public List<TaskCallbackImpl> updateCallback(TaskCallbackImpl callback) {
    if (maxConcurrency > 1) {
      // Use the completedCallbacks queue to handle the out-of-order case when max concurrency is larger than 1
      return completedCallbacks.update(callback);
    } else {
      return ImmutableList.of(callback);
    }
  }
Update the task callbacks with the new callback completed.
It uses a high-watermark model to roll the callbacks for checkpointing.
@param callback new completed callback
@return the list of callbacks for checkpointing
samza-core/src/main/java/org/apache/samza/task/TaskCallbackManager.java