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    import java.io.DataInput;
023    
024    import org.apache.hadoop.classification.InterfaceAudience;
025    import org.apache.hadoop.classification.InterfaceStability;
026    
027    /**
028     * <code>RecordReader</code> reads &lt;key, value&gt; pairs from an 
029     * {@link InputSplit}.
030     *   
031     * <p><code>RecordReader</code>, typically, converts the byte-oriented view of 
032     * the input, provided by the <code>InputSplit</code>, and presents a 
033     * record-oriented view for the {@link Mapper} & {@link Reducer} tasks for 
034     * processing. It thus assumes the responsibility of processing record 
035     * boundaries and presenting the tasks with keys and values.</p>
036     * 
037     * @see InputSplit
038     * @see InputFormat
039     */
040    @InterfaceAudience.Public
041    @InterfaceStability.Stable
042    public interface RecordReader<K, V> {
043      /** 
044       * Reads the next key/value pair from the input for processing.
045       *
046       * @param key the key to read data into
047       * @param value the value to read data into
048       * @return true iff a key/value was read, false if at EOF
049       */      
050      boolean next(K key, V value) throws IOException;
051      
052      /**
053       * Create an object of the appropriate type to be used as a key.
054       * 
055       * @return a new key object.
056       */
057      K createKey();
058      
059      /**
060       * Create an object of the appropriate type to be used as a value.
061       * 
062       * @return a new value object.
063       */
064      V createValue();
065    
066      /** 
067       * Returns the current position in the input.
068       * 
069       * @return the current position in the input.
070       * @throws IOException
071       */
072      long getPos() throws IOException;
073    
074      /** 
075       * Close this {@link InputSplit} to future operations.
076       * 
077       * @throws IOException
078       */ 
079      public void close() throws IOException;
080    
081      /**
082       * How much of the input has the {@link RecordReader} consumed i.e.
083       * has been processed by?
084       * 
085       * @return progress from <code>0.0</code> to <code>1.0</code>.
086       * @throws IOException
087       */
088      float getProgress() throws IOException;
089    }