ZooKeeper 客户端API操作

一、节点信息

前提：centos102、centos103、centos104 服务器都已经开启

pom.xml 依赖

    
        junit
        junit
        4.13.2
    

    
        org.apache.logging.log4j
        log4j-core
        2.17.1
    

    
        org.apache.zookeeper
        zookeeper
        3.5.7
    
    
        org.junit.jupiter
        junit-jupiter
        RELEASE
        compile
    

log4j.properties 配置

# 设置全局的日志记录级别为 INFO
log4j.rootLogger=INFO, stdout

# 控制台输出
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d %p [%c] - %m%n

# 文件输出
log4j.appender.logfile=org.apache.log4j.FileAppender
log4j.appender.logfile.File=target/spring.log
log4j.appender.logfile.layout=org.apache.log4j.PatternLayout
log4j.appender.logfile.layout.ConversionPattern=%d %p [%c] - %m%n

1、创建节点

zkClient.java 代码

// 注意：逗号后面不能有空格
private String connectString = "centos102:2181,centos103:2181,centos104:2181";
private int sessionTimeout = 2000;
private ZooKeeper zkClient;

// 创建客户端
@Before
public void init() throws IOException {

    zkClient = new ZooKeeper(connectString, sessionTimeout, new Watcher() {
        @Override
        public void process(WatchedEvent event) {

        }
    });
}

// 创建子节点
@Test
public void create() throws InterruptedException, KeeperException {
    String nodeCreated = zkClient.create("/frost", "cat".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
    
}

运行创建子节点，看看是否创建了该节点

2、获取子节点并监听节点变化

@Test
public void getChildren() throws InterruptedException, KeeperException {
    List<String> children = zkClient.getChildren("/", true);

    for (String child : children) {
        System.out.println(child);
    }
}

那如果此时我再创建一个节点，此时控制台没有任何变化，我想要创建一个节点控制台能够看到相关变化怎么办？此时只需要将程序保持不结束，然后将客户端查看子节点函数放入监听器中。

3、判断节点是否存在

@Test
public void exit() throws InterruptedException, KeeperException {
    Stat stat = zkClient.exists("/frost", false);
    System.out.println(stat == null ? "not exits" : "exits");
}

4、客户端向服务端写入数据

写入请求直接发给 Leader 节点

1. 客户端发送写入请求，leader节点执行写入操作
2. leader通知follow1执行写入操作
3. folllow1写入完毕给leader返回确认ack
4. 现在半数以上服务器完成写入，leader给客户端发送确认ack
5. leader通知follow2写入
6. follow2写入完毕给leader发送确认ack
   

写入请求直接发给 follow 节点

1. 客户端发送写入请求，
2. follow1 将写入请求发送给leader
3. leader节点执行写入操作，然后leader通知follow1执行写入操作
4. folllow1写入完毕给leader返回确认ack
5. 现在半数以上服务器完成写入，leader给follow1发送确认ack
6. follow1给客户端发送确认ack
7. leader通知follow2写入
8. follow2写入完毕给leader发送确认ack

二、服务器动态上下线监听

1、监听过程

以下红色字体写错，应该是下线则通知注册监听器的客户端

对于ZooKeeper集群来说，客户端和服务器都相当于客户端，区别在于：服务器在ZooKeeper集群中是创建节点，客户端在ZooKeeper是监听信息。

2、代码

服务器注册到zk集群

import org.apache.zookeeper.*;
import java.io.IOException;

public class DistributeServer {

    private String connectString = "centos102:2181,centos103:2181,centos104:2181";
    private int sessionTimeout = 2000;
    ZooKeeper zk;

    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {

        DistributeServer server = new DistributeServer();

        // 1. 获取zk连接
        server.getConnect();

        // 2. 注册服务器到 zk 集群
        server.regist(args[0]);

        // 3. 启动业务逻辑（睡觉）
        server.business();
    }

    private void business() throws InterruptedException {
        Thread.sleep(Long.MAX_VALUE);
    }

    private void regist(String hostname) throws InterruptedException, KeeperException {
        String create = zk.create("/servers", hostname.getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL); // 临时带序号的节点
        System.out.println(hostname + "is online");

    }

    private void getConnect() throws IOException {
        zk = new ZooKeeper(connectString, sessionTimeout, new Watcher() {
            @Override
            public void process(WatchedEvent event) {

            }
        });
    }
}

客户端进行监听

import org.apache.zookeeper.*;

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

public class DistributeClient {

    private String connectString = "centos102:2181,centos103:2181,centos104:2181";
    private int sessionTimeout = 2000;
    ZooKeeper zk;

    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {

        DistributeClient client = new DistributeClient();

        // 1. 获取zk连接
        client.getConnect();

        // 2. 监听/servers下子节点的增加和删除
        client.getServerList();

        // 3. 启动业务逻辑（睡觉）
        client.business();
    }

    private void business() throws InterruptedException {
        Thread.sleep(Long.MAX_VALUE);
    }

    private void getServerList() throws InterruptedException, KeeperException {
        List<String> children = zk.getChildren("/servers", true);
        ArrayList<String> servers = new ArrayList<>();
        for (String child : children) {
            byte[] data = zk.getData("/servers/" + child, false, null);
            servers.add(new String(data));
        }
        System.out.println(servers);
    }

    private void getConnect() throws IOException {
        zk = new ZooKeeper(connectString, sessionTimeout, new Watcher() {
            @Override
            public void process(WatchedEvent event) {
                try {
                    getServerList();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (KeeperException e) {
                    e.printStackTrace();
                }
            }
        });
    }
}

启动客户端，然后在服务器上进行增加节点监听

删除节点监听

因为我们服务端的代码传参了，所以我们需要设置一下这个参数：

下图代表服务端启动的是hadoop102节点

先把客户端启动起来，发现有一个节点hadoop101：

在启动服务端，hadoop102上线：

然后返回看客户端的监听，发现节点有变化，打印出所有节点 [hadoop102, hadoop101]

此时我们修改一下再此启动服务端让 hadoop103 上线：

返回客户端查看发现 hadoop102 下线，hadoop103 上线

三、分布式锁

1、什么是分布式锁?

比如说"进程1"在使用该资源的时候，会先去获得锁，"进程1"获得锁以后会对该资源保持独占，这样其他进程就无法访问该资源，"进程1"用完该资源以后就将锁释放掉，让其他进程来获得锁，那么通过这个锁机制，我们就能保证了分布式系统中多个进程能够有序的访问该临界资源。那么我们把这个分布式环境下的这个锁叫作分布式锁。

import org.apache.zookeeper.*;
import org.apache.zookeeper.data.Stat;

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

public class DistributedLock {

    private final String connectString = "centos102:2181,centos103:2181,centos104:2181";
    private final int sessionTimeout = 2000;
    ZooKeeper zk;

    private CountDownLatch connectLatch = new CountDownLatch(1);
    private CountDownLatch waitLatch = new CountDownLatch(1);

    // 前一个节点
    private String waitPath;
    // 当前节点
    String currentMode;

    public DistributedLock() throws IOException, InterruptedException, KeeperException {
        // 1. 获取连接
         zk = new ZooKeeper(connectString, sessionTimeout, new Watcher() {
            @Override
            public void process(WatchedEvent event) {
                // connectLatch，如果连接上zk，可以释放
                if (event.getState() == Event.KeeperState.SyncConnected) {
                    connectLatch.countDown();
                }

                // waitLatch，需要释放
                if (event.getType() == Event.EventType.NodeDeleted && event.getPath().equals(waitPath)) {
                    waitLatch.countDown();
                }
            }
        });

         // 等待zk正常连接后往下走
         connectLatch.await();

        // 2. 判断根节点/lock是否存在
        Stat stat = zk.exists("/locks", false);

        if (stat == null) {
            // 创建根节点(永久节点)
            zk.create("/locks", "locks".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
        }

    }

    // 对 zk 加锁
    public void zkLock() {
        // 创建对应的临时带序号的节点
        try {
            currentMode = zk.create("/locks/" + "seq-", null, ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);

            // 判断创建的节点是否是最小的序号节点，如果是，获取到锁；如果不是，监听前一个节点
            List<String> children = zk.getChildren("/locks", false);

            // 如果 children 只有一个节点，直接获取锁；如果有多个节点，需要判断，谁最小
            if (children.size() == 1) {
                return;
            }else {
                // 排序
                Collections.sort(children);

                // 获取节点名称seq00000001
                String thisNode = currentMode.substring("/locks/".length());
                // 通过seq00000001获取该节点在children当中的位置
                int index = children.indexOf(thisNode);

                if (index == -1) {
                    System.out.println("数据异常");
                }else if (index == 0) {
                    // 该节点为第一个，获取锁直接返回
                    return;
                }else {
                    // 不是第一个，监听前一个节点
                    waitPath = "/locks/" + children.get(index - 1);
                    zk.getData(waitPath, true, null);

                    // 等待监听结束
                    waitLatch.await();
                    return;
                }
            }

        } catch (KeeperException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    // 解锁
    public void unZkLock() throws InterruptedException, KeeperException {
        // 删除节点
        zk.delete(currentMode, -1);
    }
}

测试

import org.apache.zookeeper.KeeperException;

import java.io.IOException;

public class DistributedLockTest {

    public static void main(String[] args) throws IOException, InterruptedException, KeeperException {

        final DistributedLock lock1 = new DistributedLock();

        final DistributedLock lock2 = new DistributedLock();

        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    lock1.zkLock();
                    System.out.println("线程1启动，获取到锁");

                    Thread.sleep(5000);

                    lock1.unZkLock();
                    System.out.println("线程1释放锁");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (KeeperException e) {
                    e.printStackTrace();
                }
            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    lock2.zkLock();
                    System.out.println("线程2启动，获取到锁");

                    Thread.sleep(5000);

                    lock2.unZkLock();
                    System.out.println("线程2释放锁");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (KeeperException e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }

}

2、Curator 框架实现分布式锁

原生JAVA API出现的问题：
（1）会话是异步的，需要自己去连接
（2）Watch需要重复注册，不然就不能生效
（3）开发的复杂性还是比较高
（4）不支持多节点的删除和创建，需要自己去递归

Curator 是一个专门解决分布式锁的框架，解决了原生JAVA API开发分布式遇到的的问题
curator 官方文档：https://curator.apache.org/index.html

pom.xml 文件添加依赖

    org.apache.curator
    curator-framework
    4.3.0


    org.apache.curator
    curator-recipes
    4.3.0


    org.apache.curator
    curator-client
    4.3.0

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.framework.recipes.locks.InterProcessMutex;
import org.apache.curator.retry.ExponentialBackoffRetry;

public class CuratorLockTest {
    public static void main(String[] args) {
        // 创建分布式锁1
        InterProcessMutex lock1 = new InterProcessMutex(getCuratorFramework(), "/locks");

        // 创建分布式锁2
        InterProcessMutex lock2 = new InterProcessMutex(getCuratorFramework(), "/locks");

        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    lock1.acquire();
                    System.out.println("线程1获取到锁");

                    lock1.acquire();
                    System.out.println("线程1获取到锁");

                    Thread.sleep(5 * 1000);

                    lock1.release();
                    System.out.println("线程1释放锁");

                    lock1.release();
                    System.out.println("线程1再次释放锁");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    lock2.acquire();
                    System.out.println("线程2获取到锁");

                    lock2.acquire();
                    System.out.println("线程2获取到锁");

                    Thread.sleep(5 * 1000);

                    lock2.release();
                    System.out.println("线程2释放锁");

                    lock2.release();
                    System.out.println("线程2再次释放锁");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }).start();

    }

    private static CuratorFramework getCuratorFramework() {

        ExponentialBackoffRetry policy = new ExponentialBackoffRetry(3000, 3);

        CuratorFramework client = CuratorFrameworkFactory.builder().connectString("centos102:2181,centos103:2181,centos104:2181")
                .connectionTimeoutMs(2000)
                .sessionTimeoutMs(2000)
                .retryPolicy(policy).build();

        // 启动客户端
        client.start();
        System.out.println("zookeeper 启动成功");

        return client;
    }
}