Zookeeper的paxos，ZAB，watch和API

Paxos

参考文章：https://www.douban.com/note/208430424/

参考文章：https://zh.wikipedia.org/zh-cn/Paxos%E7%AE%97%E6%B3%95

参考文章：https://www.cnblogs.com/YC-L/p/14470535.html

Paxos算法是莱斯利·兰伯特，于1990年提出的一种基于消息传递且具有高度容错特性的共识（consensus）算法。

需要注意的是，Paxos常被误称为“一致性算法”。但是“一致性（consistency）”和“共识（consensus）”并不是同一个概念

Paxos是一个共识（consensus）算法

ZAB

参考文章：https://www.cnblogs.com/YC-L/p/14470604.html

参考文章：https://www.cnblogs.com/frankltf/p/10392151.html

其中一个节点挂掉之后，3个节点会不断通过3888端口通信

然后投票，版本号低的票会被淘汰，最终推选出一个合适的leader

watch

官方文档：https://zookeeper.apache.org/doc/r3.6.2/zookeeperProgrammers.html#ch_zkWatches

API示例

官方地址：https://zookeeper.apache.org/doc/r3.6.2/apidocs/zookeeper-server/index.html

package com.test.zookeeper;

import org.apache.zookeeper.*;
import org.apache.zookeeper.data.Stat;

import java.util.concurrent.CountDownLatch;

/**
 * Hello world!
 *
 */
public class App 
{
    public static void main( String[] args ) throws Exception {
        System.out.println( "Hello World!" );

        //zk是有session概念的，没有连接池的概念
        //watch:观察，回调
        //watch的注册值发生在 读类型调用，get，exites。。。
        //第一类：new zk 时候，传入的watch，这个watch，session级别的，跟path 、node没有关系。
        final CountDownLatch cd = new CountDownLatch(1);
        final ZooKeeper zk = new ZooKeeper("192.168.150.11:2181,192.168.150.12:2181,192.168.150.13:2181,192.168.150.14:2181",
                3000, new Watcher() {
            //Watch 的回调方法！
            @Override
            public void process(WatchedEvent event) {
                Event.KeeperState state = event.getState();
                Event.EventType type = event.getType();
                String path = event.getPath();
                System.out.println("new zk watch: "+ event.toString());

                switch (state) {
                    case Unknown:
                        break;
                    case Disconnected:
                        break;
                    case NoSyncConnected:
                        break;
                    case SyncConnected:
                        System.out.println("connected");
                        cd.countDown();
                        break;
                    case AuthFailed:
                        break;
                    case ConnectedReadOnly:
                        break;
                    case SaslAuthenticated:
                        break;
                    case Expired:
                        break;
                }

                switch (type) {
                    case None:
                        break;
                    case NodeCreated:
                        break;
                    case NodeDeleted:
                        break;
                    case NodeDataChanged:
                        break;
                    case NodeChildrenChanged:
                        break;
                }
            }
        });

        cd.await();
        ZooKeeper.States state = zk.getState();
        switch (state) {
            case CONNECTING:
                System.out.println("ing......");
                break;
            case ASSOCIATING:
                break;
            case CONNECTED:
                System.out.println("ed........");
                break;
            case CONNECTEDREADONLY:
                break;
            case CLOSED:
                break;
            case AUTH_FAILED:
                break;
            case NOT_CONNECTED:
                break;
        }

        String pathName = zk.create("/test", "olddata".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);

        final Stat stat = new Stat();
        byte[] node = zk.getData("/test", new Watcher() {
            @Override
            public void process(WatchedEvent event) {
                System.out.println("getData watch: " + event.toString());
                try {
                    //true   default Watch  被重新注册   new zk的那个watch
                    zk.getData("/test", this, stat);
                } catch (KeeperException e) {
                    e.printStackTrace();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, stat);

        System.out.println(new String(node));

        //触发回调
        Stat stat1 = zk.setData("/test", "newdata".getBytes(), 0);
        //还会触发吗？
        Stat stat2 = zk.setData("/test", "newdata01".getBytes(), stat1.getVersion());

        System.out.println("-------async start----------");
        zk.getData("/test", false, new AsyncCallback.DataCallback() {
            @Override
            public void processResult(int rc, String path, Object ctx, byte[] data, Stat stat) {
                System.out.println("-------async call back----------");
                System.out.println(ctx.toString());
                System.out.println(new String(data));

            }

        },"abc");
        System.out.println("-------async over----------");
        Thread.sleep(2222222);
    }
}

ACL Permission

官方文档：https://zookeeper.apache.org/doc/r3.6.2/apidocs/zookeeper-server/index.html

• ANYONE_ID_UNSAFE，This Id represents anyone
• AUTH_IDS，This Id is only usable to set ACLs，It will get substituted with the Id's the client authenticated wit
• OPEN_ACL_UNSAFE，This is a completely open ACL
• CREATOR_ALL_ACL，This ACL gives the creators authentication id's all permissions
• READ_ACL_UNSAFE，This ACL gives the world the ability to read

CoundDownLatch

参考文章：https://www.cnblogs.com/YC-L/p/14470786.html