Zookeeper是一个开放源代码的分布式协调服务，由雅虎创建，是Google Chubby的开源实现。Zookeeper的设计目标是将那些复杂且容易出错的分布式一致性服务封装起来，构成一个高效可靠的元语集，并以一系列简单易用的接口提供给用户使用。
单机模式部署与运行（Windows）
- 确保已经安装了JAVA 1.6及其以上版本的JDK
- 下载Zookeeper http://zookeeper.apache.org/releases.html 目前稳定版本即stable版本为3.4.6。将下载完成的压缩包解压到%ZK_HOME%下，其中ZK_HOME 目录可以随意设定，如：D://programes 等
- 配置文件 zoo.cfg ,单机模式下，只需要将%ZK_HOME%/conf/zoo_sample.cfg文件重命名即可
- 启动服务，在%ZK_HOME%bin目录下执行zkServer.cmd即可启动zookeeper服务器
Zookeeper 可执行脚本说明
- zkCleanup 清理ZooKeeper历史数据，包括事务日志和快照文件
- zkCli ZooKeeper的一个建议客户端
- zkEnv 设置ZooKeeper的环境变量
- zkServer ZooKeeper服务器的启动、停止和重启脚本
创建会话
客户端可以通过创建一个ZooKeeper实例来连接ZooKeeper服务器。该类中的注释这样写道：
ZooKeeper类是ZooKeeper客户端库的主类。要使用ZooKeeper服务，应用程序首先必须要实现一个ZooKeeper的实例。所有的迭代都将通过调用ZooKeeper类的方法来完成。这个类的方法都是线程安全的除非另有说明。
一旦一个与服务器的连接被建立，会分配给客户端一个Session ID. 客户端会周期性地发送心跳到服务器端，以保持会话有效。
只要Session ID 有效，应用程序都可以通过客户端调用ZooKeeper的 API
如果因为某些原因，导致客户端未能在指定的时间段内（超过SessionTimeout的值）发送心跳到服务器端，则该会话将过期，Session ID 将失效。
如果客户端对象不可用，为了调用ZooKeeper API,应用程序必须创建一个新的客户端
ZooKeeper的构造方法有：
ZooKeeper(String connectString,int sessionTimeout,Watcher watcher)
ZooKeeper(String connectString,int sessionTimeout,Watcher watcher,boolean canBeReadOnly)
ZooKeeper(String connectString,int sessionTimeout,Watcher watcher,long sessionId,boolean canBeReadOnly)
ZooKeeper(String connectString,int sessionTimeout,Watcher watcher,long sessionId,byte[] sessionPasswd, boolean canBeReadOnly)
connectString 是指ZooKeeper服务器的列表，host:prot,host:port 这样的格式构成，即主机:端口号通过英文逗号分隔。当然如果是单击模式下只需要一个;
sessionTimeout 值会话的超时时间，单位是毫秒
watcher Watcher事件的处理器，如果设置为null,表示不需要默认的Watcher事件处理器
canBeReadOnly 用于表示当前会话是否支持 read-only
sessionId,sessionPasswd 分别代表 会话的Id和密钥。这两个参数可以唯一确定一个会话。可以通过ZooKeeper 实例调用 getSesssionId()和getSessionPasswd()方法获取。
/**
*创建一个最基本的ZooKeeper会话示例
* @author zhangwei_david
* @version $Id: ZKConstructorDemo.java, v 0.1 2015年5月1日 下午5:55:21 zhangwei_david Exp $
*/
public class ZKConstructorDemo implements Watcher {
private static CountDownLatch connectedSemphore = new CountDownLatch(1);
public static void main(String[] args) throws Exception {
//创建Zookeeper会话实例
ZooKeeper zookeeper = new ZooKeeper("127.0.0.1:2181", 5000, new ZKConstructorDemo());
// 输出当前会话的状态
System.out.println("zk客户端的状态是：" + zookeeper.getState());
System.out.println("zk 客户端的sessionId=" + zookeeper.getSessionId() + ", sessionPasswd是："
+ new String(zookeeper.getSessionPasswd()));
try {
// 当前闭锁在为0之前一直等待，除非线程中断
connectedSemphore.await();
} catch (Exception e) {
System.out.println("Zookeeper session established");
}
}
/**
* @see org.apache.zookeeper.Watcher#process(org.apache.zookeeper.WatchedEvent)
*/
public void process(WatchedEvent event) {
System.out.println("Receive watched event:" + event);
//如果客户端已经处于连接状态闭锁减去1
if (KeeperState.SyncConnected == event.getState()) {
connectedSemphore.countDown();
}
}
}
结果：
2015-05-01 19:03:43 [ main:0 ] - [ INFO ] Client environment:zookeeper.version=3.4.6-1569965, built on 02/20/2014 09:09 GMT 2015-05-01 19:03:43 [ main:0 ] - [ INFO ] Client environment:host.name=David 2015-05-01 19:03:43 [ main:0 ] - [ INFO ] Client environment:java.version=1.8.0 2015-05-01 19:03:43 [ main:0 ] - [ INFO ] Client environment:java.io.tmpdir=C:\Users\Lenovo\AppData\Local\Temp\ 2015-05-01 19:03:43 [ main:0 ] - [ INFO ] Client environment:java.compiler=<NA> 2015-05-01 19:03:43 [ main:0 ] - [ INFO ] Client environment:os.name=Windows 8.1 2015-05-01 19:03:43 [ main:0 ] - [ INFO ] Client environment:os.arch=x86 2015-05-01 19:03:43 [ main:0 ] - [ INFO ] Client environment:os.version=6.3 2015-05-01 19:03:43 [ main:0 ] - [ INFO ] Client environment:user.name=Lenovo 2015-05-01 19:03:43 [ main:0 ] - [ INFO ] Client environment:user.home=C:\Users\Lenovo 2015-05-01 19:03:43 [ main:0 ] - [ INFO ] Client environment:user.dir=H:\Alipay.com\workspace4alipay\demo 2015-05-01 19:03:43 [ main:0 ] - [ INFO ] Initiating client connection, connectString=127.0.0.1:2181 sessionTimeout=5000 watcher=com.cathy.demo.zk.ZKConstructorDemo@1e717c2 zk客户端的状态是：CONNECTING zk 客户端的sessionId=0, sessionPasswd是：
**
*使用SessionId和Sessionpwd创建的ZooKeeper会话示例
* @author zhangwei_david
* @version $Id: ZKConstructorWithIdAndPasswdDemo.java, v 0.1 2015年5月1日 下午5:55:21 zhangwei_david Exp $
*/
public class ZKConstructorWithIdAndPasswdDemo implements Watcher {
private static CountDownLatch connectedSemphore = new CountDownLatch(1);
public static void main(String[] args) throws Exception {
//创建Zookeeper会话实例
ZooKeeper zookeeper = new ZooKeeper("127.0.0.1:2181", 5000,
new ZKConstructorWithIdAndPasswdDemo());
// 输出当前会话的状态
System.out.println("未使用id和密钥创建zk客户端的状态是：" + zookeeper.getState());
long sessionId = zookeeper.getSessionId();
byte[] sessionPasswd = zookeeper.getSessionPasswd();
System.out.println("zk 客户端的sessionId=" + zookeeper.getSessionId() + ", sessionPasswd是："
+ new String(zookeeper.getSessionPasswd()));
zookeeper = new ZooKeeper("127.0.0.1:2181", 5000, new ZKConstructorWithIdAndPasswdDemo(),
sessionId, sessionPasswd);
System.out.println("使用id和密钥创建zk客户端的状态是：" + zookeeper.getState());
zookeeper = new ZooKeeper("127.0.0.1:2181", 5000, new ZKConstructorWithIdAndPasswdDemo(),
1L, "test".getBytes());
System.out.println("使用错误id和密钥创建zk客户端的状态是：" + zookeeper.getState());
try {
// 当前闭锁在为0之前一直等待，除非线程中断
connectedSemphore.await();
} catch (Exception e) {
System.out.println("Zookeeper session established");
}
}
/**
* @see org.apache.zookeeper.Watcher#process(org.apache.zookeeper.WatchedEvent)
*/
public void process(WatchedEvent event) {
System.out.println("Receive watched event:" + event);
//如果客户端已经处于连接状态闭锁减去1
if (KeeperState.SyncConnected == event.getState()) {
connectedSemphore.countDown();
}
}
}
结果：
2015-05-01 19:14:34 [ main:0 ] - [ INFO ] Initiating client connection, connectString=127.0.0.1:2181 sessionTimeout=5000 watcher=com.cathy.demo.zk.ZKConstructorWithIdAndPasswdDemo@1e717c2 未使用id和密钥创建zk客户端的状态是：CONNECTING zk 客户端的sessionId=0, sessionPasswd是： 2015-05-01 19:14:34 [ main:31 ] - [ INFO ] Initiating client connection, connectString=127.0.0.1:2181 sessionTimeout=5000 watcher=com.cathy.demo.zk.ZKConstructorWithIdAndPasswdDemo@aa7bc2 sessionId=0 sessionPasswd=<hidden> 使用id和密钥创建zk客户端的状态是：CONNECTING 2015-05-01 19:14:34 [ main:31 ] - [ INFO ] Initiating client connection, connectString=127.0.0.1:2181 sessionTimeout=5000 watcher=com.cathy.demo.zk.ZKConstructorWithIdAndPasswdDemo@ccd017 sessionId=1 sessionPasswd=<hidden> 使用错误id和密钥创建zk客户端的状态是：CONNECTING 2015-05-01 19:14:34 [ main-SendThread(127.0.0.1:2181):31 ] - [ INFO ] Opening socket connection to server 127.0.0.1/127.0.0.1:2181. Will not attempt to authenticate using SASL (unknown error) 2015-05-01 19:14:34 [ main-SendThread(127.0.0.1:2181):31 ] - [ INFO ] Opening socket connection to server 127.0.0.1/127.0.0.1:2181. Will not attempt to authenticate using SASL (unknown error) 2015-05-01 19:14:34 [ main-SendThread(127.0.0.1:2181):31 ] - [ INFO ] Opening socket connection to server 127.0.0.1/127.0.0.1:2181. Will not attempt to authenticate using SASL (unknown error) 2015-05-01 19:14:34 [ main-SendThread(127.0.0.1:2181):47 ] - [ INFO ] Socket connection established to 127.0.0.1/127.0.0.1:2181, initiating session 2015-05-01 19:14:34 [ main-SendThread(127.0.0.1:2181):47 ] - [ INFO ] Socket connection established to 127.0.0.1/127.0.0.1:2181, initiating session 2015-05-01 19:14:34 [ main-SendThread(127.0.0.1:2181):47 ] - [ INFO ] Socket connection established to 127.0.0.1/127.0.0.1:2181, initiating session 2015-05-01 19:14:34 [ main-SendThread(127.0.0.1:2181):47 ] - [ INFO ] Session establishment complete on server 127.0.0.1/127.0.0.1:2181, sessionid = 0x14d0ee972c70006, negotiated timeout = 5000 2015-05-01 19:14:34 [ main-SendThread(127.0.0.1:2181):47 ] - [ INFO ] Unable to reconnect to ZooKeeper service, session 0x1 has expired, closing socket connection Receive watched event:WatchedEvent state:SyncConnected type:None path:null Receive watched event:WatchedEvent state:Expired type:None path:null 2015-05-01 19:14:34 [ main-SendThread(127.0.0.1:2181):47 ] - [ INFO ] Session establishment complete on server 127.0.0.1/127.0.0.1:2181, sessionid = 0x14d0ee972c70007, negotiated timeout = 5000 2015-05-01 19:14:34 [ main-EventThread:47 ] - [ INFO ] EventThread shut down