Flume學習——BasicChannelSemantics

devos發表於2013-11-24
public class MemoryChannel extends BasicChannelSemantics

public abstract class BasicChannelSemantics extends AbstractChannel

public abstract class AbstractChannel implements Channel, LifecycleAware, Configurable

AbstractChannel只在實現了LifecycleAware,NamedComponent和Configurable中的一些介面，除了對部分方法進行了同步，沒什麼特殊。

BasicChannelSemantics是實現事務機制的關鍵之一

 * <p>
 * An implementation of basic {@link Channel} semantics, including the
 * implied thread-local semantics of the {@link Transaction} class,
 * which is required to extend {@link BasicTransactionSemantics}.
 * </p>
 */

BasicChannelSemantics實現了基礎的Channel語法，包括了Transaction的thread-local語法。

首先BasicChannelSemantics類持有一個ThreadLocal物件，它維護了一個BasicTransactionSemantics物件。BasicTransationSementics是一個abstract class,提供了Transaction介面的基礎實現。

這些每個執行緒都包括了一個唯一的Transaction物件，保證了事務的隔離性。

  private ThreadLocal<BasicTransactionSemantics> currentTransaction
      = new ThreadLocal<BasicTransactionSemantics>();

線上程中獲取當前執行緒中的事務通過getTransaction方法，它會呼叫BasicChannelSemantics中定義的的抽象方法createTransaction()來獲取BasicTransactionSemantics的例項。

/**
   * <p>
   * Initializes the channel if it is not already, then checks to see
   * if there is an open transaction for this thread, creating a new
   * one via <code>createTransaction</code> if not.
   * @return the current <code>Transaction</code> object for the
   *     calling thread
   * </p>
   */
  @Override
  public Transaction getTransaction() {

    if (!initialized) {
      synchronized (this) {
        if (!initialized) {
          initialize();
          initialized = true;
        }
      }
    }

    BasicTransactionSemantics transaction = currentTransaction.get();
    if (transaction == null || transaction.getState().equals(
            BasicTransactionSemantics.State.CLOSED)) {
      transaction = createTransaction();
      currentTransaction.set(transaction);
    }
    return transaction;
  }

首先，Channel使用了延遲初始化的機制。只有在Channel第一次被呼叫getTransaction()時，它的initialize()方法才被呼叫。

當currentTransaction中並不包含一個BasicTransactionSemantics物件的時候，或者當前的transaction物件已經處理CLOSED狀態的時候，它就呼叫createTransaction方法來獲取一個Transaction,並設定給thread-local的currentTransaction。

至此，Transaction的thread-local機制已實現。

那麼Channel是如何利用thread-local的Transaction物件來實現訊息存取的事務呢?它只是確保事務已開啟，然後將訊息的存取功能代理給本執行緒的transaction物件。


  @Override
  public void put(Event event) throws ChannelException {
    BasicTransactionSemantics transaction = currentTransaction.get();
    Preconditions.checkState(transaction != null,
        "No transaction exists for this thread");
    transaction.put(event);
  }

  @Override
  public Event take() throws ChannelException {
    BasicTransactionSemantics transaction = currentTransaction.get();
    Preconditions.checkState(transaction != null,
        "No transaction exists for this thread");
    return transaction.take();
  }

 

 

 

 

 

 

 

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