面试题：微服务架构下Spring Cloud全局锁实现方法之高级难度问题

Question

假设在一个复杂的Spring Cloud微服务系统中，使用基于Zookeeper实现全局锁，如何处理锁的重试机制以及避免死锁情况发生？请详细阐述思路及可能涉及的代码逻辑。

Answer 1

处理锁的重试机制思路

1. 确定重试策略：
   • 固定间隔重试：每次重试间隔固定时间，例如1秒。这种方式简单，但可能在高竞争场景下效率较低。
   • 指数退避重试：随着重试次数增加，间隔时间以指数形式增长。如初始间隔1秒，下次2秒，再下次4秒等。这种方式能有效避免在高竞争环境下频繁重试导致的性能问题。
2. 设置重试次数上限：避免无限重试，消耗系统资源。根据业务场景确定合适的重试次数，比如5次。
3. 在获取锁失败时触发重试：当使用Zookeeper获取锁操作返回失败（例如节点已存在，代表锁已被占用），根据设定的重试策略进行重试。

避免死锁情况发生思路

1. 锁的持有时间限制：为获取到的锁设置一个合理的过期时间。当持有锁的服务出现异常或未主动释放锁时，Zookeeper会在过期时间后自动删除锁节点，其他服务可以重新获取锁。
2. 检查锁的依赖关系：在复杂业务场景下，如果存在多个锁之间的依赖，确保依赖关系不会形成环。可以使用有向无环图（DAG）来管理锁的获取顺序，避免死锁。
3. 监控与检测：通过Zookeeper的Watcher机制，监控锁节点的状态变化。如果发现某个锁长时间未释放，可以进行预警并采取相应措施，如强制释放锁。

可能涉及的代码逻辑（以Java和Curator客户端为例）

1. 获取锁的基本代码

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 ZookeeperLockExample {
    private static final String ZK_ADDRESS = "localhost:2181";
    private static final String LOCK_PATH = "/my_lock_path";

    public static void main(String[] args) {
        CuratorFramework client = CuratorFrameworkFactory.newClient(ZK_ADDRESS, new ExponentialBackoffRetry(1000, 3));
        client.start();

        InterProcessMutex lock = new InterProcessMutex(client, LOCK_PATH);

        try {
            if (lock.acquire(5, TimeUnit.SECONDS)) {
                try {
                    // 执行业务逻辑
                    System.out.println("Lock acquired, doing business logic...");
                } finally {
                    lock.release();
                    System.out.println("Lock released");
                }
            } else {
                System.out.println("Failed to acquire lock");
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            client.close();
        }
    }
}

2. 重试机制代码

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;
import java.util.concurrent.TimeUnit;

public class ZookeeperLockWithRetry {
    private static final String ZK_ADDRESS = "localhost:2181";
    private static final String LOCK_PATH = "/my_lock_path";
    private static final int MAX_RETRIES = 5;
    private static final int BASE_SLEEP_TIME_MS = 1000;

    public static void main(String[] args) {
        CuratorFramework client = CuratorFrameworkFactory.newClient(ZK_ADDRESS, new ExponentialBackoffRetry(BASE_SLEEP_TIME_MS, MAX_RETRIES));
        client.start();

        InterProcessMutex lock = new InterProcessMutex(client, LOCK_PATH);

        boolean acquired = false;
        int retryCount = 0;
        while (!acquired && retryCount < MAX_RETRIES) {
            try {
                acquired = lock.acquire(5, TimeUnit.SECONDS);
            } catch (Exception e) {
                e.printStackTrace();
            }
            if (!acquired) {
                try {
                    TimeUnit.SECONDS.sleep((long) Math.pow(2, retryCount));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                retryCount++;
            }
        }

        if (acquired) {
            try {
                // 执行业务逻辑
                System.out.println("Lock acquired, doing business logic...");
            } finally {
                try {
                    lock.release();
                    System.out.println("Lock released");
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        } else {
            System.out.println("Failed to acquire lock after " + MAX_RETRIES + " retries");
        }

        client.close();
    }
}

3. 设置锁的持有时间限制代码

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;
import java.util.concurrent.TimeUnit;

public class ZookeeperLockWithExpiry {
    private static final String ZK_ADDRESS = "localhost:2181";
    private static final String LOCK_PATH = "/my_lock_path";
    private static final int LOCK_EXPIRY_SECONDS = 30;

    public static void main(String[] args) {
        CuratorFramework client = CuratorFrameworkFactory.newClient(ZK_ADDRESS, new ExponentialBackoffRetry(1000, 3));
        client.start();

        InterProcessMutex lock = new InterProcessMutex(client, LOCK_PATH);

        try {
            if (lock.acquire(LOCK_EXPIRY_SECONDS, TimeUnit.SECONDS)) {
                try {
                    // 执行业务逻辑
                    System.out.println("Lock acquired, doing business logic...");
                } finally {
                    lock.release();
                    System.out.println("Lock released");
                }
            } else {
                System.out.println("Failed to acquire lock");
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            client.close();
        }
    }
}