Curator使用：(一)如何开始使用及api介绍(创建会话以及增删查改)

前言##

记录下ZK客户端的使用学习，初步想法是从几个方面来记录

1. 如何开始使用及api介绍(创建会话以及增删查改)
2. 异步调用
3. 事件监听
4. Master选举
5. 分布式锁
6. 计数器
7. Barrier

仓库介绍##

GroupID/Org	ArtifactID/Name	Description
org.apache.curator	curator-recipes	All of the recipes. Note: this artifact has dependencies on client and framework and, so, Maven (or whatever tool you're using) should pull those in automatically.
org.apache.curator	curator-async	Asynchronous DSL with O/R modeling, migrations and many other features.
org.apache.curator	curator-framework	The Curator Framework high level API. This is built on top of the client and should pull it in automatically.
org.apache.curator	curator-client	The Curator Client - replacement for the ZooKeeper class in the ZK distribution.
org.apache.curator	curator-test	Contains the TestingServer, the TestingCluster and a few other tools useful for testing.
org.apache.curator	curator-examples	Example usages of various Curator features.
org.apache.curator	curator-x-discovery	A Service Discovery implementation built on the Curator Framework.
org.apache.curator	curator-x-discovery-server	A RESTful server that can be used with Curator Discovery.

版本说明##

zk版本：

curator版本：

      <dependency>
            <groupId>org.apache.curator</groupId>
            <artifactId>curator-framework</artifactId>
            <version>4.2.0</version>
        </dependency>
 <dependency>
            <groupId>org.apache.curator</groupId>
            <artifactId>curator-recipes</artifactId>
            <version>4.2.0</version>
        </dependency>

常用API介绍###

1.创建会话

    CuratorFramework cc = CuratorFrameworkFactory.builder()
                .connectString("ip:port")
                .sessionTimeoutMs(2000)
                .connectionTimeoutMs(5000)
                .retryPolicy(new ExponentialBackoffRetry(1000,3))
                .namespace("test")
                .build();
    cc.start();    

说一下retryPolicy，重连策略，建议用其中两种

    //重连3次，每次休息3秒
    new RetryNTimes(3,3000);
    //重连3次，每次休息大约是1秒
    new ExponentialBackoffRetry(1000,3);
    //初始化一个大概的等待时间1秒，然后开始重连，最多重连3次，每次最多休息2秒
    new ExponentialBackoffRetry(1000,3,2000);

    //计算通过这个初始化的大约时间，计算实际需要睡眠多久
    long sleepMs = baseSleepTimeMs * Math.max(1, random.nextInt(1 << (retryCount + 1)));

namespace代表命名空间，注意的是，curator会自动创建。
2.创建普通节点

    cc.create().forPath("/comm_msg_nd","ctx".getBytes());
    cc.create().forPath("/comm_no_msg_nd");

需要注意的是，如果没有值，会默认把当前ip地址放进去

    return InetAddress.getLocalHost().getHostAddress().getBytes(); 

3.创建临时节点

    cc.create().withMode(CreateMode.EPHEMERAL).forPath("/ephe_nd","ff".getBytes());
    //creatingParentsIfNeeded会自动创建父节点
    cc.create().creatingParentsIfNeeded().withMode(CreateMode.EPHEMERAL).forPath("/ephe_nd","ff".getBytes());

需要注意，只有叶节点可以做临时节点，所以叶节点的父节点必须是永久节点，也就是creatingParentsIfNeeded这个方法创建的父节点必须是永久节点

4.删除节点

    //删除单独一个节点
    cc.delete().forPath("/comm_msg_nd");
    //删除多级节点
    cc.create().creatingParentsIfNeeded().forPath("/p1/p2/p3/multi_nd");
    cc.delete().deletingChildrenIfNeeded().forPath("/p1");
    //删除指定版本的节点
    cc.delete().withVersion(0).forPath("/comm_msg_nd");
    //保证删除，失败后继续执行
    cc.delete().guaranteed().forPath("/comm_msg_nd");

    //通过添加错误的回调方法来实现，错误后继续执行
    OperationAndData.ErrorCallback<String> errorCallback = null;
            if ( guaranteed )
            {
                errorCallback = new OperationAndData.ErrorCallback<String>()
                {
                    @Override
                    public void retriesExhausted(OperationAndData<String> operationAndData)
                    {
                        //将路径添加到错误处理中
                        client.getFailedDeleteManager().addFailedOperation(unfixedPath);
                    }
                };
            }
            client.processBackgroundOperation(new OperationAndData<String>(this, path, backgrounding.getCallback(), errorCallback, backgrounding.getContext(), null), null);
    
    //然后FailedDeleteManager中通过调用这个来继续请求删除操作
    @Override
    protected void executeGuaranteedOperationInBackground(String path)
            throws Exception
    {
        client.delete().guaranteed().inBackground().forPath(path);
    }

5.读数据

    Stat stat = new Stat();
    byte[] ctx = cc.getData().storingStatIn(stat).forPath("/comm_msg_nd");
    //获取节点内容为ctx
    System.out.println(new String(ctx));
    //获取该节点stat
    //78,78,1573789366124,1573789366124,0,0,0,0,3,0,78
    System.out.println(stat);

6.更新数据

    Stat stat = new Stat();
    stat = cc.setData().forPath("/comm_msg_nd","new ctx".getBytes());
    System.out.println(stat);