Flume 的失败选择机器和BACKOFF的机制

在OrderSelector.java中

默认的是round_robin，即轮询。

backoff 算法是指数回退，代码如下：

 public void informFailure(T failedObject) {
    //If there is no backoff this method is a no-op.
    if (!shouldBackOff) {
      return;
    }
    FailureState state = stateMap.get(failedObject);
    long now = System.currentTimeMillis();
    long delta = now - state.lastFail;

    /*
     * When do we increase the backoff period?
     * We basically calculate the time difference between the last failure
     * and the current one. If this failure happened within one hour of the
     * last backoff period getting over, then we increase the timeout,
     * since the object did not recover yet. Else we assume this is a fresh
     * failure and reset the count.
     */
    long lastBackoffLength = Math.min(maxTimeout, 1000 * (1 << state.sequentialFails));
    long allowableDiff = lastBackoffLength + CONSIDER_SEQUENTIAL_RANGE;
    if (allowableDiff > delta) {
      if (state.sequentialFails < EXP_BACKOFF_COUNTER_LIMIT) {
        state.sequentialFails++;
      }
    } else {
      state.sequentialFails = 1;
    }
    state.lastFail = now;
    //Depending on the number of sequential failures this component had, delay
    //its restore time. Each time it fails, delay the restore by 1000 ms,
    //until the maxTimeOut is reached.
    state.restoreTime = now + Math.min(maxTimeout, 1000 * (1 << state.sequentialFails));
  }

失败一次后，restoreTime就会左移，直到和maxTimeout.

 

/**
   *
   * @return - List of indices currently active objects
   */
  protected List<Integer> getIndexList() {
    long now = System.currentTimeMillis();

    List<Integer> indexList = new ArrayList<Integer>();

    int i = 0;
    for (T obj : stateMap.keySet()) {
      if (!isShouldBackOff() || stateMap.get(obj).restoreTime < now) {
        indexList.add(i);
      }
      i++;
    }
    return indexList;
  }

返回可用的机器

RoundRobinOrderSelector.java

public class RoundRobinOrderSelector<T> extends OrderSelector<T> {

  private int nextHead = 0;

  public RoundRobinOrderSelector(boolean shouldBackOff) {
    super(shouldBackOff);
  }

  @Override
  public Iterator<T> createIterator() {
    List<Integer> activeIndices = getIndexList();
    int size = activeIndices.size();
    // possible that the size has shrunk so gotta adjust nextHead for that
    if (nextHead >= size) {
      nextHead = 0;
    }
    int begin = nextHead++;
    if (nextHead == activeIndices.size()) {
      nextHead = 0;
    }

    int[] indexOrder = new int[size];

    for (int i = 0; i < size; i++) {
      indexOrder[i] = activeIndices.get((begin + i) % size);
    }

    return new SpecificOrderIterator<T>(indexOrder, getObjects());
  }
}