zookeeper监听服务器节点动态上下线

需求：某分布式系统中，主节点有多台，可以动态上下线，任意一台客户端都能实时感知到主节点服务器的上下线

实现思路：

1. 服务器上线后，就在zookeeper集群上注册（要在zookeeper集群上创建一个临时node，以便服务器下线时就可以在zookeeper集群上删掉该临时node）。
2. 客户端上线后，使用getChildren() 获取当前在线服务器列表，并注册监听。
3. 当服务器下线后，zookeeper集群上该临时节点就会删掉，服务器下线事件通知客户端。
4. 客户端在process()中，再重新获取在线服务器列表，并重新注册监听。

服务器端注册代码：

package com.allen.zookeeper01;

import org.apache.zookeeper.*;

import java.io.IOException;

public class DistributeServer {
    private String connectString = "hadoop02:2181,hadoop03:2181,hadoop04:2181";
    private int sessionTimeout = 2000;
    private ZooKeeper zk;

    public static void main(String[] args) {
        DistributeServer server = new DistributeServer();

        // 连接zookeeper集群连接
        try {
            server.getConnect();
        } catch (IOException e) {
            e.printStackTrace();
        }

        // 注册节点
        try {
            server.register(args[0]);
        } catch (KeeperException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // 业务逻辑处理
        try {
            server.business();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }

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

    private void register(String hostname) throws KeeperException, InterruptedException {
        zk.create("/servers/server", hostname.getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
        System.out.println(hostname + " is online. ");
    }

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

            }
        });
    }
}

客户端代码：

package com.allen.zookeeper01;

import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;

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

public class DistributedClient {
    private String connectString = "hadoop02:2181,hadoop03:2181,hadoop04:2181";
    private int sessionTimeout = 2000;
    private ZooKeeper zk;

    public static void main(String[] args) {
        DistributedClient client = new DistributedClient();

        // 获取zookeeper集群连接
        try {
            client.getConnect();
        } catch (IOException e) {
            e.printStackTrace();
        }

        // 注册监听
        try {
            client.getChildren();
        } catch (KeeperException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // 业务逻辑处理
        try {
            client.business();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }


    }

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

    private void getChildren() throws KeeperException, InterruptedException {
        List children = zk.getChildren("/servers", true);

        ArrayList hosts = new ArrayList();

        for (String child : children) {
            byte[] data = zk.getData("/servers/" + child, false, null);
            hosts.add(new String(data));
        }

        // 将所有在线的服务器打印
        System.out.println(hosts);
    }

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

效果如图：

1. 无服务器上线
2. hadoop02上线
3. hadoop03上线
4. hadoop03下线