Curator框架是最好用,最流行的zookeeper的客戶端。
它有以下三個優點
1.提供了一套非常友好的操作API;
2. 提供一些高級特性(包括但不僅限於前篇文章中提到的)的封裝
3.易測試
maven依賴如下
<dependency> <groupId>org.apache.curator</groupId> <artifactId>curator-recipes</artifactId> <version>2.5.0</version> </dependency>
按照官方給出的文檔和包結構,可以輕鬆的看出Curator功能分兩大類,一是對zookeeper的一些基本命令的封裝,比如增刪改查。是他的framework模塊,一個是他的高級特性,即recipes模塊。
一、framework模塊
Curator提供了一套Fluent風格的操作API。這在很多腳本類語言裏比較流行。
比如他創建client的代碼是這樣
CuratorFramework client = builder.connectString("192.168.11.56:2180")
.sessionTimeoutMs(30000)
.connectionTimeoutMs(30000)
.canBeReadOnly(false)
.retryPolicy(new ExponentialBackoffRetry(1000, Integer.MAX_VALUE))
.namespace(namespace)
.defaultData(null)
.build();
client.start();
增刪改查的
public class CrudExamples {
private static CuratorFramework client = ClientFactory.newClient();
private static final String PATH = "/crud";
public static void main(String[] args) {
try {
client.start();
client.create().forPath(PATH, "I love messi".getBytes());
byte[] bs = client.getData().forPath(PATH);
System.out.println("新建的節點,data爲:" + new String(bs));
client.setData().forPath(PATH, "I love football".getBytes());
// 由於是在background模式下獲取的data,此時的bs可能爲null
byte[] bs2 = client.getData().watched().inBackground().forPath(PATH);
System.out.println("修改後的data爲" + new String(bs2 != null ? bs2 : new byte[0]));
client.delete().forPath(PATH);
Stat stat = client.checkExists().forPath(PATH);
// Stat就是對zonde所有屬性的一個映射, stat=null表示節點不存在!
System.out.println(stat);
} catch (Exception e) {
e.printStackTrace();
} finally {
CloseableUtils.closeQuietly(client);
}
}
}
此外,Curator還支持事務,一組crud操作同生同滅。代碼如下
/**
* 事務操作
*
* @author shencl
*/
public class TransactionExamples {
private static CuratorFramework client = ClientFactory.newClient();
public static void main(String[] args) {
try {
client.start();
// 開啓事務
CuratorTransaction transaction = client.inTransaction();
Collection<CuratorTransactionResult> results = transaction.create()
.forPath("/a/path", "some data".getBytes()).and().setData()
.forPath("/another/path", "other data".getBytes()).and().delete().forPath("/yet/another/path")
.and().commit();
for (CuratorTransactionResult result : results) {
System.out.println(result.getForPath() + " - " + result.getType());
}
} catch (Exception e) {
e.printStackTrace();
} finally {
// 釋放客戶端連接
CloseableUtils.closeQuietly(client);
}
}
}
這段的代碼的運行結果,由於最後一步delete的節點不存在,所以整個事務commit失敗。失敗的原因會放在Collection<CuratorTransactionResult>中,非常友好。
好了framework部分的內容就這麼多,是不是特別簡單呢。下面就來看看recipes包的內容吧。。
Recipes部分提供的功能官網列的很詳細,點擊這裏。注意文章第一段:Curator宣稱,Recipes模塊實現了除二階段提交之外的所有zookeeper特性。
二、Recipes模塊
主要有
Elections(選舉),Locks(鎖),Barriers(關卡),Atomic(原子量),Caches,Queues等
1、 Elections
選舉主要依賴於LeaderSelector和LeaderLatch2個類。前者是所有存活的客戶端不間斷的輪流做Leader,大同社會。後者是一旦選舉出Leader,除非有客戶端掛掉重新觸發選舉,否則不會交出領導權。某黨?
這兩者在實現上是可以切換的,直接上代碼,怎麼切換註釋裏有。由於篇幅所限,這裏僅貼出基於LeaderSelector的選舉,更多代碼見附件
/**
* 本類基於leaderSelector實現,所有存活的client會公平的輪流做leader
* 如果不想頻繁的變化Leader,需要在takeLeadership方法裏阻塞leader的變更! 或者使用 {@link}
* LeaderLatchClient
*/
public class LeaderSelectorClient extends LeaderSelectorListenerAdapter implements Closeable {
private final String name;
private final LeaderSelector leaderSelector;
private final String PATH = "/leaderselector";
public LeaderSelectorClient(CuratorFramework client, String name) {
this.name = name;
leaderSelector = new LeaderSelector(client, PATH, this);
leaderSelector.autoRequeue();
}
public void start() throws IOException {
leaderSelector.start();
}
@Override
public void close() throws IOException {
leaderSelector.close();
}
/**
* client成爲leader後,會調用此方法
*/
@Override
public void takeLeadership(CuratorFramework client) throws Exception {
int waitSeconds = (int) (5 * Math.random()) + 1;
System.out.println(name + "是當前的leader");
try {
Thread.sleep(TimeUnit.SECONDS.toMillis(waitSeconds));
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
} finally {
System.out.println(name + " 讓出領導權\n");
}
}
/**
* leader選舉
*
* @author shencl
*/
public class LeaderSelectorExample {
public static void main(String[] args) {
List<CuratorFramework> clients = Lists.newArrayList();
List<LeaderSelectorClient> examples = Lists.newArrayList();
try {
for (int i = 0; i < 10; i++) {
CuratorFramework client = ClientFactory.newClient();
LeaderSelectorClient example = new LeaderSelectorClient(client, "Client #" + i);
clients.add(client);
examples.add(example);
client.start();
example.start();
}
System.out.println("----------先觀察一會選舉的結果-----------");
Thread.sleep(10000);
System.out.println("----------關閉前5個客戶端,再觀察選舉的結果-----------");
for (int i = 0; i < 5; i++) {
clients.get(i).close();
}
// 這裏有個小技巧,讓main程序一直監聽控制檯輸入,異步的代碼就可以一直在執行。不同於while(ture)的是,按回車或esc可退出
new BufferedReader(new InputStreamReader(System.in)).readLine();
} catch (Exception e) {
e.printStackTrace();
} finally {
for (LeaderSelectorClient exampleClient : examples) {
CloseableUtils.closeQuietly(exampleClient);
}
for (CuratorFramework client : clients) {
CloseableUtils.closeQuietly(client);
}
}
}
}
2、locks
curator lock相關的實現在recipes.locks包裏。頂級接口都是InterProcessLock。我們直接看最有代表性的InterProcessReadWriteLock 進程內部讀寫鎖(可重入讀寫鎖)。什麼叫可重入,什麼叫讀寫鎖。不清楚的先查好資料吧。總之讀寫鎖一定是成對出現的。 簡易傳送門
我們先定義兩個任務,可並行的執行的,和互斥執行的。
/**
* 並行任務
*
* @author shencl
*/
public class ParallelJob implements Runnable {
private final String name;
private final InterProcessLock lock;
// 鎖等待時間
private final int wait_time = 5;
ParallelJob(String name, InterProcessLock lock) {
this.name = name;
this.lock = lock;
}
@Override
public void run() {
try {
doWork();
} catch (Exception e) {
// ingore;
}
}
public void doWork() throws Exception {
try {
if (!lock.acquire(wait_time, TimeUnit.SECONDS)) {
System.err.println(name + "等待" + wait_time + "秒,仍未能獲取到lock,準備放棄。");
}
// 模擬job執行時間0-4000毫秒
int exeTime = new Random().nextInt(4000);
System.out.println(name + "開始執行,預計執行時間= " + exeTime + "毫秒----------");
Thread.sleep(exeTime);
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.release();
}
}
}
/**
* 互斥任務
*
* @author shencl
*/
public class MutexJob implements Runnable {
private final String name;
private final InterProcessLock lock;
// 鎖等待時間
private final int wait_time = 10;
MutexJob(String name, InterProcessLock lock) {
this.name = name;
this.lock = lock;
}
@Override
public void run() {
try {
doWork();
} catch (Exception e) {
// ingore;
}
}
public void doWork() throws Exception {
try {
if (!lock.acquire(wait_time, TimeUnit.SECONDS)) {
System.err.println(name + "等待" + wait_time + "秒,仍未能獲取到lock,準備放棄。");
}
// 模擬job執行時間0-2000毫秒
int exeTime = new Random().nextInt(2000);
System.out.println(name + "開始執行,預計執行時間= " + exeTime + "毫秒----------");
Thread.sleep(exeTime);
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.release();
}
}
}
/**
* 分佈式鎖實例
*
* @author shencl
*/
public class DistributedLockExample {
private static CuratorFramework client = ClientFactory.newClient();
private static final String PATH = "/locks";
// 進程內部(可重入)讀寫鎖
private static final InterProcessReadWriteLock lock;
// 讀鎖
private static final InterProcessLock readLock;
// 寫鎖
private static final InterProcessLock writeLock;
static {
client.start();
lock = new InterProcessReadWriteLock(client, PATH);
readLock = lock.readLock();
writeLock = lock.writeLock();
}
public static void main(String[] args) {
try {
List<Thread> jobs = Lists.newArrayList();
for (int i = 0; i < 10; i++) {
Thread t = new Thread(new ParallelJob("Parallel任務" + i, readLock));
jobs.add(t);
}
for (int i = 0; i < 10; i++) {
Thread t = new Thread(new MutexJob("Mutex任務" + i, writeLock));
jobs.add(t);
}
for (Thread t : jobs) {
t.start();
}
} catch (Exception e) {
e.printStackTrace();
} finally {
CloseableUtils.closeQuietly(client);
}
}
}
三、測試方法
curator提供了很好的測試工具,你甚至是可以在完全沒有搭建zookeeper server端的情況下,完成測試。
有2個重要的類
TestingServer 模擬單點, TestingCluster模擬集羣。
需要使用的話,得依賴
<dependency> <groupId>org.apache.curator</groupId> <artifactId>curator-test</artifactId> <version>2.5.0</version> </dependency>
附件:http://dl2.iteye.com/upload/attachment/0099/3974/33947bab-1b45-3593-8c30-616d66194a9d.rar