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Apache Curator入門實戰

Curator是Netflix公司開源的一個Zookeeper客戶端，與Zookeeper提供的原生客戶端相比，Curator的抽象層次更高，簡化了Zookeeper客戶端的開發量。

1.Zookeeper安裝部署

Zookeeper的部署很簡單，如果已經有Java運行環境的話，下載tarball解壓后即可運行。

[root@vm Temp]$ wget http://mirror.bit.edu.cn/apache/zookeeper/zookeeper-3.4.6/zookeeper-3.4.6.tar.gz [root@vm Temp]$ tar zxvf zookeeper-3.4.6.tar.gz [root@vm Temp]$ cd zookeeper-3.4.6 [root@vm zookeeper-3.4.6]$ cp conf/zoo_sample.cfg conf/zoo.cfg [root@vm zookeeper-3.4.6]$ export ZOOKEEPER_HOME=/usr/local/src/zookeeper-3.4.5 [root@vm zookeeper-3.4.6]$ export PATH=$ZOOKEEPER_HOME/bin:$PATH [root@vm zookeeper-3.4.6]$ bin/zkServer.sh start [root@vm zookeeper-3.4.6]$ bin/zkCli.sh -server 127.0.0.1:2181

2.客戶端常用操作

用zkCli.sh連接上Zookeeper服務后，用help能列出所有命令：

[root@BC-VM-edce4ac67d304079868c0bb265337bd4 zookeeper-3.4.6]# bin/zkCli.sh -127.0.0.1:2181 Connecting to localhost:2181 2015-06-11 10:55:14,387 [myid:] - INFO [main:Environment@100] - Client environment:zookeeper.version=3.4.6-1569965, built on 02/20/2014 09:09 GMT ... [zk: localhost:2181(CONNECTED) 5] help ZooKeeper -server host:port cmd args connect host:port get path [watch] ls path [watch] set path data [version] rmr path delquota [-n|-b] path quit printwatches on|off create [-s] [-e] path data acl stat path [watch] close ls2 path [watch] history listquota path setAcl path acl getAcl path sync path redo cmdno addauth scheme auth delete path [version] setquota -n|-b val path

 

 

下面就試驗一下常用的命令：

• create：創建路徑結點。
• ls：查看路徑下的所有結點。
• get：獲得結點上的值。
• set：修改結點上的值。
• delete：刪除結點。

[zk: localhost:2181(CONNECTED) 6] create /zktest mydata
Created /zktest
[zk: localhost:2181(CONNECTED) 12] ls /
[zktest, zookeeper]
[zk: localhost:2181(CONNECTED) 7] ls /zktest
[]
[zk: localhost:2181(CONNECTED) 13] get /zktest
mydata
cZxid = 0x1c ctime = Thu Jun 11 10:58:06 CST 2015 mZxid = 0x1c mtime = Thu Jun 11 10:58:06 CST 2015 pZxid = 0x1c cversion = 0 dataVersion = 0 aclVersion = 0 ephemeralOwner = 0x0 dataLength = 6 numChildren = 0 [zk: localhost:2181(CONNECTED) 14] set /zktest junk cZxid = 0x1c ctime = Thu Jun 11 10:58:06 CST 2015 mZxid = 0x1f mtime = Thu Jun 11 10:59:08 CST 2015 pZxid = 0x1c cversion = 0 dataVersion = 1 aclVersion = 0 ephemeralOwner = 0x0 dataLength = 4 numChildren = 0 [zk: localhost:2181(CONNECTED) 15] delete /zktest [zk: localhost:2181(CONNECTED) 16] ls / [zookeeper]

 

3.用Curator管理Zookeeper

Curator的Maven依賴如下，一般直接使用curator-recipes就行了，如果需要自己封裝一些底層些的功能的話，例如增加連接管理重試機制等，則可以引入curator-framework包。

    <dependency> <groupId>org.apache.curator</groupId> <artifactId>curator-recipes</artifactId> <version>2.7.0</version> </dependency>

 

3.1 Client操作

利用Curator提供的客戶端API，可以完全實現上面原生客戶端的功能。值得注意的是，Curator采用流式風格API。

package com.cdai.codebase.bigdata.hadoop.zookeeper.curator; import org.apache.curator.framework.CuratorFramework; import org.apache.curator.framework.CuratorFrameworkFactory; import org.apache.curator.retry.RetryNTimes; /** * Curator framework's client test. * Output: * $ create /zktest hello * $ ls / * [zktest, zookeeper] * $ get /zktest * hello * $ set /zktest world * $ get /zktest * world * $ delete /zktest * $ ls / * [zookeeper] */ public class CuratorClientTest { /** Zookeeper info */ private static final String ZK_ADDRESS = "192.168.1.100:2181"; private static final String ZK_PATH = "/zktest"; public static void main(String[] args) throws Exception { // 1.Connect to zk CuratorFramework client = CuratorFrameworkFactory.newClient( ZK_ADDRESS, new RetryNTimes(10, 5000) ); client.start(); System.out.println("zk client start successfully!"); // 2.Client API test // 2.1 Create node String data1 = "hello"; print("create", ZK_PATH, data1); client.create(). creatingParentsIfNeeded(). forPath(ZK_PATH, data1.getBytes()); // 2.2 Get node and data print("ls", "/"); print(client.getChildren().forPath("/")); print("get", ZK_PATH); print(client.getData().forPath(ZK_PATH)); // 2.3 Modify data String data2 = "world"; print("set", ZK_PATH, data2); client.setData().forPath(ZK_PATH, data2.getBytes()); print("get", ZK_PATH); print(client.getData().forPath(ZK_PATH)); // 2.4 Remove node print("delete", ZK_PATH); client.delete().forPath(ZK_PATH); print("ls", "/"); print(client.getChildren().forPath("/")); } private static void print(String... cmds) { StringBuilder text = new StringBuilder("$ "); for (String cmd : cmds) { text.append(cmd).append(" "); } System.out.println(text.toString()); } private static void print(Object result) { System.out.println( result instanceof byte[] ? new String((byte[]) result) : result); } }

 

3.2 監聽器

Curator提供了三種Watcher(Cache)來監聽結點的變化：

• Path Cache：監視一個路徑下1）孩子結點的創建、2）刪除，3）以及結點數據的更新。產生的事件會傳遞給注冊的PathChildrenCacheListener。
• Node Cache：監視一個結點的創建、更新、刪除，並將結點的數據緩存在本地。
• Tree Cache：Path Cache和Node Cache的“合體”，監視路徑下的創建、更新、刪除事件，並緩存路徑下所有孩子結點的數據。

下面就測試一下最簡單的Path Watcher：

package com.cdai.codebase.bigdata.hadoop.zookeeper.curator; import org.apache.curator.framework.CuratorFramework; import org.apache.curator.framework.CuratorFrameworkFactory; import org.apache.curator.framework.recipes.cache.ChildData; import org.apache.curator.framework.recipes.cache.PathChildrenCache; import org.apache.curator.framework.recipes.cache.PathChildrenCache.StartMode; import org.apache.curator.retry.RetryNTimes; /** * Curator framework watch test. */ public class CuratorWatcherTest { /** Zookeeper info */ private static final String ZK_ADDRESS = "192.168.1.100:2181"; private static final String ZK_PATH = "/zktest"; public static void main(String[] args) throws Exception { // 1.Connect to zk CuratorFramework client = CuratorFrameworkFactory.newClient( ZK_ADDRESS, new RetryNTimes(10, 5000) ); client.start(); System.out.println("zk client start successfully!"); // 2.Register watcher PathChildrenCache watcher = new PathChildrenCache( client, ZK_PATH, true // if cache data ); watcher.getListenable().addListener((client1, event) -> { ChildData data = event.getData(); if (data == null) { System.out.println("No data in event[" + event + "]"); } else { System.out.println("Receive event: " + "type=[" + event.getType() + "]" + ", path=[" + data.getPath() + "]" + ", data=[" + new String(data.getData()) + "]" + ", stat=[" + data.getStat() + "]"); } }); watcher.start(StartMode.BUILD_INITIAL_CACHE); System.out.println("Register zk watcher successfully!"); Thread.sleep(Integer.MAX_VALUE); } }

 

下面是在zkCli.sh中操作時Java程序的輸出：

Java: zk client start successfully! Java: Register zk watcher successfully! zkCli: [zk: localhost:2181(CONNECTED) 11] create /zktest/hello mydata Java: Receive event: type=[CHILD_ADDED], path=[/zktest/hello], data=[mydata], stat=[121,121,1434001221097,1434001221097,0,0,0,0,6,0,121] zkCli: [zk: localhost:2181(CONNECTED) 12] set /zktest/hello otherdata Java: Receive event: type=[CHILD_UPDATED], path=[/zktest/hello], data=[otherdata], stat=[121,122,1434001221097,1434001228467,1,0,0,0,9,0,121] zkCli: [zk: localhost:2181(CONNECTED) 13] delete /zktest/hello Java: Receive event: type=[CHILD_REMOVED], path=[/zktest/hello], data=[otherdata], stat=[121,122,1434001221097,1434001228467,1,0,0,0,9,0,121]

 

4.Curator“菜譜”

既然Maven包叫做curator-recipes，那說明Curator有它獨特的“菜譜”：

• 鎖：包括共享鎖、共享可重入鎖、讀寫鎖等。
• 選舉：Leader選舉算法。
• Barrier：阻止分布式計算直至某個條件被滿足的“柵欄”，可以看做JDK Concurrent包中Barrier的分布式實現。
• 緩存：前面提到過的三種Cache及監聽機制。
• 持久化結點：連接或Session終止后仍然在Zookeeper中存在的結點。
• 隊列：分布式隊列、分布式優先級隊列等。

4.1 分布式鎖

分布式編程時，比如最容易碰到的情況就是應用程序在線上多機部署，於是當多個應用同時訪問某一資源時，就需要某種機制去協調它們。例如，現在一台應用正在rebuild緩存內容，要臨時鎖住某個區域暫時不讓訪問；又比如調度程序每次只想一個任務被一台應用執行等等。

下面的程序會啟動兩個線程t1和t2去爭奪鎖，拿到鎖的線程會占用5秒。運行多次可以觀察到，有時是t1先拿到鎖而t2等待，有時又會反過來。Curator會用我們提供的lock路徑的結點作為全局鎖，這個結點的數據類似這種格式：[_c_64e0811f-9475-44ca-aa36-c1db65ae5350-lock-0000000005]，每次獲得鎖時會生成這種串，釋放鎖時清空數據。

package com.cdai.codebase.bigdata.hadoop.zookeeper.curator; import org.apache.curator.framework.CuratorFramework; import org.apache.curator.framework.CuratorFrameworkFactory; import org.apache.curator.framework.recipes.locks.InterProcessMutex; import org.apache.curator.retry.RetryNTimes; import java.util.concurrent.TimeUnit; /** * Curator framework's distributed lock test. */ public class CuratorDistrLockTest { /** Zookeeper info */ private static final String ZK_ADDRESS = "192.168.1.100:2181"; private static final String ZK_LOCK_PATH = "/zktest"; public static void main(String[] args) throws InterruptedException { // 1.Connect to zk CuratorFramework client = CuratorFrameworkFactory.newClient( ZK_ADDRESS, new RetryNTimes(10, 5000) ); client.start(); System.out.println("zk client start successfully!"); Thread t1 = new Thread(() -> { doWithLock(client); }, "t1"); Thread t2 = new Thread(() -> { doWithLock(client); }, "t2"); t1.start(); t2.start(); } private static void doWithLock(CuratorFramework client) { InterProcessMutex lock = new InterProcessMutex(client, ZK_LOCK_PATH); try { if (lock.acquire(10 * 1000, TimeUnit.SECONDS)) { System.out.println(Thread.currentThread().getName() + " hold lock"); Thread.sleep(5000L); System.out.println(Thread.currentThread().getName() + " release lock"); } } catch (Exception e) { e.printStackTrace(); } finally { try { lock.release(); } catch (Exception e) { e.printStackTrace(); } } } }

 

4.2 Leader選舉

當集群里的某個服務down機時，我們可能要從slave結點里選出一個作為新的master，這時就需要一套能在分布式環境中自動協調的Leader選舉方法。Curator提供了LeaderSelector監聽器實現Leader選舉功能。同一時刻，只有一個Listener會進入takeLeadership()方法，說明它是當前的Leader。注意：當Listener從takeLeadership()退出時就說明它放棄了“Leader身份”，這時Curator會利用Zookeeper再從剩余的Listener中選出一個新的Leader。autoRequeue()方法使放棄Leadership的Listener有機會重新獲得Leadership，如果不設置的話放棄了的Listener是不會再變成Leader的。

package com.cdai.codebase.bigdata.hadoop.zookeeper.curator; import org.apache.curator.framework.CuratorFramework; import org.apache.curator.framework.CuratorFrameworkFactory; import org.apache.curator.framework.recipes.leader.LeaderSelector; import org.apache.curator.framework.recipes.leader.LeaderSelectorListener; import org.apache.curator.framework.state.ConnectionState; import org.apache.curator.retry.RetryNTimes; import org.apache.curator.utils.EnsurePath; /** * Curator framework's leader election test. * Output: * LeaderSelector-2 take leadership! * LeaderSelector-2 relinquish leadership! * LeaderSelector-1 take leadership! * LeaderSelector-1 relinquish leadership! * LeaderSelector-0 take leadership! * LeaderSelector-0 relinquish leadership! * ... */ public class CuratorLeaderTest { /** Zookeeper info */ private static final String ZK_ADDRESS = "192.168.1.100:2181"; private static final String ZK_PATH = "/zktest"; public static void main(String[] args) throws InterruptedException { LeaderSelectorListener listener = new LeaderSelectorListener() { @Override public void takeLeadership(CuratorFramework client) throws Exception { System.out.println(Thread.currentThread().getName() + " take leadership!"); // takeLeadership() method should only return when leadership is being relinquished. Thread.sleep(5000L); System.out.println(Thread.currentThread().getName() + " relinquish leadership!"); } @Override public void stateChanged(CuratorFramework client, ConnectionState state) { } }; new Thread(() -> { registerListener(listener); }).start(); new Thread(() -> { registerListener(listener); }).start(); new Thread(() -> { registerListener(listener); }).start(); Thread.sleep(Integer.MAX_VALUE); } private static void registerListener(LeaderSelectorListener listener) { // 1.Connect to zk CuratorFramework client = CuratorFrameworkFactory.newClient( ZK_ADDRESS, new RetryNTimes(10, 5000) ); client.start(); // 2.Ensure path try { new EnsurePath(ZK_PATH).ensure(client.getZookeeperClient()); } catch (Exception e) { e.printStackTrace(); } // 3.Register listener LeaderSelector selector = new LeaderSelector(client, ZK_PATH, listener); selector.autoRequeue(); selector.start(); } }