ZooKeeper的：Curator框架应用

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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：

方法名描述create()开始创建操作，可以调用额外的方法(比如方式mode 或者后台执行background) 并在最后调用forPath()指定要操作的ZNodedelete()开始删除操作. 可以调用额外的方法(版本或者后台处理version or background)并在最后调用forPath()指定要操作的ZNodecheckExists()开始检查ZNode是否存在的操作. 可以调用额外的方法(监控或者后台处理)并在最后调用forPath()指定要操作的ZNodegetData()开始获得ZNode节点数据的操作. 可以调用额外的方法(监控、后台处理或者获取状态watch, background or get stat) 并在最后调用forPath()指定要操作的ZNodesetData()开始设置ZNode节点数据的操作. 可以调用额外的方法(版本或者后台处理) 并在最后调用forPath()指定要操作的ZNodegetChildren()开始获得ZNode的子节点列表。以调用额外的方法(监控、后台处理或者获取状态watch, background or get stat) 并在最后调用forPath()指定要操作的ZNodeinTransaction()开始是原子ZooKeeper事务. 可以复合create, setData, check, and/or delete 等操作然后调用commit()作为一个原子操作提交

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：

Event TypeEvent MethodsCREATEgetResultCode() and getPath()DELETEgetResultCode() and getPath()EXISTSgetResultCode(), getPath() and getStat()GETDATAgetResultCode(), getPath(), getStat() and getData()SETDATAgetResultCode(), getPath() and getStat()CHILDRENgetResultCode(), getPath(), getStat(), getChildren()WATCHEDgetWatchedEvent()

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();    }}

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