大数据学习笔记-------------------(12_1)

第11章 zookeeper应用程序接口

Zookeeper有一个官方的API绑定的Java和C。 ZooKeeper Community提供大多数语言（.NET，Python等）的API。使用ZooKeeper的API，应用程序可以连接、交互、操纵数据、协调，最终从ZooKeeper ensemble断开。

ZooKeeper API具有丰富的功能集来获得的ZooKeeper ensemble的所有功能用一个简单而安全的方式。ZooKeeper API提供同步和异步方法。

ZooKeeper ensemble和ZooKeeper API在各方面完全相得益彰，它有利于开发者用一个很好的方式。

11.1 ZooKeeper API基础

与ZooKeeper ensemble交互的应用被称为ZooKeeper Client或着客户端(Client)。

 Znode是ZooKeeper ensemble核心组件，ZooKeeper API提供了一套方法来操作ZooKeeper ensemble中znode的详细信息。

 客户端(Client)应遵循下面给出与ZooKeeper ensemble进行清晰整洁互动的步骤。

Ø  连接到ZooKeeper ensemble。ZooKeeper ensemble为客户端分配一个会话的ID。

Ø  定期发送心跳到服务器。否则，ZooKeeper ensemble中到期的会话ID和客户端需要重新连接。

Ø  Get/Set znodes只要会话ID是有效的。

Ø  一旦所有的任务都完成， ZooKeeperensemble断开。如果客户端长时间处于非活动状态，那么ZooKeeper ensemble会自动断开客户端。

11.2 Java 绑定

ZooKeeper API的核心部分是ZooKeeperclass。提供连接ZooKeeper ensemble构造的选项，连接方法如下：

Ø  connect---连接到ZooKeeper ensemble

Ø  create-------创建一个znode

Ø  exists--------检查znode是否存在以及节点信息

Ø  getData-----从指定节点获取数据

Ø  setData-----从指定节点设置数据

Ø  getChildren--在指定znode中获取所有可用的sub-nodes

Ø  delete----------删除指定znode及所有children

Ø  close-----------关闭连接

11.3 连接zookeeper ensemble

zookeeperclass通过它的constructor提供连接的函数。constructor特征如下：  

 ZooKeeper(String connectionString, int sessionTimeout, Watcher watcher)

 参数解释说明：

connectionString----zookeeper ensemble host(服务)

sessionTimeout---会话超时单位用毫秒

watcher----------对象实施“Watcher”的接口。zookeeper ensemble返回连接的status通过watcher对象。

 创建一个新的helper类:ZooKeeperConnection并添加一个方法:connect。connect方法创建一个ZooKeeper对象、连接到ZooKeeperensemble、然后返回该对象。

CountDownLatch被用于停止（等待）的主要过程，直到客户端与ZooKeeper ensemble连接到。

ZooKeeperensemble通过Watcher callback返回连接状态。一旦客户端与ZooKeeper ensemble连接，Watcher callback被调用；Watcher callback调用CountDownLatch中CountDownLatch方法释放主进程中的lock、await。

下面是与ZooKeeper ensemble连接的完整代码: ZooKeeperConnection.java 

// import java classesimport java.io.IOException;import java.util.concurrent.CountDownLatch;// import zookeeper classesimport org.apache.zookeeper.KeeperException;import org.apache.zookeeper.WatchedEvent;import org.apache.zookeeper.Watcher;import org.apache.zookeeper.Watcher.Event.KeeperState;import org.apache.zookeeper.ZooKeeper;import org.apache.zookeeper.AsyncCallback.StatCallback;import org.apache.zookeeper.KeeperException.Code;import org.apache.zookeeper.data.Stat;public class ZooKeeperConnection {// declare zookeeper instance to access ZooKeeper ensembleprivate ZooKeeper zoo;final CountDownLatch connectedSignal = new CountDownLatch(1);// Method to connect zookeeper ensemble.public ZooKeeper connect(String host) throws IOException,InterruptedException {zoo = new ZooKeeper(host,5000,new Watcher() {public void process(WatchedEvent we) {if (we.getState() == KeeperState.SyncConnected) {connectedSignal.countDown();}}});connectedSignal.await();return zoo;}// Method to disconnect from zookeeper serverpublic void close() throws InterruptedException {zoo.close();}}

11.4 创建Znode

zookeeper类提供create方法来创建ZooKeeper ensemble中新的znode。create方法的参数如下：      

          

create(String path, byte[] data, List<ACL> acl, CreateMode createMode)

参数解释说明：

path----------Znode Path。例如，/myapp1，/myapp1/mydata1

data----------在指定znode path存储data

acl-----------被创建node的接入控制列表(ACL)。zookeeper API为了解一些基本的ACL列表提供一个静态接口:ZooDefs.Ids。例如，ZooDefs.Ids.OPEN_ACL_UNSAFE返回打开节点的ACL列表

createMode-----node的类型，要么ephemeral、sequential、both。是枚举类型。

创建一个新的Java应用程序来检查ZooKeeperAPI的create功能。创建一个文件ZKCreate.java。在main方法中，创建一个类型ZooKeeperConnection的对象，并调用connect方法来连接的ZooKeeper ensemble。

connect方法将返回ZooKeeper对象ZK。现在，ZK调用对象的自定义路径(path)和数据(data)创建方法。完整的程序代码来创建znode如下：ZKCreate.java 

                                           

import java.io.IOException;import org.apache.zookeeper.WatchedEvent;import org.apache.zookeeper.Watcher;import org.apache.zookeeper.Watcher.Event.KeeperState;import org.apache.zookeeper.ZooKeeper;import org.apache.zookeeper.KeeperException;import org.apache.zookeeper.CreateMode;import org.apache.zookeeper.ZooDefs;public class ZKCreate {     // create static instance for zookeeper class.     private static ZooKeeper zk;     // create static instance for ZooKeeperConnection class.private static ZooKeeperConnection conn;// Method to create znode in zookeeper ensemblepublic static void create(String path, byte[] data) throws KeeperException,InterruptedException {zk.create(path, data, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);}public static void main(String[] args){// znode pathString path = "/MyFirstZnode"; // Assign path to znode// data in byte arraybyte[] data = "My first zookeeper app”.getBytes(); // Declare datatry {conn = new ZooKeeperConnection();zk = conn.connect("localhost");create(path, data); // Create the data to the specified pathconn.close();} catch (Exception e) {System.out.println(e.getMessage()); //Catch error message}}}   

一旦应用程序编译并执行，带有指定数据的znode被创建在zookeeperensemble。用zookeeper CLI zkCli.sh检查它：

 启动客服端：bin/zkCli.shstart

调用get：get /MyFirstZnode

11.5Exists---检查znode是否存在

zookeeper类提供exists方法来检查znode的存在。如果指定的znode存在，它返回一个znode的元数据。该exists方法如下：         

exists(String path, boolean watcher)

参数解释说明：

path-----znode 路径

watcher----布尔值，给指定znode设置watch

创建一个新的Java应用程序来检查zookeeperAPI 的行exists函数。创建一个文件ZKExists.java。在main方法中，创建ZooKeeper的对象， ZK使用ZooKeeperConnection对象。于是，使用带有自定义的path的zk对象调用exist函数。完整的列表如下：ZKExists.java      

    

import java.io.IOException;import org.apache.zookeeper.ZooKeeper;import org.apache.zookeeper.KeeperException;import org.apache.zookeeper.WatchedEvent;import org.apache.zookeeper.Watcher;import org.apache.zookeeper.Watcher.Event.KeeperState;import org.apache.zookeeper.data.Stat;public class ZKExists {private static ZooKeeper zk;private static ZooKeeperConnection conn;// Method to check existence of znode and its status, if znode is available.public static Stat znode_exists(String path) throws KeeperException,InterruptedException {return zk.exists(path, true);}public static void main(String[] args) throws InterruptedException,KeeperException {String path= "/MyFirstZnode"; // Assign znode to the specified pathtry {conn = new ZooKeeperConnection();zk = conn.connect("localhost");Stat stat = znode_exists(path); // Stat checks the path of the znodeif(stat!= null) {System.out.println("Node exists and the node version is " + stat.getVersion());} else {System.out.println("Node does not exists");}}catch(Exception e) {System.out.println(e.getMessage()); // Catches error messages}}}

一旦应用程序被编译、执行，输出信息：Nodeexists and the node version is 1.  

11.6 getData方法

zookeeper类提供的getData方法来获取连接在指定znode的数据及其status。GetData方法：

getData(String path, Watcher watcher, Stat stat)

参数解释说明：

 path-------Znode 路径

watcher----Watcher类型的回调函数。zookeeper ensemble将通过回调watcher通知，当指点节点数据变化时。一次通知

 stat--------返回znode的元数据

创建一个新的Java应用程序帮助理解ZooKeeperAPI中getData函数。创建一个文件ZKGetData.java。在main方法中，使用ZooKeeperConnection对象创建一个ZooKeeper对象ZK。然后，ZK对象调用自定义路径GetData方法。

下面是完整的程序代码，以获取从指定的节点的数据：ZKGetData.java 

import java.io.IOException;import java.util.concurrent.CountDownLatch;import org.apache.zookeeper.ZooKeeper;import org.apache.zookeeper.KeeperException;import org.apache.zookeeper.WatchedEvent;import org.apache.zookeeper.Watcher;import org.apache.zookeeper.Watcher.Event.KeeperState;import org.apache.zookeeper.data.Stat;public class ZKGetData {private static ZooKeeper zk;private static ZooKeeperConnection conn;public static Stat znode_exists(String path) throws KeeperException,InterruptedException {return zk.exists(path,true);}public static void main(String[] args) throws InterruptedException, KeeperException {String path = "/MyFirstZnode";final CountDownLatch connectedSignal = new CountDownLatch(1);try {conn = new ZooKeeperConnection();zk = conn.connect("localhost");Stat stat = znode_exists(path);if(stat != null) {byte[] b = zk.getData(path, new Watcher() {public void process(WatchedEvent we) {if (we.getType() == Event.EventType.None) {switch(we.getState()) {case Expired:connectedSignal.countDown();break;}} else {String path = "/MyFirstZnode";try {byte[] bn = zk.getData(path, false, null);String data = new String(bn, "UTF-8");System.out.println(data);connectedSignal.countDown();} catch(Exception ex) {System.out.println(ex.getMessage());}}}}, null);String data = new String(b, "UTF-8");System.out.println(data);connectedSignal.await();} else {System.out.println("Node does not exists");}}catch(Exception e) {System.out.println(e.getMessage());}}}

一旦编译并运行，输出：My first zookeeper app。用zookeeper CLIzkCli.sh检查它：

 启动客服端：bin/zkCli.shstart

调用set：set /MyFirstZnode Hello

应用程序输出"Hello"并退出。