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.List;
023
024 import org.apache.hadoop.classification.InterfaceAudience;
025 import org.apache.hadoop.classification.InterfaceStability;
026 import org.apache.hadoop.fs.FileSystem;
027 import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
028
029 /**
030 * <code>InputFormat</code> describes the input-specification for a
031 * Map-Reduce job.
032 *
033 * <p>The Map-Reduce framework relies on the <code>InputFormat</code> of the
034 * job to:<p>
035 * <ol>
036 * <li>
037 * Validate the input-specification of the job.
038 * <li>
039 * Split-up the input file(s) into logical {@link InputSplit}s, each of
040 * which is then assigned to an individual {@link Mapper}.
041 * </li>
042 * <li>
043 * Provide the {@link RecordReader} implementation to be used to glean
044 * input records from the logical <code>InputSplit</code> for processing by
045 * the {@link Mapper}.
046 * </li>
047 * </ol>
048 *
049 * <p>The default behavior of file-based {@link InputFormat}s, typically
050 * sub-classes of {@link FileInputFormat}, is to split the
051 * input into <i>logical</i> {@link InputSplit}s based on the total size, in
052 * bytes, of the input files. However, the {@link FileSystem} blocksize of
053 * the input files is treated as an upper bound for input splits. A lower bound
054 * on the split size can be set via
055 * <a href="{@docRoot}/../mapred-default.html#mapreduce.input.fileinputformat.split.minsize">
056 * mapreduce.input.fileinputformat.split.minsize</a>.</p>
057 *
058 * <p>Clearly, logical splits based on input-size is insufficient for many
059 * applications since record boundaries are to respected. In such cases, the
060 * application has to also implement a {@link RecordReader} on whom lies the
061 * responsibility to respect record-boundaries and present a record-oriented
062 * view of the logical <code>InputSplit</code> to the individual task.
063 *
064 * @see InputSplit
065 * @see RecordReader
066 * @see FileInputFormat
067 */
068 @InterfaceAudience.Public
069 @InterfaceStability.Stable
070 public abstract class InputFormat<K, V> {
071
072 /**
073 * Logically split the set of input files for the job.
074 *
075 * <p>Each {@link InputSplit} is then assigned to an individual {@link Mapper}
076 * for processing.</p>
077 *
078 * <p><i>Note</i>: The split is a <i>logical</i> split of the inputs and the
079 * input files are not physically split into chunks. For e.g. a split could
080 * be <i><input-file-path, start, offset></i> tuple. The InputFormat
081 * also creates the {@link RecordReader} to read the {@link InputSplit}.
082 *
083 * @param context job configuration.
084 * @return an array of {@link InputSplit}s for the job.
085 */
086 public abstract
087 List<InputSplit> getSplits(JobContext context
088 ) throws IOException, InterruptedException;
089
090 /**
091 * Create a record reader for a given split. The framework will call
092 * {@link RecordReader#initialize(InputSplit, TaskAttemptContext)} before
093 * the split is used.
094 * @param split the split to be read
095 * @param context the information about the task
096 * @return a new record reader
097 * @throws IOException
098 * @throws InterruptedException
099 */
100 public abstract
101 RecordReader<K,V> createRecordReader(InputSplit split,
102 TaskAttemptContext context
103 ) throws IOException,
104 InterruptedException;
105
106 }
107