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/*
 * 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
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * 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.hive.ql.stats;

import org.apache.hadoop.hive.common.classification.InterfaceAudience;
import org.apache.hadoop.hive.common.classification.InterfaceStability;

/**
 * An interface for any possible implementation for gathering statistics.
 */
@InterfaceAudience.Public
@InterfaceStability.Stable
public interface StatsAggregator {

  /**
   * This method connects to the temporary storage.
   *
   * @param hconf
   *          HiveConf that contains the connection parameters.
   * @param sourceTask
   * @return true if connection is successful, false otherwise.
   */
  public boolean connect(StatsCollectionContext scc);

  /**
   * This method aggregates a given statistic from all tasks (partial stats).
   * After aggregation, this method also automatically removes all records
   * that have been aggregated.
   *
   * @param keyPrefix
   *          a prefix of the keys used in StatsPublisher to publish stats.
   *          Any rows that starts with the same prefix will be aggregated. For example, if
   *          the StatsPublisher uses the following compound key to publish stats:
   *
   *          the output directory name (unique per FileSinkOperator) +
   *          the partition specs (only for dynamic partitions) +
   *          taskID (last component of task file)
   *
   *          The keyPrefix for aggregation could be first 2 components. This will aggregates stats
   *          across all tasks for each partition.
   *
   * @param statType
   *          a string noting the key to be published. Ex: "numRows".
   * @return a string representation of a long value, null if there are any error/exception.
   */
  public String aggregateStats(String keyPrefix, String statType);

  /**
   * This method closes the connection to the temporary storage.
   *
   * @return true if close connection is successful, false otherwise.
   */
  public boolean closeConnection(StatsCollectionContext scc);
}