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.mapred;
020    
021    import java.io.IOException;
022    
023    import org.apache.hadoop.classification.InterfaceAudience;
024    import org.apache.hadoop.classification.InterfaceStability;
025    
026    /**
027     * <code>RecordReader</code> reads &lt;key, value&gt; pairs from an 
028     * {@link InputSplit}.
029     *   
030     * <p><code>RecordReader</code>, typically, converts the byte-oriented view of 
031     * the input, provided by the <code>InputSplit</code>, and presents a 
032     * record-oriented view for the {@link Mapper} & {@link Reducer} tasks for 
033     * processing. It thus assumes the responsibility of processing record 
034     * boundaries and presenting the tasks with keys and values.</p>
035     * 
036     * @see InputSplit
037     * @see InputFormat
038     */
039    @InterfaceAudience.Public
040    @InterfaceStability.Stable
041    public interface RecordReader<K, V> {
042      /** 
043       * Reads the next key/value pair from the input for processing.
044       *
045       * @param key the key to read data into
046       * @param value the value to read data into
047       * @return true iff a key/value was read, false if at EOF
048       */      
049      boolean next(K key, V value) throws IOException;
050      
051      /**
052       * Create an object of the appropriate type to be used as a key.
053       * 
054       * @return a new key object.
055       */
056      K createKey();
057      
058      /**
059       * Create an object of the appropriate type to be used as a value.
060       * 
061       * @return a new value object.
062       */
063      V createValue();
064    
065      /** 
066       * Returns the current position in the input.
067       * 
068       * @return the current position in the input.
069       * @throws IOException
070       */
071      long getPos() throws IOException;
072    
073      /** 
074       * Close this {@link InputSplit} to future operations.
075       * 
076       * @throws IOException
077       */ 
078      public void close() throws IOException;
079    
080      /**
081       * How much of the input has the {@link RecordReader} consumed i.e.
082       * has been processed by?
083       * 
084       * @return progress from <code>0.0</code> to <code>1.0</code>.
085       * @throws IOException
086       */
087      float getProgress() throws IOException;
088    }