ZooKeeper Client Library提供了丰富直观的API供用户程序使用，下面是一些常用的API：
- create(path, data, flags): 创建一个ZNode, path是其路径，data是要存储在该ZNode上的数据，flags常用的有: PERSISTEN, PERSISTENT_SEQUENTAIL, EPHEMERAL, EPHEMERAL_SEQUENTAIL
- delete(path, version): 删除一个ZNode，可以通过version删除指定的版本, 如果version是-1的话，表示删除所有的版本
- exists(path, watch): 判断指定ZNode是否存在，并设置是否Watch这个ZNode。这里如果要设置Watcher的话，Watcher是在创建ZooKeeper实例时指定的，如果要设置特定的Watcher的话，可以调用另一个重载版本的exists(path, watcher)。以下几个带watch参数的API也都类似
- getData(path, watch): 读取指定ZNode上的数据，并设置是否watch这个ZNode
- setData(path, watch): 更新指定ZNode的数据，并设置是否Watch这个ZNode
- getChildren(path, watch): 获取指定ZNode的所有子ZNode的名字，并设置是否Watch这个ZNode
- sync(path): 把所有在sync之前的更新操作都进行同步，达到每个请求都在半数以上的ZooKeeper Server上生效。path参数目前没有用
- setAcl(path, acl): 设置指定ZNode的Acl信息
- getAcl(path): 获取指定ZNode的Acl信息
具体是如何其作用的呢？
客户端连接到服务器
启动客户端的脚本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 $JVMFLAGS \ org.apache.zookeeper.ZooKeeperMain "$@"
从上述脚本可以看出：
连接到zookeeper服务器时，需要启动org.apache.zookeeper.ZooKeeperMain类，其入口main方法如下：
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，主要方法为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); } }
我们以connect命令来看看连接的过程。
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); }
创建客户端：
/** * 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)); }
调用创建客户端方法：
/** * 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(); }
启动两个进程：
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.
*/
）线程的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与ZooKeeper之间的通信，需要创建一个Session，这个Session会有一个超时时间。因为ZooKeeper集群会把Client的Session信息持久化，所以在Session没超时之前，Client与ZooKeeper Server的连接可以在各个ZooKeeper Server之间透明地移动。
在实际的应用中，如果Client与Server之间的通信足够频繁，Session的维护就不需要其它额外的消息了。否则，ZooKeeper Client会每t/3 ms发一次心跳给Server，如果Client 2t/3 ms没收到来自Server的心跳回应，就会换到一个新的ZooKeeper Server上。这里t是用户配置的Session的超时时间。
@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支持一种Watch操作，Client可以在某个ZNode上设置一个Watcher，来Watch该ZNode上的变化。如果该ZNode上有相应的变化，就会触发这个Watcher，把相应的事件通知给设置Watcher的Client。需要注意的是，ZooKeeper中的Watcher是一次性的，即触发一次就会被取消，如果想继续Watch的话，需要客户端重新设置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(); } } }
读代码的逻辑，从响应报文中读取到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); }
创建连接：
/** * 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; }
客户端命令分析
先让我们看看客户端都提供了哪些命令形式？
ZooKeeperMain中定义了cli的各种命令：
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是所有的命令集合，commandMapCli是连接上后的命令集合，
我们以创建一个节点(CreateCommand)为例深入了解一下：
该命令形式为：create [-s] [-e] [-c] path [data] [acl]
其中 s，e，c分别代表：
sequential，ephemeral，container
ZNode根据其本身的特性，可以分为下面两类：
- Regular ZNode: 常规型ZNode, 用户需要显式的创建、删除
- Ephemeral ZNode: 临时型ZNode, 用户创建它之后，可以显式的删除，也可以在创建它的Session结束后，由ZooKeeper Server自动删除
ZNode还有一个Sequential的特性，如果创建的时候指定的话，该ZNode的名字后面会自动Append一个不断增加的SequenceNo。
执行命令的代码如下：
@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; }
创建节点的代码如下：
/** * 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()); } }
组装请求发送给zookeeper服务器，并返回响应报文：
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; }
请求进入队列，等待处理：
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; }
参考文献：
【1】http://www.wuzesheng.com/?p=2609
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