ZooKeeper-znode概念与使用

可以将ZK看作一个具有高可用性特征的文件系统。这个文件系统中没有文件和目录，而是统一使用节点（znode）的概念，称为znode。znode既可以作为保存数据的容器（如同文件），也可以作为保存其他znode的容器（如同目录）。所有的znode构成了一个层次化的命名空间，一种自然的建立组成员列表的方式就是利用这种层次结构，创建一个以组名为节点名的znode作为父节点，然后以组成员名（服务器名）为节点名来创建作为子节点的znode。

创建组

package com.zhen.zookeeper.createGroup;

import java.io.IOException;
import java.util.concurrent.CountDownLatch;

import org.apache.zookeeper.CreateMode;
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.ZooDefs.Ids;
import org.apache.zookeeper.ZooKeeper;

/**
 * @author FengZhen
 * @date 2018年10月13日
 * 创建组
 */
public class CreateGroup implements Watcher{

	private static final int SESSION_TIMEOUT = 5000;

	private ZooKeeper zk;
	private CountDownLatch connectedSignal = new CountDownLatch(1);

	public void connect(String hosts) throws IOException, InterruptedException {
		/**
		 * hosts:ZooKeeper服务的主机地址（可指定端口，默认是2181）
		 * SESSION_TIMEOUT:以毫秒为单位的会话超时参数（此处为5秒）
		 * this:Watcher对象的实例。Watcher对象接收来自于ZooKeeper的回调，以获得各种事件的通知。
		 */
		zk = new ZooKeeper(hosts, SESSION_TIMEOUT, this);
		connectedSignal.await();
	}

	/**
	 * 当客户端已经与ZK建立连接后，Watcher的process方法会被调用
	 * 参数是一个用于表示该连接的事件。
	 */
	public void process(WatchedEvent event) {
		//连接事件
		if (event.getState() == KeeperState.SyncConnected) {
			/**
			 * 通过调用CountDownLatch的countDown方法来递减它的计数器。
			 * 锁存器（latch）被创建时带有一个值为1的计数器，用于表示在它释放所有等待线程之前需要发生的事件数。
			 * 在调用一次countDown方法之后，计数器的值变为0，则await方法返回。
			 */
			connectedSignal.countDown();
		}
	}

	public void create(String groupName) throws KeeperException, InterruptedException {
		String path = "/" + groupName;
		/**
		 * 用ZK的create方法创建一个新的ZK的znode
		 * path：路径（用字符串表示）
		 * null：znode的内容（字节数组，此处为空值）
		 * Ids.OPEN_ACL_UNSAFE：访问控制列表(简称ACL，此处为完全开放的ACL，允许任何客户端对znode进行读写)
		 * CreateMode.PERSISTENT：znode类型
		 * znode类型可以分为两种：1.短暂的(ephemeral)	2.持久的(persistent)
		 * 创建znode的客户端断开连接时，无论客户端是明确断开还是因为任何原因而终止，短暂znode都会被ZK服务删除。持久znode不会被删除。
		 * create方法的返回值是ZK所创建的节点路径
		 */
		String createdPath = zk.create(path, null, Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
		System.out.println("Created " + createdPath);
	}

	public void close() throws InterruptedException {
		zk.close();
	}

	public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
		String hosts = "localhost:2181";
		String groupName = "zoo";
		CreateGroup createGroup = new CreateGroup();
		createGroup.connect(hosts);
		createGroup.create(groupName);
		createGroup.close();
	}

}

　　

提供一个连接类

package com.zhen.zookeeper;

import java.io.IOException;
import java.util.concurrent.CountDownLatch;

import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.Watcher.Event.KeeperState;

/**
 * @author FengZhen
 * @date 2018年10月13日
 * 等待与ZK建立连接
 */
public class ConnectionWatcher implements Watcher{

	private static final int SESSION_TIMEOUT = 5000;

	protected ZooKeeper zk;
	private CountDownLatch connectedSignal = new CountDownLatch(1);

	public void connect(String hosts) throws IOException, InterruptedException {
		/**
		 * hosts:ZooKeeper服务的主机地址（可指定端口，默认是2181）
		 * SESSION_TIMEOUT:以毫秒为单位的会话超时参数（此处为5秒）
		 * this:Watcher对象的实例。Watcher对象接收来自于ZooKeeper的回调，以获得各种事件的通知。
		 */
		zk = new ZooKeeper(hosts, SESSION_TIMEOUT, this);
		connectedSignal.await();
	}

	/**
	 * 当客户端已经与ZK建立连接后，Watcher的process方法会被调用
	 * 参数是一个用于表示该连接的事件。
	 */
	public void process(WatchedEvent event) {
		//连接事件
		if (event.getState() == KeeperState.SyncConnected) {
			/**
			 * 通过调用CountDownLatch的countDown方法来递减它的计数器。
			 * 锁存器（latch）被创建时带有一个值为1的计数器，用于表示在它释放所有等待线程之前需要发生的事件数。
			 * 在调用一次countDown方法之后，计数器的值变为0，则await方法返回。
			 */
			connectedSignal.countDown();
		}
	}

	public void close() throws InterruptedException {
		zk.close();
	}

}

　　

加入组

package com.zhen.zookeeper.joinGroup;

import java.io.IOException;

import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.ZooDefs.Ids;

import com.zhen.zookeeper.ConnectionWatcher;

/**
 * @author FengZhen
 * @date 2018年10月13日
 * 加入组-注册组成员
 */
public class JoinGroup extends ConnectionWatcher{

 	public void join(String groupName, String memberName) throws KeeperException, InterruptedException {
		String path = "/" + groupName + "/" + memberName;
		//创建短暂znode作为组znode的子节点
		String createdPath = zk.create(path, null, Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);
		System.out.println("Created " + createdPath);
	}

 	public static void main(String[] args) throws IOException, InterruptedException, KeeperException {
 		String hosts = "localhost:2181";
		String groupName = "zoo";
		String memberName = "member1";
		JoinGroup joinGroup = new JoinGroup();
		joinGroup.connect(hosts);
		joinGroup.join(groupName, memberName);
		Thread.sleep(Long.MAX_VALUE);
	}

 	/**
 	 * echo dump | nc localhost 2181
		SessionTracker dump:
		Session Sets (3):
		0 expire at Sat Oct 13 15:57:48 CST 2018:
		0 expire at Sat Oct 13 15:57:50 CST 2018:
		1 expire at Sat Oct 13 15:57:52 CST 2018:
			0x166594274c90001
		ephemeral nodes dump:
		Sessions with Ephemerals (1):
		0x166594274c90001:
			/zoo/member1
 	 */
}

　　

列出组成员

package com.zhen.zookeeper.listGroup;

import java.io.IOException;
import java.util.List;

import org.apache.zookeeper.KeeperException;

import com.zhen.zookeeper.ConnectionWatcher;

/**
 * @author FengZhen
 * @date 2018年10月13日
 * 列出组成员
 */
public class ListGroup extends ConnectionWatcher{

	public void list(String groupName) {
		String path = "/" + groupName;
		try {
			/**
			 * 输出一个znode的子节点列表
			 * path：该znode的路径
			 * false:观察标志
			 * 如果在一个znode上设置了观察标志，那么一旦该znode的状态改变，关联的观察（Watcher）会被触发。
			 * 虽然此处没有使用观察，但在查看一个znode的子节点时，通过设置观察可以让应用程序接收到组成员加入、退出和组被删除的有关通知
			 */
			List<String> children = zk.getChildren(path, false);
			if (children.isEmpty()) {
				System.out.printf("No members in group %s\n", groupName);
				System.exit(1);
			}
			for (String child : children) {
				System.out.println(child);
			}
		} catch (KeeperException e) {
			System.out.printf("Group %s does not exist\n", groupName);
			System.exit(1);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

	public static void main(String[] args) throws IOException, InterruptedException {
		String hosts = "localhost:2181";
		String groupName = "zoo";
		ListGroup listGroup = new ListGroup();
		listGroup.connect(hosts);
		listGroup.list(groupName);
		listGroup.close();
	}

	/**
	 * 也可使用命令行工具
	 * ./zkCli.sh -server localhost:2181 ls /zoo
	 * Connecting to localhost:2181
		2018-10-13 16:26:10,856 [myid:] - INFO  [main:Environment@100] - Client environment:zookeeper.version=3.4.9-1757313, built on 08/23/2016 06:50 GMT
		2018-10-13 16:26:10,860 [myid:] - INFO  [main:Environment@100] - Client environment:host.name=192.168.1.109
		2018-10-13 16:26:10,860 [myid:] - INFO  [main:Environment@100] - Client environment:java.version=1.8.0_162
		2018-10-13 16:26:10,863 [myid:] - INFO  [main:Environment@100] - Client environment:java.vendor=Oracle Corporation
		2018-10-13 16:26:10,863 [myid:] - INFO  [main:Environment@100] - Client environment:java.home=/Library/Java/JavaVirtualMachines/jdk1.8.0_162.jdk/Contents/Home/jre
		2018-10-13 16:26:10,863 [myid:] - INFO  [main:Environment@100] - Client environment:java.class.path=/Users/FengZhen/Desktop/Hadoop/zookeeper-3.4.9/bin/../build/classes:/Users/FengZhen/Desktop/Hadoop/zookeeper-3.4.9/bin/../build/lib/*.jar:/Users/FengZhen/Desktop/Hadoop/zookeeper-3.4.9/bin/../lib/slf4j-log4j12-1.6.1.jar:/Users/FengZhen/Desktop/Hadoop/zookeeper-3.4.9/bin/../lib/slf4j-api-1.6.1.jar:/Users/FengZhen/Desktop/Hadoop/zookeeper-3.4.9/bin/../lib/netty-3.10.5.Final.jar:/Users/FengZhen/Desktop/Hadoop/zookeeper-3.4.9/bin/../lib/log4j-1.2.16.jar:/Users/FengZhen/Desktop/Hadoop/zookeeper-3.4.9/bin/../lib/jline-0.9.94.jar:/Users/FengZhen/Desktop/Hadoop/zookeeper-3.4.9/bin/../zookeeper-3.4.9.jar:/Users/FengZhen/Desktop/Hadoop/zookeeper-3.4.9/bin/../src/java/lib/*.jar:/Users/FengZhen/Desktop/Hadoop/zookeeper-3.4.9/bin/../conf:
		2018-10-13 16:26:10,863 [myid:] - INFO  [main:Environment@100] - Client environment:java.library.path=/Users/FengZhen/Library/Java/Extensions:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java:.
		2018-10-13 16:26:10,863 [myid:] - INFO  [main:Environment@100] - Client environment:java.io.tmpdir=/var/folders/tr/vc5hfpsd2c56s7m9wh93401h0000gn/T/
		2018-10-13 16:26:10,863 [myid:] - INFO  [main:Environment@100] - Client environment:java.compiler=<NA>
		2018-10-13 16:26:10,863 [myid:] - INFO  [main:Environment@100] - Client environment:os.name=Mac OS X
		2018-10-13 16:26:10,864 [myid:] - INFO  [main:Environment@100] - Client environment:os.arch=x86_64
		2018-10-13 16:26:10,864 [myid:] - INFO  [main:Environment@100] - Client environment:os.version=10.13.4
		2018-10-13 16:26:10,864 [myid:] - INFO  [main:Environment@100] - Client environment:user.name=FengZhen
		2018-10-13 16:26:10,864 [myid:] - INFO  [main:Environment@100] - Client environment:user.home=/Users/FengZhen
		2018-10-13 16:26:10,864 [myid:] - INFO  [main:Environment@100] - Client environment:user.dir=/Users/FengZhen/Desktop/Hadoop/zookeeper-3.4.9/bin
		2018-10-13 16:26:10,865 [myid:] - INFO  [main:ZooKeeper@438] - Initiating client connection, connectString=localhost:2181 sessionTimeout=30000 watcher=org.apache.zookeeper.ZooKeeperMain$MyWatcher@799f7e29
		2018-10-13 16:26:10,898 [myid:] - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1032] - Opening socket connection to server localhost/0:0:0:0:0:0:0:1:2181. Will not attempt to authenticate using SASL (unknown error)
		2018-10-13 16:26:11,014 [myid:] - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@876] - Socket connection established to localhost/0:0:0:0:0:0:0:1:2181, initiating session
		2018-10-13 16:26:11,024 [myid:] - INFO  [main-SendThread(localhost:2181):ClientCnxn$SendThread@1299] - Session establishment complete on server localhost/0:0:0:0:0:0:0:1:2181, sessionid = 0x166594274c90006, negotiated timeout = 30000

		WATCHER::

		WatchedEvent state:SyncConnected type:None path:null
		[member1]
	 */
}

　　

删除组

package com.zhen.zookeeper.deleteGroup;

import java.io.IOException;
import java.util.List;

import org.apache.zookeeper.KeeperException;

import com.zhen.zookeeper.ConnectionWatcher;

/**
 * @author FengZhen
 * @date 2018年10月13日
 * 删除组
 *
 */
public class DeleteGroup extends ConnectionWatcher{

	public void delete(String groupName) {
		String path = "/" + groupName;
		try {
			List<String> children = zk.getChildren(path, false);
			for (String child : children) {
				zk.delete(path + "/" + child, -1);
			}
			/**
			 * delete方法有两个参数
			 * path：节点路径
			 * -1：版本号
			 * 如果所提供的版本号与znode的版本号一致，ZK会删除这个znode。
			 * 这是一种乐观的加锁机制，使客户端能够检测出对znode的修改冲突。
			 * 通过将版本号设置为-1，可以绕过这个版本检测机制，不管znode的版本号是什么而直接将其删除。
			 * ZK不支持递归删除，因此在删除父节点之前必须先删除子节点
			 */
			zk.delete(path, -1);
		} catch (KeeperException e) {
			System.out.printf("Group %s does not exist\n", groupName);
			System.exit(1);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

	public static void main(String[] args) throws IOException, InterruptedException {
		String hosts = "localhost:2181";
		String groupName = "zoo";
		DeleteGroup deleteGroup = new DeleteGroup();
		deleteGroup.connect(hosts);
		deleteGroup.delete(groupName);
		deleteGroup.close();
	}

}