ZooKeeper 概述

import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.Stat;

public class ConfigWatcher implements Watcher {
    private ZooKeeper zk;
    private String configPath;
    private volatile String currentDeadline;

    public ConfigWatcher(String connectString, String configPath) throws Exception {
        this.configPath = configPath;
        this.zk = new ZooKeeper(connectString, 3000, this);
        loadConfig(); // 初次加载
    }

    /** 加载配置并重新注册 Watcher（一次性触发机制需要反复注册） */
    public void loadConfig() throws Exception {
        Stat stat = new Stat();
        byte[] data = zk.getData(configPath, this, stat); // this=Watcher, 重新注册
        this.currentDeadline = new String(data, "UTF-8");
        System.out.println("[配置更新] 当前选课截止时间: " + currentDeadline
                + " | 版本号: " + stat.getVersion());
    }

    @Override
    public void process(WatchedEvent event) {
        if (event.getType() == Event.EventType.NodeDataChanged
                && event.getPath().equals(configPath)) {
            try {
                loadConfig(); // 收到通知后重新读取并注册
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public String getDeadline() {
        return currentDeadline;
    }
}

// 操作前：CourseService.java —— 硬编码学生服务地址
public class CourseService {
    // 问题：实例 2 挂了，调用方却不知道，每次都失败
    private static final String[] STUDENT_SERVICE_URLS = {
        "http://192.168.1.101:8081",
        "http://192.168.1.102:8082",  // 这台已经挂了
        "http://192.168.1.103:8083"
    };

    public Student getStudent(int studentId) {
        String url = STUDENT_SERVICE_URLS[random.nextInt(3)]; // 随机负载均衡
        return restTemplate.getForObject(url + "/student/" + studentId, Student.class);
        // 如果随机到 8082，抛 ConnectionException！
    }
}

import org.apache.zookeeper.*;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ThreadLocalRandom;

public class ServiceRegistry implements Watcher {
    private ZooKeeper zk;
    private String servicePath;
    private volatile List<String> instances = new ArrayList<>();

    /** 服务实例注册（随会话存活） */
    public void register(String serviceName, String instanceAddress) throws Exception {
        String path = "/services/" + serviceName;
        // 确保父节点存在
        if (zk.exists(path, false) == null) {
            zk.create(path, new byte[0],
                    ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
        }
        // 创建临时节点——会话断开后自动删除
        String nodePath = path + "/" + instanceAddress;
        zk.create(nodePath, instanceAddress.getBytes(),
                ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);
        System.out.println("[注册] " + instanceAddress + " 已注册到 " + path);
    }

    /** 服务发现：获取实例列表并持续监听变更 */
    public void discover(String serviceName) throws Exception {
        String path = "/services/" + serviceName;
        refreshInstances(path); // 首次加载
    }

    private void refreshInstances(String path) throws Exception {
        List<String> children = zk.getChildren(path, this); // this=Watcher, 注册监听
        this.instances = children;
        System.out.println("[发现] 当前可用实例: " + instances);
    }

    @Override
    public void process(WatchedEvent event) {
        if (event.getType() == Event.EventType.NodeChildrenChanged) {
            try {
                refreshInstances(event.getPath()); // 子节点变化，刷新列表
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    /** 随机负载均衡 */
    public String getRandomInstance() {
        if (instances.isEmpty()) return null;
        int index = ThreadLocalRandom.current().nextInt(instances.size());
        return instances.get(index);
    }
}

// ❌ 错误：将大文件内容写入 ZNode
byte[] hugeFile = Files.readAllBytes(Paths.get("student_photos.zip")); // 5MB
zk.create("/files/student_photos", hugeFile,
        ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
// 运行时报错...

// ✅ 正确：ZNode 只指向外部存储
String fileId = uploadToMinio(hugeFile); // 上传到对象存储
zk.create("/files/student_photos", fileId.getBytes(),
        ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
// ZNode 中存储的是 "minio://bucket/student_photos_20260613.zip"

// ❌ 错误：Watcher 触发后不再重新注册
byte[] data = zk.getData("/config", true, null); // 注册 Watcher（一次性）
System.out.println("初始配置: " + new String(data));

// 后续更新不会收到通知！因为 Watcher 已被自动删除

// ✅ 正确：Watcher 回调中重新注册
public void watchConfig(ZooKeeper zk, String path) throws Exception {
    Stat stat = new Stat();
    byte[] data = zk.getData(path, event -> {
        if (event.getType() == Watcher.Event.EventType.NodeDataChanged) {
            try {
                watchConfig(zk, path);  // 递归重新注册
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }, stat);
    System.out.println("配置更新: " + new String(data) + " (版本: " + stat.getVersion() + ")");
}