This
     * notification can come at any point before or after the constructor call
     * has returned.
     * <p>
     * The instantiated ZooKeeper client object will pick an arbitrary server
     * from the connectString and attempt to connect to it. If establishment of
     * the connection fails, another server in the connect string will be tried
     * (the order is non-deterministic, as we random shuffle the list), until a
     * connection is established. The client will continue attempts until the
     * session is explicitly closed.
     * <p>
     * Added in 3.2.0: An optional "chroot" suffix may also be appended to the
     * connection string. This will run the client commands while interpreting
     * all paths relative to this root (similar to the unix chroot command).
     * <p>
     * For backward compatibility, there is another version
     * {@link #ZooKeeper(String, int, Watcher, boolean)} which uses
     * default {@link StaticHostProvider}
     *
     * @param connectString
     *            comma separated host:port pairs, each corresponding to a zk
     *            server. e.g. "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002" If
     *            the optional chroot suffix is used the example would look
     *            like: "127.0.0.1:3000,127.0.0.1:3001,127.0.0.1:3002/app/a"
     *            where the client would be rooted at "/app/a" and all paths
     *            would be relative to this root - ie getting/setting/etc...
     *            "/foo/bar" would result in operations being run on
     *            "/app/a/foo/bar" (from the server perspective).
     * @param sessionTimeout
     *            session timeout in milliseconds
     * @param watcher
     *            a watcher object which will be notified of state changes, may
     *            also be notified for node events
     * @param canBeReadOnly
     *            (added in 3.4) whether the created client is allowed to go to
     *            read-only mode in case of partitioning. Read-only mode
     *            basically means that if the client can't find any majority
     *            servers but there's partitioned server it could reach, it
     *            connects to one in read-only mode, i.e. read requests are
     *            allowed while write requests are not. It continues seeking for
     *            majority in the background.
     * @param aHostProvider
     *            use this as HostProvider to enable custom behaviour.
     *
     * @throws IOException
     *             in cases of network failure
     * @throws IllegalArgumentException
     *             if an invalid chroot path is specified
     */
    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();
    }

啓動兩個進程：

    public void start() {
        sendThread.start();
        eventThread.start();
    }

eventThread的run方法：

 @Override
        public void run() { try { isRunning = true; while (true) { Object event = waitingEvents.take(); if (event == eventOfDeath) { wasKilled = true; } else { processEvent(event); } if (wasKilled) synchronized (waitingEvents) { if (waitingEvents.isEmpty()) { isRunning = false; break; } } } } catch (InterruptedException e) { LOG.error("Event thread exiting due to interruption", e); } LOG.info("EventThread shut down for session: 0x{}", Long.toHexString(getSessionId())); } private void processEvent(Object event) { try { if (event instanceof WatcherSetEventPair) { // each watcher will process the event WatcherSetEventPair pair = (WatcherSetEventPair) event; for (Watcher watcher : pair.watchers) { try { watcher.process(pair.event); } catch (Throwable t) { LOG.error("Error while calling watcher ", t); } } } else if (event instanceof LocalCallback) { LocalCallback lcb = (LocalCallback) event; if (lcb.cb instanceof StatCallback) { ((StatCallback) lcb.cb).processResult(lcb.rc, lcb.path, lcb.ctx, null); } else if (lcb.cb instanceof DataCallback) { ((DataCallback) lcb.cb).processResult(lcb.rc, lcb.path, lcb.ctx, null, null); } else if (lcb.cb instanceof ACLCallback) { ((ACLCallback) lcb.cb).processResult(lcb.rc, lcb.path, lcb.ctx, null, null); } else if (lcb.cb instanceof ChildrenCallback) { ((ChildrenCallback) lcb.cb).processResult(lcb.rc, lcb.path, lcb.ctx, null); } else if (lcb.cb instanceof Children2Callback) { ((Children2Callback) lcb.cb).processResult(lcb.rc, lcb.path, lcb.ctx, null, null); } else if (lcb.cb instanceof StringCallback) { ((StringCallback) lcb.cb).processResult(lcb.rc, lcb.path, lcb.ctx, null); } else { ((VoidCallback) lcb.cb).processResult(lcb.rc, lcb.path, lcb.ctx); } } else { Packet p = (Packet) event; int rc = 0; String clientPath = p.clientPath; if (p.replyHeader.getErr() != 0) { rc = p.replyHeader.getErr(); } if (p.cb == null) { LOG.warn("Somehow a null cb got to EventThread!"); } else if (p.response instanceof ExistsResponse || p.response instanceof SetDataResponse || p.response instanceof SetACLResponse) { StatCallback cb = (StatCallback) p.cb; if (rc == 0) { if (p.response instanceof ExistsResponse) { cb.processResult(rc, clientPath, p.ctx, ((ExistsResponse) p.response) .getStat()); } else if (p.response instanceof SetDataResponse) { cb.processResult(rc, clientPath, p.ctx, ((SetDataResponse) p.response) .getStat()); } else if (p.response instanceof SetACLResponse) { cb.processResult(rc, clientPath, p.ctx, ((SetACLResponse) p.response) .getStat()); } } else { cb.processResult(rc, clientPath, p.ctx, null); } } else if (p.response instanceof GetDataResponse) { DataCallback cb = (DataCallback) p.cb; GetDataResponse rsp = (GetDataResponse) p.response; if (rc == 0) { cb.processResult(rc, clientPath, p.ctx, rsp .getData(), rsp.getStat()); } else { cb.processResult(rc, clientPath, p.ctx, null, null); } } else if (p.response instanceof GetACLResponse) { ACLCallback cb = (ACLCallback) p.cb; GetACLResponse rsp = (GetACLResponse) p.response; if (rc == 0) { cb.processResult(rc, clientPath, p.ctx, rsp .getAcl(), rsp.getStat()); } else { cb.processResult(rc, clientPath, p.ctx, null, null); } } else if (p.response instanceof GetChildrenResponse) { ChildrenCallback cb = (ChildrenCallback) p.cb; GetChildrenResponse rsp = (GetChildrenResponse) p.response; if (rc == 0) { cb.processResult(rc, clientPath, p.ctx, rsp .getChildren()); } else { cb.processResult(rc, clientPath, p.ctx, null); } } else if (p.response instanceof GetChildren2Response) { Children2Callback cb = (Children2Callback) p.cb; GetChildren2Response rsp = (GetChildren2Response) p.response; if (rc == 0) { cb.processResult(rc, clientPath, p.ctx, rsp .getChildren(), rsp.getStat()); } else { cb.processResult(rc, clientPath, p.ctx, null, null); } } else if (p.response instanceof CreateResponse) { StringCallback cb = (StringCallback) p.cb; CreateResponse rsp = (CreateResponse) p.response; if (rc == 0) { cb.processResult(rc, clientPath, p.ctx, (chrootPath == null ? rsp.getPath() : rsp.getPath() .substring(chrootPath.length()))); } else { cb.processResult(rc, clientPath, p.ctx, null); } } else if (p.response instanceof Create2Response) { Create2Callback cb = (Create2Callback) p.cb; Create2Response rsp = (Create2Response) p.response; if (rc == 0) { cb.processResult(rc, clientPath, p.ctx, (chrootPath == null ? rsp.getPath() : rsp.getPath() .substring(chrootPath.length())), rsp.getStat()); } else { cb.processResult(rc, clientPath, p.ctx, null, null); } } else if (p.response instanceof MultiResponse) { MultiCallback cb = (MultiCallback) p.cb; MultiResponse rsp = (MultiResponse) p.response; if (rc == 0) { List<OpResult> results = rsp.getResultList(); int newRc = rc; for (OpResult result : results) { if (result instanceof ErrorResult && KeeperException.Code.OK.intValue() != (newRc = ((ErrorResult) result) .getErr())) { break; } } cb.processResult(newRc, clientPath, p.ctx, results); } else { cb.processResult(rc, clientPath, p.ctx, null); } } else if (p.cb instanceof VoidCallback) { VoidCallback cb = (VoidCallback) p.cb; cb.processResult(rc, clientPath, p.ctx); } } } catch (Throwable t) { LOG.error("Caught unexpected throwable", t); } } }

 

sendThread（

/**
* This class services the outgoing request queue and generates the heart
* beats. It also spawns the ReadThread.
*/

）線程的run方法：

 @Override
        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()) {
                        // 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) {
                        throw new SessionTimeoutException(
                                "Client session timed out, have not heard from server in "
                                        + clientCnxnSocket.getIdleRecv() + "ms"
                                        + " for sessionid 0x"
                                        + Long.toHexString(sessionId));
                    }
                    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);
                } 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與ZooKeeper之間的通訊，須要建立一個Session，這個Session會有一個超時時間。由於ZooKeeper集羣會把Client的Session信息持久化，因此在Session沒超時以前，Client與ZooKeeper Server的鏈接能夠在各個ZooKeeper Server之間透明地移動。

在實際的應用中，若是Client與Server之間的通訊足夠頻繁，Session的維護就不須要其它額外的消息了。不然，ZooKeeper Client會每t/3 ms發一次心跳給Server，若是Client 2t/3 ms沒收到來自Server的心跳回應，就會換到一個新的ZooKeeper Server上。這裏t是用戶配置的Session的超時時間。

 @Override
    void doTransport(int waitTimeOut, List<Packet> pendingQueue, ClientCnxn cnxn)
            throws IOException, InterruptedException {
        selector.select(waitTimeOut);
        Set<SelectionKey> selected;
        synchronized (this) {
            selected = selector.selectedKeys();
        }
        // Everything below and until we get back to the select is
        // non blocking, so time is effectively a constant. That is
        // Why we just have to do this once, here
        updateNow();
        for (SelectionKey k : selected) {
            SocketChannel sc = ((SocketChannel) k.channel());
            if ((k.readyOps() & SelectionKey.OP_CONNECT) != 0) {
                if (sc.finishConnect()) {
                    updateLastSendAndHeard();
                    updateSocketAddresses();
                    sendThread.primeConnection();
                }
            } else if ((k.readyOps() & (SelectionKey.OP_READ | SelectionKey.OP_WRITE)) != 0) {
                doIO(pendingQueue, cnxn);
            }
        }
        if (sendThread.getZkState().isConnected()) {
            if (findSendablePacket(outgoingQueue,
                    sendThread.tunnelAuthInProgress()) != null) {
                enableWrite();
            }
        }
        selected.clear();
    }

ZooKeeper支持一種Watch操做，Client能夠在某個ZNode上設置一個Watcher，來Watch該ZNode上的變化。若是該ZNode上有相應的變化，就會觸發這個Watcher，把相應的事件通知給設置Watcher的Client。須要注意的是，ZooKeeper中的Watcher是一次性的，即觸發一次就會被取消，若是想繼續Watch的話，須要客戶端從新設置Watcher。

  /**
     * @return true if a packet was received
     * @throws InterruptedException
     * @throws IOException
     */
    void doIO(List<Packet> pendingQueue, ClientCnxn cnxn)
      throws InterruptedException, IOException {
        SocketChannel sock = (SocketChannel) sockKey.channel();
        if (sock == null) {
            throw new IOException("Socket is null!");
        }
        if (sockKey.isReadable()) {
            int rc = sock.read(incomingBuffer);
            if (rc < 0) {
                throw new EndOfStreamException(
                        "Unable to read additional data from server sessionid 0x"
                                + Long.toHexString(sessionId)
                                + ", likely server has closed socket");
            }
            if (!incomingBuffer.hasRemaining()) {
                incomingBuffer.flip();
                if (incomingBuffer == lenBuffer) {
                    recvCount++;
                    readLength();
                } else if (!initialized) {
                    readConnectResult();
                    enableRead();
                    if (findSendablePacket(outgoingQueue,
                            sendThread.tunnelAuthInProgress()) != null) {
                        // Since SASL authentication has completed (if client is configured to do so),
                        // outgoing packets waiting in the outgoingQueue can now be sent.
                        enableWrite();
                    }
                    lenBuffer.clear();
                    incomingBuffer = lenBuffer;
                    updateLastHeard();
                    initialized = true;
                } else {
                    sendThread.readResponse(incomingBuffer);
                    lenBuffer.clear();
                    incomingBuffer = lenBuffer;
                    updateLastHeard();
                }
            }
        }
        if (sockKey.isWritable()) {
            Packet p = findSendablePacket(outgoingQueue,
                    sendThread.tunnelAuthInProgress());

            if (p != null) {
                updateLastSend();
                // If we already started writing p, p.bb will already exist
                if (p.bb == null) {
                    if ((p.requestHeader != null) &&
                            (p.requestHeader.getType() != OpCode.ping) &&
                            (p.requestHeader.getType() != OpCode.auth)) {
                        p.requestHeader.setXid(cnxn.getXid());
                    }
                    p.createBB();
                }
                sock.write(p.bb);
                if (!p.bb.hasRemaining()) {
                    sentCount++;
                    outgoingQueue.removeFirstOccurrence(p);
                    if (p.requestHeader != null
                            && p.requestHeader.getType() != OpCode.ping
                            && p.requestHeader.getType() != OpCode.auth) {
                        synchronized (pendingQueue) {
                            pendingQueue.add(p);
                        }
                    }
                }
            }
            if (outgoingQueue.isEmpty()) {
                // No more packets to send: turn off write interest flag.
                // Will be turned on later by a later call to enableWrite(),
                // from within ZooKeeperSaslClient (if client is configured
                // to attempt SASL authentication), or in either doIO() or
                // in doTransport() if not.
                disableWrite();
            } else if (!initialized && p != null && !p.bb.hasRemaining()) {
                // On initial connection, write the complete connect request
                // packet, but then disable further writes until after
                // receiving a successful connection response.  If the
                // session is expired, then the server sends the expiration
                // response and immediately closes its end of the socket.  If
                // the client is simultaneously writing on its end, then the
                // TCP stack may choose to abort with RST, in which case the
                // client would never receive the session expired event.  See
                // http://docs.oracle.com/javase/6/docs/technotes/guides/net/articles/connection_release.html
                disableWrite();
            } else {
                // Just in case
                enableWrite();
            }
        }
    }

 讀代碼的邏輯，從響應報文中讀取到sessionId：

void readConnectResult() throws IOException {
        if (LOG.isTraceEnabled()) {
            StringBuilder buf = new StringBuilder("0x[");
            for (byte b : incomingBuffer.array()) {
                buf.append(Integer.toHexString(b) + ",");
            }
            buf.append("]");
            LOG.trace("readConnectResult " + incomingBuffer.remaining() + " "
                    + buf.toString());
        }
        ByteBufferInputStream bbis = new ByteBufferInputStream(incomingBuffer);
        BinaryInputArchive bbia = BinaryInputArchive.getArchive(bbis);
        ConnectResponse conRsp = new ConnectResponse();
        conRsp.deserialize(bbia, "connect");

        // read "is read-only" flag
        boolean isRO = false;
        try {
            isRO = bbia.readBool("readOnly");
        } catch (IOException e) {
            // this is ok -- just a packet from an old server which
            // doesn't contain readOnly field
            LOG.warn("Connected to an old server; r-o mode will be unavailable");
        }

        this.sessionId = conRsp.getSessionId();
        sendThread.onConnected(conRsp.getTimeOut(), this.sessionId, conRsp.getPasswd(), isRO);
    }

建立鏈接：

/**
         * Callback invoked by the ClientCnxnSocket once a connection has been
         * established.
         * 
         * @param _negotiatedSessionTimeout
         * @param _sessionId
         * @param _sessionPasswd
         * @param isRO
         * @throws IOException
         */
        void onConnected(int _negotiatedSessionTimeout, long _sessionId,
                byte[] _sessionPasswd, boolean isRO) throws IOException {
            negotiatedSessionTimeout = _negotiatedSessionTimeout;
            if (negotiatedSessionTimeout <= 0) {
                state = States.CLOSED;

                eventThread.queueEvent(new WatchedEvent(
                        Watcher.Event.EventType.None,
                        Watcher.Event.KeeperState.Expired, null));
                eventThread.queueEventOfDeath();

                String warnInfo;
                warnInfo = "Unable to reconnect to ZooKeeper service, session 0x"
                    + Long.toHexString(sessionId) + " has expired";
                LOG.warn(warnInfo);
                throw new SessionExpiredException(warnInfo);
            }
            if (!readOnly && isRO) {
                LOG.error("Read/write client got connected to read-only server");
            }
            readTimeout = negotiatedSessionTimeout * 2 / 3;
            connectTimeout = negotiatedSessionTimeout / hostProvider.size();
            hostProvider.onConnected();
            sessionId = _sessionId;
            sessionPasswd = _sessionPasswd;
            state = (isRO) ?
                    States.CONNECTEDREADONLY : States.CONNECTED;
            seenRwServerBefore |= !isRO;
            LOG.info("Session establishment complete on server "
                    + clientCnxnSocket.getRemoteSocketAddress()
                    + ", sessionid = 0x" + Long.toHexString(sessionId)
                    + ", negotiated timeout = " + negotiatedSessionTimeout
                    + (isRO ? " (READ-ONLY mode)" : ""));
            KeeperState eventState = (isRO) ?
                    KeeperState.ConnectedReadOnly : KeeperState.SyncConnected;
            eventThread.queueEvent(new WatchedEvent(
                    Watcher.Event.EventType.None,
                    eventState, null));
        }

保持索引：

    public synchronized void onConnected() {
        lastIndex = currentIndex;
        reconfigMode = false;
    }

 

 客戶端命令分析

　　先讓咱們看看客戶端都提供了哪些命令形式？

ZooKeeperMain中定義了cli的各類命令：

 protected static final Map<String,String> commandMap = new HashMap<String,String>( );
    protected static final Map<String,CliCommand> commandMapCli = 
            new HashMap<String,CliCommand>( );

    protected MyCommandOptions cl = new MyCommandOptions();
    protected HashMap<Integer,String> history = new HashMap<Integer,String>( );
    protected int commandCount = 0;
    protected boolean printWatches = true;

    protected ZooKeeper zk;
    protected String host = "";

    public boolean getPrintWatches( ) {
        return printWatches;
    }

    static {
        commandMap.put("connect", "host:port");
        commandMap.put("history","");
        commandMap.put("redo","cmdno");
        commandMap.put("printwatches", "on|off");
        commandMap.put("quit", "");

        new CloseCommand().addToMap(commandMapCli);
        new CreateCommand().addToMap(commandMapCli);
        new DeleteCommand().addToMap(commandMapCli);
        new DeleteAllCommand().addToMap(commandMapCli);
        // Depricated: rmr
        new DeleteAllCommand("rmr").addToMap(commandMapCli);
        new SetCommand().addToMap(commandMapCli);
        new GetCommand().addToMap(commandMapCli);
        new LsCommand().addToMap(commandMapCli);
        new Ls2Command().addToMap(commandMapCli);
        new GetAclCommand().addToMap(commandMapCli);
        new SetAclCommand().addToMap(commandMapCli);
        new StatCommand().addToMap(commandMapCli);
        new SyncCommand().addToMap(commandMapCli);
        new SetQuotaCommand().addToMap(commandMapCli);
        new ListQuotaCommand().addToMap(commandMapCli);
        new DelQuotaCommand().addToMap(commandMapCli);
        new AddAuthCommand().addToMap(commandMapCli);
        new ReconfigCommand().addToMap(commandMapCli);
        new GetConfigCommand().addToMap(commandMapCli);
        new RemoveWatchesCommand().addToMap(commandMapCli);
        
        // add all to commandMap
        for (Entry<String, CliCommand> entry : commandMapCli.entrySet()) {
            commandMap.put(entry.getKey(), entry.getValue().getOptionStr());
    }
    }

其中，commandMap是全部的命令集合，commandMapCli是鏈接上後的命令集合，

咱們以建立一個節點(CreateCommand)爲例深刻了解一下：

該命令形式爲：create [-s] [-e] [-c] path [data] [acl]

其中 s，e，c分別表明：

sequential，ephemeral，container

ZNode根據其自己的特性，能夠分爲下面兩類：

• Regular ZNode: 常規型ZNode, 用戶須要顯式的建立、刪除
• Ephemeral ZNode: 臨時型ZNode, 用戶建立它以後，能夠顯式的刪除，也能夠在建立它的Session結束後，由ZooKeeper Server自動刪除

ZNode還有一個Sequential的特性，若是建立的時候指定的話，該ZNode的名字後面會自動Append一個不斷增長的SequenceNo。

執行命令的代碼以下：

 @Override
    public boolean exec() throws KeeperException, InterruptedException {
        CreateMode flags = CreateMode.PERSISTENT;
        boolean hasE = cl.hasOption("e");
        boolean hasS = cl.hasOption("s");
        boolean hasC = cl.hasOption("c");
        if (hasC && (hasE || hasS)) {
            err.println("-c cannot be combined with -s or -e. Containers cannot be ephemeral or sequential.");
            return false;
        }

        if(hasE && hasS) {
            flags = CreateMode.EPHEMERAL_SEQUENTIAL;
        } else if (hasE) {
            flags = CreateMode.EPHEMERAL;
        } else if (hasS) {
            flags = CreateMode.PERSISTENT_SEQUENTIAL;
        } else if (hasC) {
            flags = CreateMode.CONTAINER;
        }
        String path = args[1];
        byte[] data = null;
        if (args.length > 2) {
            data = args[2].getBytes();
        }
        List<ACL> acl = ZooDefs.Ids.OPEN_ACL_UNSAFE;
        if (args.length > 3) {
            acl = AclParser.parse(args[3]);
        }
        try {
            String newPath = zk.create(path, data, acl, flags);
            err.println("Created " + newPath);
        } catch(KeeperException.EphemeralOnLocalSessionException e) {
            err.println("Unable to create ephemeral node on a local session");
            return false;
        } catch (KeeperException.InvalidACLException ex) {
            err.println(ex.getMessage());
            return false;
        }
        return true;
    }

建立節點的代碼以下：

 /**
     * Create a node with the given path. The node data will be the given data,
     * and node acl will be the given acl.
     * <p>
     * The flags argument specifies whether the created node will be ephemeral
     * or not.
     * <p>
     * An ephemeral node will be removed by the ZooKeeper automatically when the
     * session associated with the creation of the node expires.
     * <p>
     * The flags argument can also specify to create a sequential node. The
     * actual path name of a sequential node will be the given path plus a
     * suffix "i" where i is the current sequential number of the node. The sequence
     * number is always fixed length of 10 digits, 0 padded. Once
     * such a node is created, the sequential number will be incremented by one.
     * <p>
     * If a node with the same actual path already exists in the ZooKeeper, a
     * KeeperException with error code KeeperException.NodeExists will be
     * thrown. Note that since a different actual path is used for each
     * invocation of creating sequential node with the same path argument, the
     * call will never throw "file exists" KeeperException.
     * <p>
     * If the parent node does not exist in the ZooKeeper, a KeeperException
     * with error code KeeperException.NoNode will be thrown.
     * <p>
     * An ephemeral node cannot have children. If the parent node of the given
     * path is ephemeral, a KeeperException with error code
     * KeeperException.NoChildrenForEphemerals will be thrown.
     * <p>
     * This operation, if successful, will trigger all the watches left on the
     * node of the given path by exists and getData API calls, and the watches
     * left on the parent node by getChildren API calls.
     * <p>
     * If a node is created successfully, the ZooKeeper server will trigger the
     * watches on the path left by exists calls, and the watches on the parent
     * of the node by getChildren calls.
     * <p>
     * The maximum allowable size of the data array is 1 MB (1,048,576 bytes).
     * Arrays larger than this will cause a KeeperExecption to be thrown.
     *
     * @param path
     *                the path for the node
     * @param data
     *                the initial data for the node
     * @param acl
     *                the acl for the node
     * @param createMode
     *                specifying whether the node to be created is ephemeral
     *                and/or sequential
     * @return the actual path of the created node
     * @throws KeeperException if the server returns a non-zero error code
     * @throws KeeperException.InvalidACLException if the ACL is invalid, null, or empty
     * @throws InterruptedException if the transaction is interrupted
     * @throws IllegalArgumentException if an invalid path is specified
     */
    public String create(final String path, byte data[], List<ACL> acl,
            CreateMode createMode)
        throws KeeperException, InterruptedException
    {
        final String clientPath = path;
        PathUtils.validatePath(clientPath, createMode.isSequential());

        final String serverPath = prependChroot(clientPath);

        RequestHeader h = new RequestHeader();
        h.setType(createMode.isContainer() ? ZooDefs.OpCode.createContainer : ZooDefs.OpCode.create);
        CreateRequest request = new CreateRequest();
        CreateResponse response = new CreateResponse();
        request.setData(data);
        request.setFlags(createMode.toFlag());
        request.setPath(serverPath);
        if (acl != null && acl.size() == 0) {
            throw new KeeperException.InvalidACLException();
        }
        request.setAcl(acl);
        ReplyHeader r = cnxn.submitRequest(h, request, response, null);
        if (r.getErr() != 0) {
            throw KeeperException.create(KeeperException.Code.get(r.getErr()),
                    clientPath);
        }
        if (cnxn.chrootPath == null) {
            return response.getPath();
        } else {
            return response.getPath().substring(cnxn.chrootPath.length());
        }
    }

組裝請求發送給zookeeper服務器，並返回響應報文：

    public ReplyHeader submitRequest(RequestHeader h, Record request,
            Record response, WatchRegistration watchRegistration,
            WatchDeregistration watchDeregistration)
            throws InterruptedException {
        ReplyHeader r = new ReplyHeader();
 Packet packet = queuePacket(h, r, request, response, null, null, null,
                null, watchRegistration, watchDeregistration);
        synchronized (packet) {
            while (!packet.finished) {
                packet.wait();
            }
        }
        return r;
    }

請求進入隊列，等待處理：

Packet queuePacket(RequestHeader h, ReplyHeader r, Record request,
            Record response, AsyncCallback cb, String clientPath,
            String serverPath, Object ctx, WatchRegistration watchRegistration,
            WatchDeregistration watchDeregistration) {
        Packet packet = null;

        // Note that we do not generate the Xid for the packet yet. It is
        // generated later at send-time, by an implementation of ClientCnxnSocket::doIO(),
        // where the packet is actually sent.
        packet = new Packet(h, r, request, response, watchRegistration);
        packet.cb = cb;
        packet.ctx = ctx;
        packet.clientPath = clientPath;
        packet.serverPath = serverPath;
        packet.watchDeregistration = watchDeregistration;
        // The synchronized block here is for two purpose:
        // 1. synchronize with the final cleanup() in SendThread.run() to avoid race
        // 2. synchronized against each packet. So if a closeSession packet is added,
        // later packet will be notified.
        synchronized (state) {
            if (!state.isAlive() || closing) {
                conLossPacket(packet);
            } else {
                // If the client is asking to close the session then
                // mark as closing
                if (h.getType() == OpCode.closeSession) {
                    closing = true;
                }
                outgoingQueue.add(packet);
            }
        }
        sendThread.getClientCnxnSocket().packetAdded(); return packet;
    }

 參考文獻：

【1】http://www.wuzesheng.com/?p=2609