前言

在分布式系统中，为了保持数据操作的一致性，我们会看到锁在里面会有广泛的应用。简单一点的，我们可能就用一个简单的对象锁来做线程安全的同步。再细粒度一些的，我们会用到读写锁，然后对具体操作的属性（Read Or Write）在进行读写锁的分离控制。但今天笔者想对此话题再展开细致的探讨，相比较于使用单一锁模式，我们还是可以做出更进一步的改善的。

单一锁控制的弊端

如前文所提到的，不管说是普通的object对象锁还是说ReadWriteLock来说，它们本质上还是Single Lock（单一锁）。这种lock在客户端请求巨大的情况下会有严重的锁竞争问题，一个典型的例子是HDFS NN里的FSN锁。一旦某个请求获得了这个锁，那么其它的请求就会被block住了。所以说单一锁的问题在于，它的粒度有些时候还是太粗了。所以这里就涉及到一个锁粒度的问题了。

单一锁的优化：锁粒度的细化

那么怎么取做锁粒度的细化呢？读写锁的分离是一种常见的手段。但更好的一种做法是在实际资源上进行锁控制。因为我们使用锁去做线程安全同步控制时的初衷，是为了避免同时又多个操作在修改某个共同的资源（比如元数据的更新）。但其实不同的资源的相互更新其实是不影响的，那这时我们可以将大的锁拆分为A资源锁以及B资源锁。

用一个更具象的例子来解释这个问题：比如在分布式存储系统的元数据更新里，我们可以将DB级别的lock控制拆分为以table级别的lock。这里的Resource就从DB细粒度为Table。

锁的管理

如果锁的粒度已经按照对于实际的资源进行控制的话，这时我们可以考虑在上面包装出一个锁的管理器，由此管理器对外“借出”或“归还”锁。在这个管理类里，我们还能对当前的活跃锁做引用计数的更新。如果某个锁在某次unlock后没有再被外部引用了，我们就可以将其从锁管理器中移除出去了。

锁的容量控制

当然我们还需要对锁管理器做一个最大容量的控制，如果当前活跃锁达到容量阈值时，则后面的新锁申请也将会被Block住。这在一定程度上控制的是客户端的concurrency的行为度。

锁的资源优先级问题

这里还有一个隐蔽的问题：锁的资源优先级问题。比如说我们有3个资源，它们的资源范围关系如下：

A，资源最大(包含多个B类资源）
B，资源适中（包含多个C类资源）
C，最小粒度资源

假设我们对上述3类资源做锁控制的话，可以遵循以下原则：

在持有低优先级资源锁的情况下，不能继续再获取其上资源的锁。持有高优先级资源锁的情况下，还可以继续获取其下低一级资源的锁。

这个原则对应上面ABC资源的情况就是，用户在获取A锁资源的情况下，可以继续获取B或C资源的锁，而假如说目前已经获取C锁的情况下，是无法获取B锁和A锁的。

锁管理的demo样例

以下是摘自Hadoop Ozone里的一个锁管理器的实现，里面涉及到了上文提到的一些要点，供大家学习参考。

/** * Licensed to the Apache Software Foundation (ASF) under one or more * contributor license agreements.  See the NOTICE file distributed with this * work for additional information regarding copyright ownership.  The ASF * licenses this file to you under the Apache License, Version 2.0 (the * "License"); you may not use this file except in compliance with the License. * You may obtain a copy of the License at * <p> * http://www.apache.org/licenses/LICENSE-2.0 * <p> * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS,WITHOUT * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the * License for the specific language governing permissions and limitations under * the License. */package org.apache.hadoop.ozone.lock;import org.apache.commons.pool2.impl.GenericObjectPool;import org.apache.hadoop.conf.Configuration;import org.apache.hadoop.hdds.HddsConfigKeys;import org.slf4j.Logger;import org.slf4j.LoggerFactory;import java.util.Map;import java.util.concurrent.ConcurrentHashMap;/** * Manages the locks on a given resource. A new lock is created for each * and every unique resource. Uniqueness of resource depends on the * {@code equals} implementation of it. */public class LockManager<T> {     private static final Logger LOG = LoggerFactory.getLogger(LockManager.class);  private final Map<T, ActiveLock> activeLocks = new ConcurrentHashMap<>();  private final GenericObjectPool<ActiveLock> lockPool =      new GenericObjectPool<>(new PooledLockFactory());  /**   * Creates new LockManager instance.   *   * @param conf Configuration object   */  public LockManager(Configuration conf) {       int maxPoolSize = conf.getInt(HddsConfigKeys.HDDS_LOCK_MAX_CONCURRENCY,        HddsConfigKeys.HDDS_LOCK_MAX_CONCURRENCY_DEFAULT);    lockPool.setMaxTotal(maxPoolSize);  }  /**   * Acquires the lock on given resource.   *   * <p>If the lock is not available then the current thread becomes   * disabled for thread scheduling purposes and lies dormant until the   * lock has been acquired.   */  public void lock(T resource) {       activeLocks.compute(resource, (k, v) -> {         ActiveLock lock;      try {           if (v == null) {             // 锁为空，则在锁对象池中创建并借出          lock = lockPool.borrowObject();        } else {             lock = v;        }        // 增加锁引用计数        lock.incrementActiveCount();      } catch (Exception ex) {           LOG.error("Unable to obtain lock.", ex);        throw new RuntimeException(ex);      }      return lock;      //得到锁后进行锁的lock操作    }).lock();  }  /**   * Releases the lock on given resource.   */  public void unlock(T resource) {       ActiveLock lock = activeLocks.get(resource);    if (lock == null) {         // Someone is releasing a lock which was never acquired. Log and return.      LOG.warn("Trying to release the lock on {}, which was never acquired.",          resource);      return;    }    lock.unlock();    activeLocks.computeIfPresent(resource, (k, v) -> {         v.decrementActiveCount();      if (v.getActiveLockCount() != 0) {           return v;      }      // 如果锁没有被引用了，则在锁对象池中移出此锁      lockPool.returnObject(v);      return null;    });  }}

上面的ActiveLock实质上是一个经过简单包装的ReentrantLock，

/** * Licensed to the Apache Software Foundation (ASF) under one or more * contributor license agreements.  See the NOTICE file distributed with this * work for additional information regarding copyright ownership.  The ASF * licenses this file to you under the Apache License, Version 2.0 (the * "License"); you may not use this file except in compliance with the License. * You may obtain a copy of the License at * <p> * http://www.apache.org/licenses/LICENSE-2.0 * <p> * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS,WITHOUT * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the * License for the specific language governing permissions and limitations under * the License. */package org.apache.hadoop.ozone.lock;import java.util.concurrent.atomic.AtomicInteger;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;/** * Lock implementation which also maintains counter. */public final class ActiveLock {     private Lock lock;  private AtomicInteger count;  /**   * Use ActiveLock#newInstance to create instance.   */  private ActiveLock() {       this.lock = new ReentrantLock();    this.count = new AtomicInteger(0);  }  /**   * Creates a new instance of ActiveLock.   *   * @return new ActiveLock   */  public static ActiveLock newInstance() {       return new ActiveLock();  }  /**   * Acquires the lock.   *   * <p>If the lock is not available then the current thread becomes   * disabled for thread scheduling purposes and lies dormant until the   * lock has been acquired.   */  public void lock() {       lock.lock();  }  /**   * Releases the lock.   */  public void unlock() {       lock.unlock();  }  /**   * Increment the active count of the lock.   */  void incrementActiveCount() {       count.incrementAndGet();  }  /**   * Decrement the active count of the lock.   */  void decrementActiveCount() {       count.decrementAndGet();  }  /**   * Returns the active count on the lock.   *   * @return Number of active leases on the lock.   */  int getActiveLockCount() {       return count.get();  }  /**   * Resets the active count on the lock.   */  void resetCounter() {       count.set(0);  }  @Override  public String toString() {       return lock.toString();  }}

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