基于curator实现leader选举 博客分类： zookeeper zookeepercuratorleader选举

1.前言
curator由Netflix的工程师开发，主要目的为了基于zookeeper的应用变得简单可靠，在2013年成为apache的*项目。curator基于zookeeper，但提供了更高级别的
API抽象以及工具集，并对zookeeper提供的常用功能进行了封装和扩充，例如leader选举、分布式锁、服务发现、缓存等功能，从而使开发者在实现这些功能时不用在实现
哪些无聊的程式化代码段，也减少出错的可能性。

2.本文主要讲解如何利用curator实现leader选举，并对curator提供的两种实现形式进行对比
2.1 通过LeaderSelector进行leader选举
建议通过LeaderSelectorListenerAdapter实现，当某个实例成为leader后，会调用对应实例的takeLeadership方法，此方法执行期间，此实例一直占着leader权，
当takeLeadership方法执行结束，实例自动释放leader权，所有实例重新进行leader选举

package chengf.falcon.curator.leader;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.recipes.leader.LeaderSelectorListenerAdapter;
import org.apache.curator.framework.recipes.leader.LeaderSelector;
import java.io.Closeable;
import java.io.IOException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * @author: 作者： chengaofeng
 * @date: 创建时间：2018-06-07 15:26:54
 * @Description: TODO
 * @version V1.0
 */
public class SelectorClient extends LeaderSelectorListenerAdapter implements Closeable {
	private final String name;
	private final LeaderSelector leaderSelector;
	private final AtomicInteger leaderCount = new AtomicInteger();

	public SelectorClient(CuratorFramework client, String path, String name) {
		this.name = name;

		// 利用一个给定的路径创建一个leader selector
		// 执行leader选举的所有参与者对应的路径必须一样
		// 本例中SelectorClient也是一个LeaderSelectorListener，但这不是必须的。
		leaderSelector = new LeaderSelector(client, path, this);

		// 在大多数情况下，我们会希望一个selector放弃leader后还要重新参与leader选举
		leaderSelector.autoRequeue();
	}

	/**
	 * 启动当前实例参与leader选举
	 * 
	 * @throws IOException
	 */
	public void start() throws IOException {
		// leader选举是在后台处理的，所以这个方法会立即返回
		leaderSelector.start();
	}

	@Override
	public void close() throws IOException {
		leaderSelector.close();
	}

	/**
	 * 当前实例成为leader时，会执行下面的方法，这个方法执行结束后，当前实例就自动释放leader了，所以在想放弃leader前此方法不能结束
	 */
	@Override
	public void takeLeadership(CuratorFramework client) throws Exception {

		final int waitSeconds = (int) (5 * Math.random()) + 1;

		System.out.println(name + " 现在是leader了，持续成为leader " + waitSeconds + " 秒.");
		System.out.println(name + " 之前已经成为了 " + leaderCount.getAndIncrement() + " 次leader.");
		try {
			Thread.sleep(TimeUnit.SECONDS.toMillis(waitSeconds));
		} catch (InterruptedException e) {
			System.err.println(name + " was interrupted.");
			Thread.currentThread().interrupt();
		} finally {
			System.out.println(name + " 释放leader权.\n");
		}
	}
}

执行类

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package chengf.falcon.curator.leader;

import com.google.common.collect.Lists;
import org.apache.curator.utils.CloseableUtils;
import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.retry.ExponentialBackoffRetry;
import org.apache.curator.test.TestingServer;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.List;

public class LeaderSelectorExample {
	private static final int CLIENT_QTY = 10;

	private static final String PATH = "/examples/leader";

	public static void main(String[] args) throws Exception {
		// all of the useful sample code is in ExampleClient.java

		System.out.println("创建 " + CLIENT_QTY
				+ " 个客户端, 公平的参与leader选举，成为leader后，会等待一个随机的时间（几秒中），之后释放leader权，所有实例重新进行leader选举。");

		List<CuratorFramework> clients = Lists.newArrayList();
		List<SelectorClient> examples = Lists.newArrayList();
		TestingServer server = new TestingServer();
		try {
			for (int i = 0; i < CLIENT_QTY; ++i) {
				CuratorFramework client = CuratorFrameworkFactory.newClient(server.getConnectString(),
						new ExponentialBackoffRetry(1000, 3));
				clients.add(client);

				SelectorClient example = new SelectorClient(client, PATH, "Client #" + i);
				examples.add(example);

				client.start();
				example.start();
			}

			System.out.println("按 enter/return 来退出\n");
			new BufferedReader(new InputStreamReader(System.in)).readLine();
		} finally {
			System.out.println("关闭程序...");

			for (SelectorClient exampleClient : examples) {
				CloseableUtils.closeQuietly(exampleClient);
			}
			for (CuratorFramework client : clients) {
				CloseableUtils.closeQuietly(client);
			}

			CloseableUtils.closeQuietly(server);
		}
	}
}

执行结果

创建 10 个客户端, 公平的参与leader选举，成为leader后，会等待一个随机的时间（几秒中），之后释放leader权，所有实例重新进行leader选举。
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
SLF4J: Failed to load class "org.slf4j.impl.StaticMDCBinder".
SLF4J: Defaulting to no-operation MDCAdapter implementation.
SLF4J: See http://www.slf4j.org/codes.html#no_static_mdc_binder for further details.
按 enter/return 来退出

Client #2 现在是leader了，持续成为leader 3 秒.
Client #2 之前已经成为了 0 次leader.
Client #2 释放leader权.

Client #4 现在是leader了，持续成为leader 3 秒.
Client #4 之前已经成为了 0 次leader.
Client #4 释放leader权.

Client #5 现在是leader了，持续成为leader 5 秒.
Client #5 之前已经成为了 0 次leader.
Client #5 释放leader权.

Client #9 现在是leader了，持续成为leader 3 秒.
Client #9 之前已经成为了 0 次leader.
Client #9 释放leader权.

Client #6 现在是leader了，持续成为leader 3 秒.
Client #6 之前已经成为了 0 次leader.
Client #6 释放leader权.

Client #1 现在是leader了，持续成为leader 4 秒.
Client #1 之前已经成为了 0 次leader.
Client #1 释放leader权.

Client #8 现在是leader了，持续成为leader 1 秒.
Client #8 之前已经成为了 0 次leader.
Client #8 释放leader权.

Client #7 现在是leader了，持续成为leader 3 秒.
Client #7 之前已经成为了 0 次leader.
Client #7 释放leader权.

Client #0 现在是leader了，持续成为leader 2 秒.
Client #0 之前已经成为了 0 次leader.
Client #0 释放leader权.

Client #3 现在是leader了，持续成为leader 5 秒.
Client #3 之前已经成为了 0 次leader.
Client #3 释放leader权.

Client #2 现在是leader了，持续成为leader 4 秒.
Client #2 之前已经成为了 1 次leader.
Client #2 释放leader权.

Client #4 现在是leader了，持续成为leader 3 秒.
Client #4 之前已经成为了 1 次leader.
Client #4 释放leader权.

Client #5 现在是leader了，持续成为leader 3 秒.
Client #5 之前已经成为了 1 次leader.
Client #5 释放leader权.

Client #9 现在是leader了，持续成为leader 3 秒.
Client #9 之前已经成为了 1 次leader.
Client #9 释放leader权.

Client #6 现在是leader了，持续成为leader 1 秒.
Client #6 之前已经成为了 1 次leader.
Client #6 释放leader权.

Client #1 现在是leader了，持续成为leader 4 秒.
Client #1 之前已经成为了 1 次leader.
Client #1 释放leader权.

Client #8 现在是leader了，持续成为leader 5 秒.
Client #8 之前已经成为了 1 次leader.
关闭程序...
Client #8 was interrupted.
Client #8 释放leader权.

2.2 通过LeaderLatch实现leader选举，具体实现LeaderLatchListener来实现，
当实例成为leader后，会调用isLeader()方法，之后除非此实例连接不到zookeeper，否侧将一直占着leader权，当失去leader权后会调用notLeader()方法，为了模拟
选举过程，我们追加了一个ScheduledExecutorService来周期性的自己释放leader权

/**
 * 
 */
package chengf.falcon.curator.leader;

import java.io.IOException;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.recipes.leader.LeaderLatch;
import org.apache.curator.framework.recipes.leader.LeaderLatchListener;

/**
 * @author: 作者： chengaofeng
 * @date: 创建时间：2018-06-07 15:26:54
 * @Description: TODO
 * @version V1.0
 */
public class LatchClient implements LeaderLatchListener {

	private final String name;

	private LeaderLatch leaderLatch;

	private CuratorFramework client;
	public LatchClient(CuratorFramework client, String path, String name) throws Exception {
		this.name = name;
		this.client = client;
		leaderLatch = new LeaderLatch(client, path);
		leaderLatch.addListener(this);
		leaderLatch.start();
	}

	/*
	 * (non-Javadoc)
	 * 
	 * @see
	 * org.apache.curator.framework.recipes.leader.LeaderLatchListener#isLeader(
	 * )
	 */
	/**
	 * 成为leader后会执行下面的方法，方法执行完后不会释放leader权
	 */
	@Override
	public void isLeader() {
		System.out.println(name + " is now the leader. ");
	}

	/**
	 * 失去leader后执行下面的方法
	 */
	@Override
	public void notLeader() {
		System.out.println(name + " is not the leader. ");

	}
	
	public void stop() {
		try {
			leaderLatch.close();
		} catch (IOException e) {
		}
	}
	
	public boolean hasLeadership() {
		return leaderLatch.hasLeadership();
	}
	public CuratorFramework getClient() {
		return client;
	}
	
	public String getName() {
		return name;
	}

}

启动类

/**
 * 
 */
package chengf.falcon.curator.leader;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.List;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.retry.ExponentialBackoffRetry;
import org.apache.curator.test.TestingServer;
import org.apache.curator.utils.CloseableUtils;

import com.google.common.collect.Lists;

/**
 * @author: 作者： chengaofeng
 * @date: 创建时间：2018-06-07 15:25:47
 * @Description: TODO
 * @version V1.0
 */
public class LeaderLatchExample {
	private static final int CLIENT_QTY = 10;

	private static final String PATH = "/examples/leader";

	public static void main(String[] args) throws Exception {
		// all of the useful sample code is in ExampleClient.java

		System.out.println("创建 " + CLIENT_QTY
				+ " 个客户端, 公平的参与leader选举，成为leader后，将一直占用此leader权。直到四秒中后主动放弃leader权");


		List<CuratorFramework> clients = Lists.newArrayList();
		TestingServer server = new TestingServer();
		List<LatchClient> examples = Lists.newArrayList();
		int i = 0;
		try {
			for (; i < CLIENT_QTY; ++i) {
				CuratorFramework client = CuratorFrameworkFactory.newClient(server.getConnectString(),
						new ExponentialBackoffRetry(1000, 3));
				clients.add(client);

				LatchClient example = new LatchClient(client, PATH, "Client #" + i);
				examples.add(example);
				client.start();
				
			}
			
			// 不调用该方法则某实例成为leader后将一直持续占有leader权
			ScheduledExecutorService se = giveUpLeader(examples);
			
			System.out.println("按 enter/return 来退出\n");
			new BufferedReader(new InputStreamReader(System.in)).readLine();
			se.shutdown();
		} finally {
			System.out.println("关闭程序...");

			for (LatchClient exampleClient : examples) {
				exampleClient.stop();
			}
			for (CuratorFramework client : clients) {
				CloseableUtils.closeQuietly(client);
			}

			CloseableUtils.closeQuietly(server);
		}
	}

	/**
	 * 主动放弃leader权，并新创建一个实例参与leader选举
	 * @param examples
	 */
	private static ScheduledExecutorService giveUpLeader(List<LatchClient> examples) {
		ScheduledExecutorService se = Executors.newScheduledThreadPool(1);
		se.scheduleAtFixedRate(()->{
			examples.stream().filter(t-> t.hasLeadership()).findFirst().ifPresent(t-> {
				examples.remove(t);
				LatchClient example;
				try {
					example = new LatchClient(t.getClient(), PATH, t.getName());
					examples.add(example);
				} catch (Exception e) {
					
				}
				//当前实例主动放弃leader权
				System.out.println(t.getName() + " 现在主动放弃leader权");
				t.stop();});
		}, 4, 4, TimeUnit.SECONDS);
		return se;
	}
	
	
}

执行结果

创建 10 个客户端, 公平的参与leader选举，成为leader后，将一直占用此leader权。直到四秒中后主动放弃leader权
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
SLF4J: Failed to load class "org.slf4j.impl.StaticMDCBinder".
SLF4J: Defaulting to no-operation MDCAdapter implementation.
SLF4J: See http://www.slf4j.org/codes.html#no_static_mdc_binder for further details.
按 enter/return 来退出

Client #3 is now the leader. 
Client #3 现在主动放弃leader权
Client #2 is now the leader. 
Client #2 现在主动放弃leader权
Client #9 is now the leader. 
Client #9 现在主动放弃leader权
Client #4 is now the leader. 
Client #4 现在主动放弃leader权
Client #1 is now the leader. 
Client #1 现在主动放弃leader权
Client #5 is now the leader. 
Client #5 现在主动放弃leader权
Client #7 is now the leader. 
Client #7 现在主动放弃leader权
Client #8 is now the leader. 
Client #8 现在主动放弃leader权
Client #6 is now the leader. 
Client #6 现在主动放弃leader权
Client #0 is now the leader. 

关闭程序...