Zookeeper开发小结

plantegg

　　一.实例化zookeeper与自动重连代码样例

public class ZkClient {
private ZooKeeper zooKeeper;
private String connectString;
private Integer sessionTimeout;
private Object waiter = new Object();//simple object-lock
//"zk-client.properties";//classpath下
public ZkClient(String configLocation) throws  Exception{
Properties config = new Properties();
config.load(ClassLoader.getSystemResourceAsStream(configLocation));
connectString = config.getProperty("zk.connectString");
if(connectString == null){
throw new NullPointerException("'zk.connectString' cant be empty.. ");
}
sessionTimeout = Integer.parseInt(config.getProperty("zk.sessionTimeout","-1"));
connectZK();
}
/**
* core method,启动zk服务 本实例基于自动重连策略,如果zk连接没有建立成功或者在运行时断开,将会自动重连.
*/
private void connectZK() {
synchronized (waiter) {
try {
SessionWatcher watcher = new SessionWatcher();
// session的构建是异步的
this.zooKeeper = new ZooKeeper(connectString, sessionTimeout, watcher, false);
} catch (Exception e) {
e.printStackTrace();
}
waiter.notifyAll();
}
}

class SessionWatcher implements Watcher {
public void process(WatchedEvent event) {
// 如果是“数据变更”事件
if (event.getType() != Event.EventType.None) {
return;
}
// 如果是链接状态迁移
// 参见keeperState
synchronized (waiter) {
switch (event.getState()) {
// zk连接建立成功,或者重连成功
case SyncConnected:
System.out.println("Connected...");
waiter.notifyAll();
break;
// session过期,这是个非常严重的问题,有可能client端出现了问题,也有可能zk环境故障
// 此处仅仅是重新实例化zk client
case Expired:
System.out.println("Expired...");
// 重连
connectZK();
break;
// session过期
case Disconnected:
// 链接断开，或session迁移
System.out.println("Connecting....");
break;
case AuthFailed:
close();
throw new RuntimeException("ZK Connection auth failed...");
default:
break;
}
}
}
}
private void close(){
try {
synchronized (waiter) {
if (this.zooKeeper != null) {
zooKeeper.close();
}
waiter.notifyAll();
}
} catch (Exception e) {
e.printStackTrace();
}
}
}

　　二.授权与验证

String auth = "admin:admin";
//anywhere,but before znode operation
//can addauth more than once
zooKeeper.addAuthInfo("digest", auth.getBytes("UTF-8"));

Id id = new Id("digest", DigestAuthenticationProvider.generateDigest(auth));
ACL acl = new ACL(ZooDefs.Perms.ALL, id);
List<ACL> acls = Collections.singletonList(acl);
//如果不需要访问控制,可以使用acls = ZooDefs.Ids.OPEN_ACL_UNSAFE
zooKeeper.create("/singleWorker", null, acls, CreateMode.PERSISTENT);
　　三.创建znode节点

try{
Id id = new Id("digest", DigestAuthenticationProvider.generateDigest(auth));
ACL acl = new ACL(ZooDefs.Perms.ALL, id);
List<ACL> acls = Collections.singletonList(acl);
zooKeeper.create("/workers", null, acls, CreateMode.PERSISTENT);
}catch(KeeperException.NodeExistsException e){
//在并发环境中,节点创建有可能已经被创建,即使使用exist方法检测也不能确保.
} catch (Exception e){
e.printStackTrace();
}
　　四.删除节点

//zookeeper不允许直接删除含有子节点的节点;
//如果你需要删除当前节点以及其所有子节点,需要递归来做
private void deletePath(String path,ZooKeeper zooKeeper) throws Exception{
List<String> children = zooKeeper.getChildren(path,false);
for(String child : children){
String childPath = path + "/" + child;
deletePath(childPath,zooKeeper);
}
try{
zooKeeper.delete(path,-1);
}catch(KeeperException.NoNodeException e){
//ignore
}
}
//删除节点,删除时比较version,避免删除时被其他client修改
public boolean delete(String path){
try{
Stat stat = zooKeeper.exists(path,false);
//如果节点已经存在
if(stat != null){
zooKeeper.delete(path,stat.getVersion());
}
}catch(KeeperException.NoNodeException e){
//igore
}catch (Exception e){
e.printStackTrace();
return false;
}
return true;
}
　　五.修改数据

public boolean update(String path,byte[] data){
try{
Stat stat = zooKeeper.exists(path,false);
//如果节点已经存在
if(stat != null){
zooKeeper.setData(path,data,stat.getVersion());
return true;
}
}catch (KeeperException.NoNodeException e){
//ignore
}catch (Exception e){
e.printStackTrace();
}
return false;
}
　　五.事务

public boolean create(String name){
try{
Transaction tx = zooKeeper.transaction();
tx.create("/workers/servers/" + name,null, ZooDefs.Ids.OPEN_ACL_UNSAFE,null);
tx.create("/workers/schedule/" + name,null, ZooDefs.Ids.OPEN_ACL_UNSAFE,null);
tx.commit();
return true;
} catch (Exception e){
e.printStackTrace();
}
return false;
}

　　备注:zookeeper中的watcher机制非常好,但是重要的数据变更,不能完全依赖watcher通知,因为对于zkClient而言,网络异常都将会导致watcher有丢失的潜在风险,而且watcher是"即发即失",当你接收到watcher通知之后,在处理过程中,数据仍然有变更的可能,因此在时间线上,不可能做到完全准确.