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ZooKeeper在线API
http://zookeeper.apache.org/doc/r3.4.6/api/index.html
ZooKeeper API共包含5个包,分别为:org.apache.zookeeper,org.apache.zookeeper.data,org.apache.zookeeper.server,org.apache.zookeeper.server.quorum和org.apache.zookeeper.server.upgrade。其中org.apache.zookeeper包含ZooKeeper类,它是我们编程时最常用的类文件。
这个ZooKeeper类是ZooKeeper客户端库的主要类文件。如果要使用ZooKeeper服务,应用程序首先必须创建一个Zookeeper实例,这时就需要使用此类。一旦客户端和ZooKeeper服务建立起连接,ZooKeeper系统将会分配给此连接回话一个ID值,并且客户端将会周期地向服务器发送心跳来维持会话的连接。只要连接有效,客户端就可以调用ZooKeeper API来做相应的处理。
使用ZooKeeper的Java API来实现简单的使用zookeeper,并完成一个简单的经典小需求。
示例:假设有一组服务器用于为客户端提供某种服务。我们希望每个客户端都能找到其中一台服务器,这样它们就可以使用这项服务。
有一个挑战是如何维护这组服务器的列表。这组服务器的成员列表显然不能存储在网络中的单个节点上,否则该节点的故障将意味着整个系统的故障,我们希望这个成员列表是高可用的。
我们还希望当其中一台服务器出现故障时,能够从这组服务器成员列表中删除该节点,避免提供不可用的服务。
相应的实现代码对象以下几个类:
ConnectionWatcher.java 维护客户端和ZooKeeper服务之间的连接;
CreateGroup.java 创建指定的服务组在ZooKeeper上;
DeleteGroup.java 删除指定的服务组在ZooKeeper上;
JoinGroup.java 添加成员在指定的服务组;
ListGroup.java 列出指定服务组下的所有成员;
源代码如下:
public class ConnectionWatcher implements Watcher{
private static final int SESSION_TIMEOUT = 5000;
protected ZooKeeper zk;
private CountDownLatch connectedSignal = new CountDownLatch(1);
public void connect(String hosts) throws Exception{
//创建zookeeper实例的时候会启动一个线程连接到zookeeper服务。
zk = new ZooKeeper(hosts, SESSION_TIMEOUT, this);
connectedSignal.await();
}
//当客户端已经与zookeeper建立连接后,Watcher的process方法会被调用。
public void process(WatchedEvent event) {
if(event.getState() == KeeperState.SyncConnected){
connectedSignal.countDown();
}
}
public void close() throws Exception{
zk.close();
}
}public class CreateGroup extends ConnectionWatcher{
public void create(String groupName) throws Exception{
String path = "/"+groupName;
//创建一个完全开发的ACL(访问控制列表)、持久的znode
String createdPath = zk.create(path, null, Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
System.out.println("Created "+createdPath);
}
/**
* @param args
* @throws Exception
*/
public static void main(String[] args) throws Exception {
// TODO Auto-generated method stub
CreateGroup createGroup = new CreateGroup();
createGroup.connect("192.168.44.231");
createGroup.create("myzoo");
createGroup.close();
}
}public class DeleteGroup extends ConnectionWatcher{
public void delete(String groupName) throws Exception{
String path = "/" + groupName;
try {
//ZooKeeper不支持递归删除操作,因此在删除父节点之前必须先删除子节点。
List children = zk.getChildren(path, false);
for(String child : children){
zk.delete(path+"/"+child, -1);
}
zk.delete(path, -1);
System.out.printf("Delete Group %s\n", path);
} catch (KeeperException.NoNodeException e) {
System.out.printf("Group %s does not exist\n", groupName);
System.exit(1);
}
}
public static void main(String[] args) throws Exception {
DeleteGroup deleteGroup = new DeleteGroup();
deleteGroup.connect("192.168.44.231");
deleteGroup.delete("myzoo");
deleteGroup.close();
}
}public class JoinGroup extends ConnectionWatcher{
public void join(String groupName, String memberName) throws Exception{
String path = "/" + groupName + "/" + memberName;
String createdPath = zk.create(path, null, Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);
System.out.println("Created "+ createdPath);
}
/**
* @param args
* @throws Exception
*/
public static void main(String[] args) throws Exception {
// TODO Auto-generated method stub
JoinGroup joinGroup = new JoinGroup();
joinGroup.connect("192.168.44.231");
joinGroup.join("myzoo", "myservice2");
//stay alive until process is killed or thread is interrupted
Thread.sleep(Long.MAX_VALUE);
}
}public class ListGroup extends ConnectionWatcher{
public void list(String groupName) throws Exception{
String path = "/" + groupName;
try {
List children = zk.getChildren(path, false);
if(children.isEmpty()){
System.out.printf("No members in group %s\n", groupName);
System.exit(1);
}
for(String child : children){
System.out.println(child);
}
} catch (KeeperException.NoNodeException e) {
System.out.printf("Group %s does not exist\n", groupName);
System.exit(1);
}
}
/**
* @param args
* @throws Exception
*/
public static void main(String[] args) throws Exception {
// TODO Auto-generated method stub
ListGroup listGroup = new ListGroup();
listGroup.connect("192.168.44.231");
listGroup.list("myzoo");
listGroup.close();
}
}
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