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    
019    package org.apache.hadoop.mapreduce;
020    
021    import java.io.IOException;
022    import java.util.Iterator;
023    
024    import org.apache.hadoop.classification.InterfaceAudience;
025    import org.apache.hadoop.classification.InterfaceStability;
026    
027    /**
028     * The context passed to the {@link Reducer}.
029     * @param <KEYIN> the class of the input keys
030     * @param <VALUEIN> the class of the input values
031     * @param <KEYOUT> the class of the output keys
032     * @param <VALUEOUT> the class of the output values
033     */
034    @InterfaceAudience.Public
035    @InterfaceStability.Evolving
036    public interface ReduceContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT>
037        extends TaskInputOutputContext<KEYIN,VALUEIN,KEYOUT,VALUEOUT> {
038    
039      /** Start processing next unique key. */
040      public boolean nextKey() throws IOException,InterruptedException;
041    
042      /**
043       * Iterate through the values for the current key, reusing the same value 
044       * object, which is stored in the context.
045       * @return the series of values associated with the current key. All of the 
046       * objects returned directly and indirectly from this method are reused.
047       */
048      public Iterable<VALUEIN> getValues() throws IOException, InterruptedException;
049    
050      /**
051       * {@link Iterator} to iterate over values for a given group of records.
052       */
053      interface ValueIterator<VALUEIN> extends MarkableIteratorInterface<VALUEIN> {
054    
055        /**
056         * This method is called when the reducer moves from one key to 
057         * another.
058         * @throws IOException
059         */
060        void resetBackupStore() throws IOException;
061      }
062    }