跟着实例学习ZooKeeper的用法： Curator框架应用

前面的几篇文章介绍了一些ZooKeeper的应用方法， 本文将介绍Curator访问ZooKeeper的一些基本方法， 而不仅仅限于指定的Recipes， 你可以使用Curator API任意的访问ZooKeeper。

CuratorFramework

Curator框架提供了一套高级的API， 简化了ZooKeeper的操作。 它增加了很多使用ZooKeeper开发的特性，可以处理ZooKeeper集群复杂的连接管理和重试机制。 这些特性包括：

• 自动化的连接管理: 重新建立到ZooKeeper的连接和重试机制存在一些潜在的错误case。 Curator帮助你处理这些事情，对你来说是透明的。
• 清理API:
  • 简化了原生的ZooKeeper的方法，事件等
  • 提供了一个现代的流式接口
• 提供了Recipes实现： 如前面的文章介绍的那样，基于这些Recipes可以创建很多复杂的分布式应用

Curator框架通过CuratorFrameworkFactory以工厂模式和builder模式创建CuratorFramework实 例。 CuratorFramework实例都是线程安全的，你应该在你的应用中共享同一个CuratorFramework实例.

工厂方法newClient()提供了一个简单方式创建实例。 而Builder提供了更多的参数控制。一旦你创建了一个CuratorFramework实例，你必须调用它的start()启动，在应用退出时调用close()方法关闭.

下面的例子演示了两种创建Curator的方法：

package com.colobu.zkrecipe.framework;

import org.apache.curator.RetryPolicy;
import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.CuratorFrameworkFactory;
import org.apache.curator.retry.ExponentialBackoffRetry;
import org.apache.curator.test.TestingServer;
import org.apache.curator.utils.CloseableUtils;

public class CreateClientExample {
    private static final String PATH = "/example/basic";

    public static void main(String[] args) throws Exception {
        TestingServer server = new TestingServer();
        CuratorFramework client = null;
        try {
            client = createSimple(server.getConnectString());
            client.start();
            client.create().creatingParentsIfNeeded().forPath(PATH, "test".getBytes());
            CloseableUtils.closeQuietly(client);

            client = createWithOptions(server.getConnectString(), new ExponentialBackoffRetry(1000, 3), 1000, 1000);
            client.start();
            System.out.println(new String(client.getData().forPath(PATH)));
        } catch (Exception ex) {
            ex.printStackTrace();
        } finally {
            CloseableUtils.closeQuietly(client);
            CloseableUtils.closeQuietly(server);
        }

    }

    public static CuratorFramework createSimple(String connectionString) {
        // these are reasonable arguments for the ExponentialBackoffRetry. 
        // The first retry will wait 1 second - the second will wait up to 2 seconds - the
        // third will wait up to 4 seconds.
        ExponentialBackoffRetry retryPolicy = new ExponentialBackoffRetry(1000, 3);
        // The simplest way to get a CuratorFramework instance. This will use default values.
        // The only required arguments are the connection string and the retry policy
        return CuratorFrameworkFactory.newClient(connectionString, retryPolicy);
    }

    public static CuratorFramework createWithOptions(String connectionString, RetryPolicy retryPolicy, int connectionTimeoutMs, int sessionTimeoutMs) {
        // using the CuratorFrameworkFactory.builder() gives fine grained control
        // over creation options. See the CuratorFrameworkFactory.Builder javadoc details
        return CuratorFrameworkFactory.builder().connectString(connectionString)
                .retryPolicy(retryPolicy)
                .connectionTimeoutMs(connectionTimeoutMs)
                .sessionTimeoutMs(sessionTimeoutMs)
                // etc. etc.
                .build();
    }
}

Curator框架提供了一种流式接口。 操作通过builder串联起来， 这样方法调用类似语句一样。

client.create().forPath("/head", new byte[0]);
client.delete().inBackground().forPath("/head");
client.create().withMode(CreateMode.EPHEMERAL_SEQUENTIAL).forPath("/head/child", new byte[0]);
client.getData().watched().inBackground().forPath("/test");

CuratorFramework提供的方法：

后台操作的通知和监控可以通过ClientListener接口发布. 你可以在CuratorFramework实例上通过addListener()注册listener, Listener实现了下面的方法:

• eventReceived() 一个后台操作完成或者一个监控被触发

事件类型以及事件的方法如下：

还可以通过ConnectionStateListener接口监控连接的状态。 强烈推荐你增加这个监控器。

你可以使用命名空间Namespace避免多个应用的节点的名称冲突。 CuratorFramework提供了命名空间的概念，这样CuratorFramework会为它的API调用的path加上命名空间：

CuratorFramework    client = CuratorFrameworkFactory.builder().namespace("MyApp") ... build();
 ...
client.create().forPath("/test", data);
// node was actually written to: "/MyApp/test"

Curator还提供了临时的CuratorFramework： CuratorTempFramework， 一定时间不活动后连接会被关闭。这hi基于Camille Fournier的一篇文章： http://whilefalse.blogspot.com/2012/12/building-global-highly-available.html.

创建builder时不是调用build()而是调用buildTemp()。 3分钟不活动连接就被关闭，你也可以指定不活动的时间。 它只提供了下面几个方法：

    public void     close();
    public CuratorTransaction inTransaction() throws Exception;
    public TempGetDataBuilder getData() throws Exception;

操作方法

上面的表格列出了CuratorFramework可以用的操作。 下面就是一个例子：

package com.colobu.zkrecipe.framework;

import java.util.List;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.api.BackgroundCallback;
import org.apache.curator.framework.api.CuratorEvent;
import org.apache.curator.framework.api.CuratorListener;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.Watcher;

public class CrudExample {

    public static void main(String[] args) {

    }

    public static void create(CuratorFramework client, String path, byte[] payload) throws Exception {
        // this will create the given ZNode with the given data
        client.create().forPath(path, payload);
    }

    public static void createEphemeral(CuratorFramework client, String path, byte[] payload) throws Exception {
        // this will create the given EPHEMERAL ZNode with the given data
        client.create().withMode(CreateMode.EPHEMERAL).forPath(path, payload);
    }

    public static String createEphemeralSequential(CuratorFramework client, String path, byte[] payload) throws Exception {
        // this will create the given EPHEMERAL-SEQUENTIAL ZNode with the given
        // data using Curator protection.
        return client.create().withProtection().withMode(CreateMode.EPHEMERAL_SEQUENTIAL).forPath(path, payload);
    }

    public static void setData(CuratorFramework client, String path, byte[] payload) throws Exception {
        // set data for the given node
        client.setData().forPath(path, payload);
    }

    public static void setDataAsync(CuratorFramework client, String path, byte[] payload) throws Exception {
        // this is one method of getting event/async notifications
        CuratorListener listener = new CuratorListener() {
            @Override
            public void eventReceived(CuratorFramework client, CuratorEvent event) throws Exception {
                // examine event for details
            }
        };
        client.getCuratorListenable().addListener(listener);
        // set data for the given node asynchronously. The completion
        // notification
        // is done via the CuratorListener.
        client.setData().inBackground().forPath(path, payload);
    }

    public static void setDataAsyncWithCallback(CuratorFramework client, BackgroundCallback callback, String path, byte[] payload) throws Exception {
        // this is another method of getting notification of an async completion
        client.setData().inBackground(callback).forPath(path, payload);
    }

    public static void delete(CuratorFramework client, String path) throws Exception {
        // delete the given node
        client.delete().forPath(path);
    }

    public static void guaranteedDelete(CuratorFramework client, String path) throws Exception {
        // delete the given node and guarantee that it completes
        client.delete().guaranteed().forPath(path);
    }

    public static List<String> watchedGetChildren(CuratorFramework client, String path) throws Exception {
        /**
         * Get children and set a watcher on the node. The watcher notification
         * will come through the CuratorListener (see setDataAsync() above).
         */
        return client.getChildren().watched().forPath(path);
    }

    public static List<String> watchedGetChildren(CuratorFramework client, String path, Watcher watcher) throws Exception {
        /**
         * Get children and set the given watcher on the node.
         */
        return client.getChildren().usingWatcher(watcher).forPath(path);
    }
}

事务

上面也提到， CuratorFramework提供了事务的概念，可以将一组操作放在一个原子事务中。 什么叫事务？ 事务是原子的， 一组操作要么都成功，要么都失败。

下面的例子演示了事务的操作：

package com.colobu.zkrecipe.framework;

import java.util.Collection;

import org.apache.curator.framework.CuratorFramework;
import org.apache.curator.framework.api.transaction.CuratorTransaction;
import org.apache.curator.framework.api.transaction.CuratorTransactionFinal;
import org.apache.curator.framework.api.transaction.CuratorTransactionResult;

public class TransactionExample {

    public static void main(String[] args) {

    }

    public static Collection<CuratorTransactionResult> transaction(CuratorFramework client) throws Exception {
        // this example shows how to use ZooKeeper's new transactions
        Collection<CuratorTransactionResult> results = client.inTransaction().create().forPath("/a/path", "some data".getBytes())
                .and().setData().forPath("/another/path", "other data".getBytes())
                .and().delete().forPath("/yet/another/path")
                .and().commit(); // IMPORTANT!
                                                                                                                                // called
        for (CuratorTransactionResult result : results) {
            System.out.println(result.getForPath() + " - " + result.getType());
        }
        return results;
    }

    /*
     * These next four methods show how to use Curator's transaction APIs in a
     * more traditional - one-at-a-time - manner
     */
    public static CuratorTransaction startTransaction(CuratorFramework client) {
        // start the transaction builder
        return client.inTransaction();
    }

    public static CuratorTransactionFinal addCreateToTransaction(CuratorTransaction transaction) throws Exception {
        // add a create operation
        return transaction.create().forPath("/a/path", "some data".getBytes()).and();
    }

    public static CuratorTransactionFinal addDeleteToTransaction(CuratorTransaction transaction) throws Exception {
        // add a delete operation
        return transaction.delete().forPath("/another/path").and();
    }

    public static void commitTransaction(CuratorTransactionFinal transaction) throws Exception {
        // commit the transaction
        transaction.commit();
    }
}