使用Curator实现多台机器中只有一台机器执行任务

来源：互联网发布：万方数据库编辑：程序博客网时间：2024/06/11 19:38

最近在项目中需要在多台机器中，固定一台执行相关任务，本来是写死ip的，但是想想这实在是太硬了，就想到用zk获取锁来搞。因为第一次用zk，碰见了一些常识性问题，在这里纪录一下。

想法一：
使用可重入共享锁，然后监听加锁的节点。
理由：如果机器挂掉了，或者在发布，重新部署，这个锁会被释放，节点发生改变。

TestingServer server = new TestingServer();        server.start();        RetryPolicy retryPolicy = new ExponentialBackoffRetry(1000, 3);        CuratorFramework client1 = CuratorFrameworkFactory.builder().connectString(server.getConnectString())                .sessionTimeoutMs(5000)                .connectionTimeoutMs(5000)                .retryPolicy(retryPolicy)                .build();client1.start();InterProcessMutex lock = new InterProcessMutex(client1, PATH);lock.acquire(10, TimeUnit.SECONDS)

CuratorFramework client2 = CuratorFrameworkFactory.builder().connectString(server.getConnectString())                .sessionTimeoutMs(5000)                .connectionTimeoutMs(5000)                .retryPolicy(retryPolicy)                .build();client2.start();final NodeCache cache = new NodeCache(client2, PATH);        NodeCacheListener listener = new NodeCacheListener() {            public void nodeChanged() throws Exception {                ChildData data = cache.getCurrentData();                if (null != data) {                    System.out.println("节点数据：" + new String(cache.getCurrentData().getData()));                    System.out.println(data.getPath());                } else {                    System.out.println("节点被删除!");                }            }        };cache.getListenable().addListener(listener);cache.start();

结果：NodeCache只监听节点的增加，删除，数据变更。而加锁的情况是，在PATH路径下新增一个lock节点。释放锁是删掉lock节点。

想法二：
既然是PATH路径下新增节点，那就监听child，path下所有节点好了

PathChildrenCache cache = new PathChildrenCache(client2, PATH, true);cache.start();PathChildrenCacheListener cacheListener = new PathChildrenCacheListener() {public void childEvent(CuratorFramework curatorFramework, PathChildrenCacheEvent pathChildrenCacheEvent) throws Exception {                System.out.println("事件类型：" + pathChildrenCacheEvent.getType());                if (null != pathChildrenCacheEvent.getData()) {                    System.out.println("节点数据：" + pathChildrenCacheEvent.getData().getPath() + " = " + new String(pathChildrenCacheEvent.getData().getData()));                }            }        };cache.getListenable().addListener(cacheListener);

根据测试代码打印的结果

client1 startclient2 startclient2 check node not exists after node cache事件类型：CHILD_ADDED节点数据：/base/test/_c_13f15fc3-03cd-4152-9632-11aa98a9b708-lock-0000000000 = 192.168.0.105client1 get lock事件类型：CHILD_ADDED节点数据：/base/test/_c_568dfd7e-6bbf-408b-b1c8-ee278ab1f745-lock-0000000001 = 192.168.0.105事件类型：CHILD_REMOVED节点数据：/base/test/_c_568dfd7e-6bbf-408b-b1c8-ee278ab1f745-lock-0000000001 = 192.168.0.105client2 does not get lock after client1 get lockclient1 find node exists after lockclient2 can check node if exists without lock事件类型：CHILD_REMOVED节点数据：/base/test/_c_13f15fc3-03cd-4152-9632-11aa98a9b708-lock-0000000000 = 192.168.0.105client1 closedclient2 get lock after client closed事件类型：CHILD_ADDED节点数据：/base/test/_c_97f0605e-c1aa-45c6-a6ad-dfe82eab64d0-lock-0000000002 = 192.168.0.105

根据打印的日志，我们可以发现，当一个client试图去获取锁的时候，都会在PATH路径下增加一个lock节点，如果没有拿到锁，相关的节点就会被删除。这就是前面的两个ADD,一个REMOVE
后面的REMOVE就是关闭client1时发生的，接着的ADD就是client2获取锁时增加的。

测试代码：

public  static  void main(String[] args) throws Exception {        final String  PATH = "/base/test";        TestingServer server = new TestingServer();        server.start();        RetryPolicy retryPolicy = new ExponentialBackoffRetry(1000, 3);        CuratorFramework client1 = CuratorFrameworkFactory.builder().connectString(server.getConnectString())                .sessionTimeoutMs(5000)                .connectionTimeoutMs(5000)                .retryPolicy(retryPolicy)                .build();        CuratorFramework client2 = CuratorFrameworkFactory.builder().connectString(server.getConnectString())                .sessionTimeoutMs(5000)                .connectionTimeoutMs(5000)                .retryPolicy(retryPolicy)                .build();        client1.start();        System.out.println("client1 start");        client2.start();        System.out.println("client2 start");//        final NodeCache cache = new NodeCache(client2, PATH);//        NodeCacheListener listener = new NodeCacheListener() {//            public void nodeChanged() throws Exception {//                ChildData data = cache.getCurrentData();//                if (null != data) {//                    System.out.println("节点数据：" + new String(cache.getCurrentData().getData()));//                    System.out.println(data.getPath());//                } else {//                    System.out.println("节点被删除!");//                }//            }//        };//        cache.getListenable().addListener(listener);//        cache.start();        PathChildrenCache cache = new PathChildrenCache(client2, PATH, true);        cache.start();        PathChildrenCacheListener cacheListener = new PathChildrenCacheListener() {            public void childEvent(CuratorFramework curatorFramework, PathChildrenCacheEvent pathChildrenCacheEvent) throws Exception {                System.out.println("事件类型：" + pathChildrenCacheEvent.getType());                if (null != pathChildrenCacheEvent.getData()) {                    System.out.println("节点数据：" + pathChildrenCacheEvent.getData().getPath() + " = " + new String(pathChildrenCacheEvent.getData().getData()));                }            }        };        cache.getListenable().addListener(cacheListener);        if(client2.checkExists().forPath(PATH) == null){            System.out.println("client2 check node not exists after node cache");        }else{            System.out.println("client2 check node exists after node cache");        }        InterProcessMutex lock = new InterProcessMutex(client1, PATH);        InterProcessMutex lock2 = new InterProcessMutex(client2, PATH);        if(lock.acquire(10, TimeUnit.SECONDS)){            System.out.println("client1 get lock");            if(lock2.acquire(10, TimeUnit.SECONDS)){                System.out.println("client2 get lock after client1 get lock");            }else{                System.out.println("client2 does not get lock after client1 get lock");            }            Stat stat = client1.checkExists().forPath(PATH);            if(stat == null){                System.out.println("client1 find node not exists after lock");                client1.inTransaction().create().withMode(CreateMode.EPHEMERAL).forPath(PATH)                        .and().setData().forPath(PATH,String.valueOf(System.currentTimeMillis()).getBytes())                        .and().commit();                if(client1.checkExists().forPath(PATH) == null){                    System.out.println("client1 create node failed");                    return;                }else {                    System.out.println("client1 create node success");                }            }else{                System.out.println("client1 find node exists after lock");            }            if(client2.checkExists().forPath(PATH) == null){                System.out.println("client2 check node not exists");            }else{                System.out.println("client2 can check node if exists without lock");                CloseableUtils.closeQuietly(client1);                System.out.println("client1 closed");                if(lock2.acquire(10, TimeUnit.SECONDS)){                    System.out.println("client2 get lock after client closed");                }else{                    System.out.println("client2 does not get lock after client closed");                }            }        }else{            System.out.println("client1 do not get lock");            return;        }    }

这种做法需要自己去监控节点的变化，判断eventType，有没有其他更简单的方法呢。

想法三：
最后选择了curator整理好的leaderselector。这个地方就没什么坑了，网上的资料也是一大堆。
值得注意的就是：
1、takeLeadership方法，如果执行完，当前的实例会推出leader，重新进行选举，如果要一直保持leader，就写个死循环吧。
2、可以调用autoRequeue方法，将当前实例加入重新选举的队列，还有可能再次成为leader，否则就再也没有机会了。
3、最好做一下连接变化时的异常处理，可以继承LeaderSelectorListenerAdapter类。

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