Zookeeper开发小结
一.实例化zookeeper与自动重连代码样例
<span style="font-size:18px;">public class ZkClient { private ZooKeeper zooKeeper; private String connectString; private Integer sessionTimeout; private Object <span style="color:#ff0000;"><strong>waiter = new Object();//simple object-lock</strong></span> //"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<span style="color:#ff0000;"><strong>,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. */</strong></span> private void connectZK() {</span>
<span style="font-size:18px;">//<span style="font-size:18px;">private Object <span style="color:#ff0000;background-color: rgb(240, 240, 240);"><strong>waiter = new Object();//simple object-lock</strong></span></span></span>
<span style="font-size:18px;"> synchronized (waiter) { try { SessionWatcher watcher =<span style="color:#ff0000;"><strong> new SessionWatcher();</strong></span> // session的构建是异步的 this.zooKeeper = new ZooKeeper(connectString, sessionTimeout, watcher, false); } catch (Exception e) { e.printStackTrace(); } <span style="color:#ff0000;"><strong>waiter.notifyAll();</strong></span> } } class SessionWatcher implements Watcher { public void process(WatchedEvent<strong> event)</strong> { // 如果是“数据变更”事件 if (event.getType() != Event.EventType.None) { return; } <span style="color:#ff0000;"><strong>// 如果是链接状态迁移 // 参见keeperState synchronized (waiter) {</strong></span> 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(); } }}</span>
二.授权与验证
<span style="font-size:18px;">String auth = "admin:admin";//anywhere,but before znode operation//can addauth more than oncezooKeeper.addAuthInfo("digest", auth.getBytes("UTF-8"));</span>
<span style="font-size:18px;">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_UNSAFEzooKeeper.create("/singleWorker", null, acls, CreateMode.PERSISTENT);</span>
三.创建znode节点
<span style="font-size:18px;">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();}</span>
四.删除节点
<span style="font-size:18px;">//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<span style="color:#ff0000;"><strong>节点,需要递归来做</strong></span>private void deletePath(String path,ZooKeeper zooKeeper) throws Exception{ List<String> children = zooKeeper.getChildren(path,false); for(String child : children){ <strong><span style="color:#ff0000;">String childPath = path + "/" + child; deletePath(childPath,zooKeeper);</span></strong> } try{ zooKeeper.delete(path,-1); }catch(KeeperException.NoNodeException e){ //ignore }}</span>
<span style="font-size:18px;">//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;}</span>
五.修改数据
<span style="font-size:18px;">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;}</span>
五.事务
<span style="font-size:18px;">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;}</span>
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.