学步园

Curator是Netflix开源的一套ZooKeeper客户端框架. Netflix在使用ZooKeeper的过程中发现ZooKeeper自带的客户端太底层, 应用方在使用的时候需要自己处理很多事情, 于是在它的基础上包装了一下, 提供了一套更好用的客户端框架.
Netflix在用ZooKeeper的过程中遇到的问题, 我们也遇到了, 所以开始研究一下, 首先从他在github上的源码, wiki文档以及Netflix的技术blog入手. 

看完官方的文档之后, 发现Curator主要解决了三类问题: 

• 封装ZooKeeper client与ZooKeeper server之间的连接处理;
• 提供了一套Fluent风格的操作API;
• 提供ZooKeeper各种应用场景(recipe, 比如共享锁服务, 集群领导选举机制)的抽象封装.

先大概讲一下代码结构和主要的类，Curator包含了以下文件夹(组件)：

Recipes    Implementations of some of the common ZooKeeper"recipes". The implementations are built on top of the CuratorFramework.

构建在Curator Framework框架之上，实现了原生ZooKeeper中的分布式工具特征recipes。

Framework  The Curator Framework is a high-level API thatgreatly simplifies using ZooKeeper. It adds many features that build onZooKeeper and handles the complexity
of managing connections to the ZooKeepercluster and retrying operations.

提供高层抽象API简化ZooKeeper的使用，更重要的是封装了管理到ZooKeeper集群的连接以及重试机制的复杂性，是Curator对外提供的接口。

Utilities  Various utilities that are useful when usingZooKeeper.

一些使用ZooKeeper过程中有用的工具。

Client A replacement for the bundled ZooKeeper class that takes care ofsome low-level housekeeping and provides some useful utilities.

替换原生ZooKeeper类，封装操作原生ZooKeeper的底层细节，主要包括前面说的实现了复杂的到ZooKeeper集群的连接以及重试机制。

Errors How Curator deals with errors, connection issues, recoverableexceptions, etc.

Curator如何处理错误，连接问题以及可恢复异常。

Extensions The curator-recipes package implements thecommon recipes that are described in the ZooKeeper documentation. To avoidbloating that package, recipes/applications
that have a vertical appeal will beput in separate "extension" packages using the naming convention curator-x-name.

Curator基于ZooKeeper实现的拓展功能，包括直接操作ZooKeeper的服务的动态发现，封装后以Restful接口提供出来的服务动态发现，以及RPC调用等功能，具体以后的文章会讲到。

一个典型的Curator的例子就是：

/**
 * An example of the PathChildrenCache. The example "harness" is a command processor
 * that allows adding/updating/removed nodes in a path. A PathChildrenCache keeps a
 * cache of these changes and outputs when updates occurs.
 */
public class PathCacheExample
{
    private static final String     PATH = "/example/cache";
    public static void main(String[] args) throws Exception
    {
        TestingServer       server = new TestingServer();
        CuratorFramework    client = null;
        PathChildrenCache   cache = null;
        try
        {
            client = CuratorFrameworkFactory.newClient(server.getConnectString(), new ExponentialBackoffRetry(1000, 3));
            client.start();

            // in this example we will cache data. Notice that this is optional.
            cache = new PathChildrenCache(client, PATH, true);
            cache.start();

            setValue(client, command, args);
        }
        finally
        {
            CloseableUtils.closeQuietly(cache);
            CloseableUtils.closeQuietly(client);
            CloseableUtils.closeQuietly(server);
        }
}

private static void setValue(CuratorFramework client, String command, String[] args) throws Exception
    {
        if ( args.length != 2 )
        {
            System.err.println("syntax error (expected set <path> <value>): " + command);
            return;
        }

        String      name = args[0];
        if ( name.contains("/") )
        {
            System.err.println("Invalid node name" + name);
            return;
        }
        String      path = ZKPaths.makePath(PATH, name);

        byte[]      bytes = args[1].getBytes();
        try
        {
            client.setData().forPath(path, bytes);
        }
        catch ( KeeperException.NoNodeException e )
        {
            client.create().creatingParentsIfNeeded().forPath(path, bytes);
        }
    }

下一篇文章开始将会通过这个例子详细分析Curator源码。