HBase行锁原理及实现

  hbase mutation操作,比如delete put等,都需要先获取行锁,然后再进行操作,在获取行锁时,是通过HRegion.getRowLockInternal(byte[] row, boolean waitForLock)进行的,因此,我们先大体浏览一下这个方法的流程,如下。可以看到,该方法中主要涉及到行锁相关的内容为RowLock和RowLockContext两个类。这两个都是HRegion的内部类,下面详细看一下这两个类是咋实现的。

protected RowLock getRowLockInternal(byte[] row, boolean waitForLock) throws IOException {
    HashedBytes rowKey = new HashedBytes(row);
    RowLockContext rowLockContext = new RowLockContext(rowKey);

    // loop until we acquire the row lock (unless !waitForLock)
    while (true) {
      RowLockContext existingContext = lockedRows.putIfAbsent(rowKey, rowLockContext);
      if (existingContext == null) {
        // Row is not already locked by any thread, use newly created context.
        break;
      } else if (existingContext.ownedByCurrentThread()) {
        // Row is already locked by current thread, reuse existing context instead.
        rowLockContext = existingContext;
        break;
      } else {
        if (!waitForLock) {
          return null;
        }
        try {
          // Row is already locked by some other thread, give up or wait for it
          if (!existingContext.latch.await(this.rowLockWaitDuration, TimeUnit.MILLISECONDS)) {
            throw new IOException("Timed out waiting for lock for row: " + rowKey);
          }
        } catch (InterruptedException ie) {
          LOG.warn("Thread interrupted waiting for lock on row: " + rowKey);
          InterruptedIOException iie = new InterruptedIOException();
          iie.initCause(ie);
          throw iie;
        }
      }
    }

    // allocate new lock for this thread
    return rowLockContext.newLock();
  }

  首先看RowLock类,该类主要逻辑是release方法,是用来释放行锁的。同时有一个布尔类型参数release,默认为false,代表该行锁是否被释放掉了。

public static class RowLock {
    @VisibleForTesting final RowLockContext context;
    private boolean released = false;

    @VisibleForTesting RowLock(RowLockContext context) {
      this.context = context;
    }

    /**
     * Release the given lock.  If there are no remaining locks held by the current thread
     * then unlock the row and allow other threads to acquire the lock.
     * @throws IllegalArgumentException if called by a different thread than the lock owning thread
     */
    public void release() {
      if (!released) {
        context.releaseLock();
        released = true;
      }
    }
  }

  但是在RowLock中,并没有看到实际涉及到锁的信息,这是咋回事呢,别急,细细看下release方法,里面有一个context,是RowLockContext类型。同时其构造方法中也传了一个context对象,因此怀疑是在RowLockContext中new出了一个rowlock,进RowLockContext中看下:

@VisibleForTesting class RowLockContext {
    private final HashedBytes row;
  //通过计数以及CountDownLatch实现对行锁的condition。这里之所以将countdownlatch设置为一,是因为hbase自己也不知道到底有多少condition来竞争锁,所以加一个计数lockCount,
  //当lockCount为零时,再把latch.coutDown。否则会在getRowLockInternal中await。
private final CountDownLatch latch = new CountDownLatch(1); private final Thread thread; private int lockCount = 0; RowLockContext(HashedBytes row) { this.row = row; this.thread = Thread.currentThread(); } boolean ownedByCurrentThread() { return thread == Thread.currentThread(); } RowLock newLock() { lockCount++; return new RowLock(this); } void releaseLock() { if (!ownedByCurrentThread()) { throw new IllegalArgumentException("Lock held by thread: " + thread + " cannot be released by different thread: " + Thread.currentThread()); } lockCount--; if (lockCount == 0) { // no remaining locks by the thread, unlock and allow other threads to access RowLockContext existingContext = lockedRows.remove(row); if (existingContext != this) { throw new RuntimeException( "Internal row lock state inconsistent, should not happen, row: " + row); } latch.countDown(); } } }

  通过计数以及CountDownLatch实现对行锁的condition。这里之所以将countdownlatch设置为一,是因为hbase自己也不知道到底有多少condition来竞争锁,所以加一个计数lockCount,
当lockCount为零时,再把latch.coutDown。否则会在getRowLockInternal中await。
  在HRegion中还有一个关键的成员变量: lockedrows,用来存储当前已经获取了行锁的所有行信息,key为rowkey,value为RowLockContext。
// map from a locked row to the context for that lock including:
  // - CountDownLatch for threads waiting on that row
  // - the thread that owns the lock (allow reentrancy)
  // - reference count of (reentrant) locks held by the thread
  // - the row itself
  private final ConcurrentHashMap<HashedBytes, RowLockContext> lockedRows =
      new ConcurrentHashMap<HashedBytes, RowLockContext>();
  好啦,行锁涉及到的内容,我们都大体浏览了,再从getRowLockInternal中开始通一遍逻辑:
  1. 根据rowkey构建RowLockContext对象
  2. while循环,直到获取到行锁,或者wait超时
    1. 首先判断lockedrows中是否有该rowkey的行锁信息,此处利用的是concurrentMap的putIfAbsent
      1. 如果不存在,以为着这行锁还没有其他线程拿到,将行锁信息加入到lockedrows中,直接break跳出循环,然后now一个行锁。
      2. 如果存在,则以为着该行锁已经被占有了,逻辑如下
        1. 判断持有该行锁的线程是否是自己本身,如果是,则直接覆盖rowLockContext,跳出循环
        2. 判断是否需要wait 行锁,通过参数waitForLock,如果不wait直接return;如果wait,则调用latch.await等待,如果超时则抛出异常。
    2. 如果跳出了循环,则意味着获取成功,则newLock并返回。

   上面是获取行锁的流程,释放行锁呢,是通过HRegion的releaseRowLocks方式实现,我们看下代码:

 /**
   * If the given list of row locks is not null, releases all locks.
   */
  public void releaseRowLocks(List<RowLock> rowLocks) {
    if (rowLocks != null) {
      for (RowLock rowLock : rowLocks) {
        rowLock.release();
      }
      rowLocks.clear();
    }
  }

  可见是调用RowLock.release实现,该方法代码在上面有,具体的逻辑如下:

      在lockedrows中将该行锁删除。

      判断release是否为false,如果为false,则调用context.releaseLock,context.releaseLock逻辑如下

        首先判断释放该行锁的线程是否是该行锁的持有者,若不是则抛出异常

        将count--;

        如果count==0了,则直接调用latch.countDown,这个方法会触发其他线程去获取行锁。当count==0了也就是说该线程已经不需要改行锁,已经释放

        将release设置为true。

注意

这里在getRowLockInternal中,只要lockedRows.putIfAbsent(rowKey, rowLockContext)成功,其他线程将不会获取成功,由concurrentMap保证。

posted @ 2019-08-08 18:36  Evil_XJZ  阅读(...)  评论(...编辑  收藏