在上一篇文章，我們已經實現了分散式鎖。今天更進一步，在分散式鎖的基礎之上，實現讀寫鎖。

完整程式碼在 https://github.com/SeemSilly/codestory/tree/master/research-zoo-keeper

1 讀寫鎖的概念

參考維基百科的條目: https://zh.wikipedia.org/wiki/讀寫鎖

讀寫鎖是計算機程式的併發控制的一種同步機制，用於解決讀寫問題，讀操作可併發重入，寫操作是互斥的。 讀寫鎖有多種讀寫許可權的優先順序策略，可以設計為讀優先、寫優先或不指定優先順序。

• 讀優先：允許最大併發的讀操作，但可能會餓死寫操作；因為寫操作必須在沒有任何讀操作的時候才能夠執行。
• 寫優先：只要排隊佇列中有寫操作，讀操作就必須等待；
• 不指定優先順序：對讀操作和寫操作不做任何優先順序的假設

不指定優先順序的策略，最適合使用ZooKeeper的子節點模式來實現，今天就來嘗試這種策略。

2 鎖設計

同前面介紹的普通分散式鎖，也使用子節點模式實現。先用容器模式（CreateMode.CONTAINER）建立唯一的鎖節點，每個鎖客戶端在鎖節點下使用臨時循序模式（CreateMode. SEQUENTIAL）建立子節點。這些子節點會自動在名稱後面追加10位數字。

2.1 如何標識讀鎖還是寫鎖？

有兩種簡單的方案：在子節點名中標識、在節點的值中標識。如果採用在值中標識，每次子節點列表後，還需要再分別讀一下子節點的值，才能判斷是讀鎖還是寫鎖，會比較耗時。如果在子節點名稱中標識，會面臨一個問題：在同一個節點中建立的子節點，如果給定的名稱不同，追加的10位數字是否仍然是遞迴的？

寫個測試用例驗證一下。

public class SequentialTest extends TestBase {
  @Test
  public void testSequential() throws Exception {
    String rootNodeName = "/container-" + System.currentTimeMillis();
    ZooKeeperBase zooKeeper = new ZooKeeperBase(address);
    zooKeeper.createRootNode(rootNodeName, CreateMode.CONTAINER);

    Random random = new SecureRandom();
    long lastNumber = -1L;
    String[] prefixs = new String[] {"/a", "/b", "/c", "/d", "/e", "/f", "/g"};
    for (int i = 0; i < 10; i++) {
      int index = random.nextInt(prefixs.length);
      String childNodeName = rootNodeName + prefixs[index];
      String fullNodeName = zooKeeper.getZooKeeper().create(childNodeName, new byte[0],
          ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL_SEQUENTIAL);
      long number = Long.parseLong(fullNodeName.substring(childNodeName.length()));
      assert number == lastNumber + 1;
      lastNumber = number;
    }
  }
}

測試用例通過，說明在同一個Container中建立的子節點，不論提供的節點名是什麼，後續追加的10位數字都是順序遞增的。這樣，就可以使用節點名來區分讀鎖和寫鎖。

2.2   類設計

介紹分散式鎖的時候，已經建立了阻塞鎖 ChildrenBlockingLock，讀寫鎖正好可以基於這個類做過載。

2.3   獲取鎖的邏輯

寫鎖是一個獨佔鎖，邏輯跟普通分散式鎖相同，只要它之前有鎖就必須等待。所以，完全沿用阻塞鎖的邏輯即可。

讀鎖允許併發，它之前可以有任意讀鎖，但不能有寫鎖。所以只需要判斷有沒有寫鎖即可。

3      關鍵程式碼

3.1   ChildrenNodeLock.java

這個類，主要是增加了一個獲取排序後子節點列表的方法，這樣方便實現讀寫鎖的程式碼。當然，這個操作會增加一些耗時，如果子節點數量太大，可能不適用。

首先定義一個函式，用來返回子節點的字首

/** 子節點的字首，預設是element，子類可以過載 */
protected String getChildPrefix() {
  return "element";
}

然後定義一個內部類，子節點排序時會用到

/** 子節點名稱比較 */
private class StringCompare implements Comparator<String> {
  @Override
  public int compare(String string1, String string2) {
    return string1.substring(string1.length() - 10)
        .compareTo(string2.substring(string2.length() - 10));
  }
}

最後實現子節點排序方法，用於代替 getChildren 函式

/** 獲取排好序的子節點列表 */
final public List<String> getOrderedChildren(String path, boolean watch)
    throws KeeperException, InterruptedException {
  List<String> children = getZooKeeper().getChildren(path, watch);
  Collections.sort(children, new StringCompare());
  return children;
}

3.2   ChildrenBlockingLock.java

在多客戶端隨機測試時，經常出現程式卡死的情況，無法正常退出。經過新增日誌跟蹤，發現WatchedEvent可能會丟失，也可能會發送給並不是註冊事件的ZooKeeper客戶端。在網上搜索，發現很多人也碰到類似問題。

簡單修改了一下ChildrenBlockingLock#isLockSuccess等待訊號的程式碼，從無引數的死等變成設定一定超時時間等待。關鍵程式碼如下

protected boolean isLockSuccess() {
  boolean lockSuccess;
  try {
    while (true) {
      String prevElementName = getPrevElementName();
      if (prevElementName == null) {
        log.trace("{} 沒有更靠前的子節點，加鎖成功", elementNodeName);
        lockSuccess = true;
        break;
      } else {
        // 有更小的節點，說明當前節點沒搶到鎖，註冊前一個節點的監聽。
        log.trace("{} 監控 {} 的事件", elementNodeName, prevElementName);
        getZooKeeper().exists(this.guidNodeName + "/" + prevElementName, true);
        synchronized (mutex) {
          // 等待最多一秒
          mutex.wait(1000);
          log.trace("{} 監控的 {} 有子節點變化", elementNodeName, guidNodeName);
        }
      }
    }
  } catch (KeeperException e) {
    lockSuccess = false;
  } catch (InterruptedException e) {
    lockSuccess = false;
  }
  return lockSuccess;
}

3.3   寫鎖 ZooKeeperWriteLock.java

程式碼基本是沿用父類，只需要過載getChildPrefix()方法，

/** 返回寫鎖的字首 */
protected String getChildPrefix() {
  return "w-lock-";
}

3.4   讀鎖 ZooKeeperReadLock.java

同寫鎖相比，除了過載getChildPrefix()方法，還過載了getPrevElementName()用來查詢最近一個寫鎖。

/** 返回讀鎖的字首 */
protected String getChildPrefix() {
  return "r-lock-";
}

/** 是寫鎖 */
private boolean isWriteLock(String elementName) {
  return elementName.startsWith(ZooKeeperWriteLock.FLAG);
}

/** 讀取前一個寫鎖 */
protected String getPrevElementName() throws KeeperException, InterruptedException {
  List<String> elementNames = super.getOrderedChildren(this.guidNodeName, false);
  super.traceOrderedChildren(this.guidNodeName, elementNames);
  String prevWriteElementName = null;
  for (String oneElementName : elementNames) {
    if (this.elementNodeFullName.endsWith(oneElementName)) {
      // 已經到了當前節點
      break;
    }
    if (isWriteLock(oneElementName)) {
      prevWriteElementName = oneElementName;
    }
  }
  return prevWriteElementName;
}

4      測試用例

測試用例沒想到好的判斷方法，很難使用assert判斷結果，因此做了簡化，根據日誌輸出，靠人眼判斷是否正確。

4.1   測試執行緒類

分別為都鎖和寫鎖構建了兩個內部類

/** 寫鎖執行緒 */
class WriteLockClient extends Thread {
  ZooKeeperWriteLock writeLock;

  public WriteLockClient() {
    try {
      this.writeLock = new ZooKeeperWriteLock(address);
    } catch (IOException e) {
    }
  }

  public void run() {
    writeLock.lock(guidNodeName, this.getName());
    try {
      Thread.sleep(1000 + random.nextInt(20) * 100);
    } catch (InterruptedException e) {
    }
    writeLock.release(guidNodeName, this.getName());
  }
}

/** 讀鎖執行緒 */
class ReadLockClient extends Thread {
  ZooKeeperReadLock readLock;

  public ReadLockClient() {
    try {
      this.readLock = new ZooKeeperReadLock(address);
    } catch (IOException e) {
    }
  }

  public void run() {
    readLock.lock(guidNodeName, this.getName());
    try {
      Thread.sleep(1000 + random.nextInt(20) * 100);
    } catch (InterruptedException e) {
    }
    readLock.release(guidNodeName, this.getName());
    try {
      readLock.getZooKeeper().close();
    } catch (InterruptedException e) {
    }
  }
}

4.2   讀-讀鎖測試

程式碼

@Test
public void testReadRead() throws IOException, InterruptedException {
  ReadLockClient readLock1 = new ReadLockClient();
  ReadLockClient readLock2 = new ReadLockClient();
  readLock1.start();
  readLock2.start();
  readLock1.join();
  readLock2.join();
}

測試結果可以看到，兩個讀鎖併發執行

22:18.861 [Thread-2 INFO] r-lock-0000000000 get read lock : true
22:18.865 [Thread-1 INFO] r-lock-0000000001 get read lock : true
22:20.065 [Thread-2 INFO] r-lock-0000000000 release read lock
22:21.366 [Thread-1 INFO] r-lock-0000000001 release read lock

4.3   讀-寫鎖測試

程式碼

@Test
public void testReadWrite() throws IOException, InterruptedException {
  ReadLockClient readLock1 = new ReadLockClient();
  WriteLockClient writeLock1 = new WriteLockClient();
  readLock1.start();
  Thread.sleep(50);
  writeLock1.start();
  readLock1.join();
  writeLock1.join();
}

測試結果可以看到，首先獲取讀鎖，釋放之後才獲取到寫鎖。

27:40.800 [Thread-1 INFO] r-lock-0000000000 get read lock : true
27:43.310 [Thread-1 INFO] r-lock-0000000000 release read lock
27:43.423 [Thread-2 INFO] w-lock-0000000001 get write lock : true
27:44.423 [Thread-2 INFO] w-lock-0000000001 release write lock

4.4   寫-讀鎖測試

程式碼

@Test
public void testWriteRead() throws IOException, InterruptedException {
  ReadLockClient readLock1 = new ReadLockClient();
  WriteLockClient writeLock1 = new WriteLockClient();
  writeLock1.start();
  Thread.sleep(50);
  readLock1.start();
  writeLock1.join();
  readLock1.join();
}

測試結果可以看到，首先獲取寫鎖，釋放之後才獲取到讀鎖。

29:17.661 [Thread-2 INFO] w-lock-0000000000 get write lock : true
29:19.966 [Thread-2 INFO] w-lock-0000000000 release write lock
29:19.976 [Thread-1 INFO] r-lock-0000000001 get read lock : true
29:22.476 [Thread-1 INFO] r-lock-0000000001 release read lock

4.5   多客戶端隨機讀寫鎖測試

測試程式碼

@Test
public void testRandomReadWriteLock() throws IOException, InterruptedException {
  int threadCount = 20;
  Thread[] lockThreads = new Thread[threadCount];
  for (int i = 0; i < threadCount; i++) {
    // 一定概率是寫鎖
    boolean writeLock = random.nextInt(5) == 0;
    if (writeLock) {
      lockThreads[i] = new WriteLockClient();
    } else {
      lockThreads[i] = new ReadLockClient();
    }
    lockThreads[i].start();
  }
 

  for (int i = 0; i < threadCount; i++) {
    lockThreads[i].join();
  }
}

測試結果可以看出，如果連續多個讀鎖會併發執行。為了方便檢視，我添加了一些橫線分隔。

30:31.317 [Thread-1 INFO] w-lock-0000000000 get write lock : true
30:32.824 [Thread-1 INFO] w-lock-0000000000 release write lock
------------------------------------------------------------------
30:32.834 [Thread-17 INFO] r-lock-0000000004 get read lock : true
30:32.835 [Thread-19 INFO] r-lock-0000000002 get read lock : true
30:32.835 [Thread-20 INFO] r-lock-0000000001 get read lock : true
30:32.836 [Thread-18 INFO] r-lock-0000000003 get read lock : true
30:34.135 [Thread-20 INFO] r-lock-0000000001 release read lock
30:34.634 [Thread-17 INFO] r-lock-0000000004 release read lock
30:34.935 [Thread-19 INFO] r-lock-0000000002 release read lock
30:35.036 [Thread-18 INFO] r-lock-0000000003 release read lock
------------------------------------------------------------------
30:35.053 [Thread-16 INFO] w-lock-0000000005 get write lock : true
30:36.154 [Thread-16 INFO] w-lock-0000000005 release write lock
------------------------------------------------------------------
30:36.160 [Thread-14 INFO] r-lock-0000000007 get read lock : true
30:36.160 [Thread-15 INFO] r-lock-0000000006 get read lock : true
30:38.160 [Thread-14 INFO] r-lock-0000000007 release read lock
30:38.661 [Thread-15 INFO] r-lock-0000000006 release read lock
------------------------------------------------------------------
30:38.669 [Thread-13 INFO] w-lock-0000000008 get write lock : true
30:39.969 [Thread-13 INFO] w-lock-0000000008 release write lock
------------------------------------------------------------------
30:39.976 [Thread-12 INFO] r-lock-0000000009 get read lock : true
30:39.977 [Thread-8 INFO] r-lock-0000000014 get read lock : true
30:39.977 [Thread-6 INFO] r-lock-0000000015 get read lock : true
30:39.984 [Thread-10 INFO] r-lock-0000000011 get read lock : true
30:39.985 [Thread-3 INFO] r-lock-0000000018 get read lock : true
30:39.984 [Thread-7 INFO] r-lock-0000000013 get read lock : true
30:39.984 [Thread-11 INFO] r-lock-0000000010 get read lock : true
30:39.983 [Thread-9 INFO] r-lock-0000000012 get read lock : true
30:39.983 [Thread-2 INFO] r-lock-0000000019 get read lock : true
30:39.982 [Thread-5 INFO] r-lock-0000000016 get read lock : true
30:39.986 [Thread-4 INFO] r-lock-0000000017 get read lock : true
30:40.986 [Thread-3 INFO] r-lock-0000000018 release read lock
30:41.086 [Thread-2 INFO] r-lock-0000000019 release read lock
30:41.285 [Thread-6 INFO] r-lock-0000000015 release read lock
30:41.576 [Thread-12 INFO] r-lock-0000000009 release read lock
30:42.185 [Thread-10 INFO] r-lock-0000000011 release read lock
30:42.186 [Thread-5 INFO] r-lock-0000000016 release read lock
30:42.187 [Thread-11 INFO] r-lock-0000000010 release read lock
30:42.286 [Thread-9 INFO] r-lock-0000000012 release read lock
30:42.586 [Thread-7 INFO] r-lock-0000000013 release read lock
30:42.677 [Thread-8 INFO] r-lock-0000000014 release read lock
30:42.887 [Thread-4 INFO] r-lock-0000000017 release read lock

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