001/**
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements.  See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership.  The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License.  You may obtain a copy of the License at
009 *
010 *     http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing, software
013 * distributed under the License is distributed on an "AS IS" BASIS,
014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015 * See the License for the specific language governing permissions and
016 * limitations under the License.
017 */
018
019package org.apache.hadoop.mapred.lib.aggregate;
020
021import java.io.IOException;
022import java.util.Iterator;
023
024import org.apache.hadoop.classification.InterfaceAudience;
025import org.apache.hadoop.classification.InterfaceStability;
026import org.apache.hadoop.io.Text;
027import org.apache.hadoop.io.Writable;
028import org.apache.hadoop.io.WritableComparable;
029import org.apache.hadoop.mapred.OutputCollector;
030import org.apache.hadoop.mapred.Reporter;
031
032/**
033 * This class implements the generic reducer of Aggregate.
034 */
035@InterfaceAudience.Public
036@InterfaceStability.Stable
037public class ValueAggregatorReducer<K1 extends WritableComparable,
038                                    V1 extends Writable>
039  extends ValueAggregatorJobBase<K1, V1> {
040
041  /**
042   * @param key
043   *          the key is expected to be a Text object, whose prefix indicates
044   *          the type of aggregation to aggregate the values. In effect, data
045   *          driven computing is achieved. It is assumed that each aggregator's
046   *          getReport method emits appropriate output for the aggregator. This
047   *          may be further customiized.
048   * @param values
049   *          the values to be aggregated
050   */
051  public void reduce(Text key, Iterator<Text> values,
052                     OutputCollector<Text, Text> output, Reporter reporter) throws IOException {
053    String keyStr = key.toString();
054    int pos = keyStr.indexOf(ValueAggregatorDescriptor.TYPE_SEPARATOR);
055    String type = keyStr.substring(0, pos);
056    keyStr = keyStr.substring(pos
057                              + ValueAggregatorDescriptor.TYPE_SEPARATOR.length());
058
059    ValueAggregator aggregator = ValueAggregatorBaseDescriptor
060      .generateValueAggregator(type);
061    while (values.hasNext()) {
062      aggregator.addNextValue(values.next());
063    }
064
065    String val = aggregator.getReport();
066    key = new Text(keyStr);
067    output.collect(key, new Text(val));
068  }
069
070  /**
071   * Do nothing. Should not be called
072   */
073  public void map(K1 arg0, V1 arg1, OutputCollector<Text, Text> arg2,
074                  Reporter arg3) throws IOException {
075    throw new IOException ("should not be called\n");
076  }
077}