NLineInputFormat - About NLineInputFormat, uses, and a sample program

00-NLineInputFormat

**********************
Gist
**********************

A common interview question for a Hadoop developer position is whether we can control the number of 
mappers for a job.  We can - there are a few ways of controlling the number of mappers, as needed. 
Using NLineInputFormat is one way.

About NLineInputFormat
----------------------
With this functionality, you can specify exactly how many lines should go to a mapper.
E.g. If your file has 500 lines, and you set number of lines per mapper to 10, you have 50 mappers 
(instead of one - assuming the file is smaller than a HDFS block size).

When would you use NLineInputFormat?
------------------------------------
Some examples from Hadoop the definitive guide-
1. In applications that take a small amount of input data and run an extensive (that is, CPU-intensive) 
computation for it, then emit their output.
2. Another example...you create a “seed” input file that lists the data sources, one per line. Then 
each mapper is allocated a single data source, and it loads the data from that source into HDFS.

Sample program
---------------
The sample program below demonstrates the functionality.
The mapper merely emits the input key-value pairs.
The input is a file with ~224,000 records.
The output is files containing 10,000 records each (so a total of 23 files).

01-DataAndCodeDownload

*******************************
*Data and code download
*******************************
 
Data and code:
--------------
 
gitHub:
<<To be added>>
 
Email me at airawat.blog@gmail.com if you encounter any issues
 
 
Directory structure
-------------------
 
  formatProject
        data
            employees_tsv
                employees_tsv
 
        formatNLineInputFormat
            src
                NLineInputFormat.java  //Original Apache source code
                MapperNLineInputFormat.java //Mapper
                DriverNLineInputFormat.java //Driver
          
            jar
                formatNLineInputFormat.jar

02-DataReview

*******************************
*Sample Data
*******************************
EmpID DOB         FName     LName     Gender  Hire date     DeptID
10003 1959-12-03  Parto     Bamford   M       1986-08-28    d004
10004 1954-05-01  Chirstian Koblick   M       1986-12-01    d004
10005 1955-01-21  Kyoichi   Maliniak  M       1989-09-12    d003
.....

03-Mapper

/*******************************************************************
* Mapper
* MapperNLineInputFormat.java
*******************************************************************/

import java.io.IOException;

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

public class MapperNLineInputFormat extends
  	Mapper<LongWritable, Text, LongWritable, Text> {

	@Override
	public void map(LongWritable key, Text value, Context context)
			throws IOException, InterruptedException {

		context.write(key, value);
	}

}

04-Driver

/*******************************************************************
* Driver
* DriverNLineInputFormat.java
*******************************************************************/

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.lib.output.LazyOutputFormat;
import org.apache.hadoop.mapreduce.lib.output.TextOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;

public class DriverNLineInputFormat extends Configured implements Tool {

  @Override
	public int run(String[] args) throws Exception {

		if (args.length != 2) {
			System.out
					.printf("Two parameters are required for DriverNLineInputFormat- <input dir> <output dir>\n");
			return -1;
		}

		Job job = new Job(getConf());
		job.setJobName("NLineInputFormat example");
		job.setJarByClass(DriverNLineInputFormat.class);

		job.setInputFormatClass(NLineInputFormat.class);
		NLineInputFormat.addInputPath(job, new Path(args[0]));
		job.getConfiguration().setInt(
				"mapreduce.input.lineinputformat.linespermap", 10000);

		LazyOutputFormat.setOutputFormatClass(job, TextOutputFormat.class);
		FileOutputFormat.setOutputPath(job, new Path(args[1]));

		job.setMapperClass(MapperNLineInputFormat.class);
		job.setNumReduceTasks(0);

		boolean success = job.waitForCompletion(true);
		return success ? 0 : 1;
	}

	public static void main(String[] args) throws Exception {
		int exitCode = ToolRunner.run(new Configuration(),
				new DriverNLineInputFormat(), args);
		System.exit(exitCode);
	}
}

05-CommandToLoadData

***********************************************
** Commands to load data
***********************************************

$ hadoop fs -mkdir formatProject
$ hadoop fs -put formatProject/data formatProject/

06-CommandToRunProgram

***********************************************
** Commands to run the program
***********************************************

hadoop jar ~/Blog/formatProject/formatNLineInputFormat/jar/formatNLineInputFormat.jar DriverNLineInputFormat /user/akhanolk/sortProject/data/employees/employees_tsv /user/akhanolk/formatProject/data/output-formatNLineInputFormat

07-Results

***********************************************
** Results
***********************************************

$ for filename in `hadoop fs -ls -R formatProject/data/output-formatNLineInputFormat/part* | awk '{print $8}'`  
do
    echo "Filename:  " $filename "	[Record count:" 	`hadoop fs -cat $filename | wc -l` "]"
done

Filename: formatProject/data/output-formatNLineInputFormat/part-m-00000 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00001 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00002 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00003 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00004 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00005 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00006 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00007 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00008 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00009 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00010 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00011 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00012 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00013 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00014 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00015 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00016 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00017 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00018 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00019 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00020 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00021 [Record count: 10000 ]
Filename: formatProject/data/output-formatNLineInputFormat/part-m-00022 [Record count: 4683 ]

$ hadoop fs -cat formatProject/data/output-formatNLineInputFormat/part-* | wc -l
224683

$ hadoop fs -cat formatProject/data/output-formatNLineInputFormat/part-m-00022
...
11474355  499977	1956-06-05	Martial	Weisert	F	1996-09-17	d002
11474407	499979	1962-10-29	Prasadram	Waleschkowski	M	1994-01-04	d005
11474467	499980	1959-06-28	Gino	Usery	M	1991-02-11	d007
..


 

08-NLineInputFormat.java

/******************************************************
 * NLineInputFormat.java
 * Had to add this to the project, as the version of
 * Hadoop I have does not include the NLineInputFormat
 * functionality as part of the new API
 *****************************************************/
 
 /**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

//import org.apache.hadoop.classification.InterfaceAudience;
//import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FSDataInputStream;
import org.apache.hadoop.fs.FileStatus;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.InputSplit;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.JobContext;
import org.apache.hadoop.mapreduce.RecordReader;
import org.apache.hadoop.mapreduce.TaskAttemptContext;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.input.FileSplit;
import org.apache.hadoop.mapreduce.lib.input.LineRecordReader;
import org.apache.hadoop.util.LineReader;

/**
 * NLineInputFormat which splits N lines of input as one split.
 * 
 * In many "pleasantly" parallel applications, each process/mapper processes the
 * same input file (s), but with computations are controlled by different
 * parameters.(Referred to as "parameter sweeps"). One way to achieve this, is
 * to specify a set of parameters (one set per line) as input in a control file
 * (which is the input path to the map-reduce application, where as the input
 * dataset is specified via a config variable in JobConf.).
 * 
 * The NLineInputFormat can be used in such applications, that splits the input
 * file such that by default, one line is fed as a value to one map task, and
 * key is the offset. i.e. (k,v) is (LongWritable, Text). The location hints
 * will span the whole mapred cluster.
 */
// @InterfaceAudience.Public
// @InterfaceStability.Stable
public class NLineInputFormat extends FileInputFormat<LongWritable, Text> {
  public static final String LINES_PER_MAP = "mapreduce.input.lineinputformat.linespermap";

	public RecordReader<LongWritable, Text> createRecordReader(
			InputSplit genericSplit, TaskAttemptContext context)
			throws IOException {
		context.setStatus(genericSplit.toString());
		return new LineRecordReader();
	}

	/**
	 * Logically splits the set of input files for the job, splits N lines of
	 * the input as one split.
	 * 
	 * @see FileInputFormat#getSplits(JobContext)
	 */
	public List<InputSplit> getSplits(JobContext job) throws IOException {
		List<InputSplit> splits = new ArrayList<InputSplit>();
		int numLinesPerSplit = getNumLinesPerSplit(job);
		for (FileStatus status : listStatus(job)) {
			splits.addAll(getSplitsForFile(status, job.getConfiguration(),
					numLinesPerSplit));
		}
		return splits;
	}

	public static List<FileSplit> getSplitsForFile(FileStatus status,
			Configuration conf, int numLinesPerSplit) throws IOException {
		List<FileSplit> splits = new ArrayList<FileSplit>();
		Path fileName = status.getPath();
		if (status.isDir()) {
			throw new IOException("Not a file: " + fileName);
		}
		FileSystem fs = fileName.getFileSystem(conf);
		LineReader lr = null;
		try {
			FSDataInputStream in = fs.open(fileName);
			lr = new LineReader(in, conf);
			Text line = new Text();
			int numLines = 0;
			long begin = 0;
			long length = 0;
			int num = -1;
			while ((num = lr.readLine(line)) > 0) {
				numLines++;
				length += num;
				if (numLines == numLinesPerSplit) {
					// NLineInputFormat uses LineRecordReader, which always
					// reads
					// (and consumes) at least one character out of its upper
					// split
					// boundary. So to make sure that each mapper gets N lines,
					// we
					// move back the upper split limits of each split
					// by one character here.
					if (begin == 0) {
						splits.add(new FileSplit(fileName, begin, length - 1,
								new String[] {}));
					} else {
						splits.add(new FileSplit(fileName, begin - 1, length,
								new String[] {}));
					}
					begin += length;
					length = 0;
					numLines = 0;
				}
			}
			if (numLines != 0) {
				splits.add(new FileSplit(fileName, begin, length,
						new String[] {}));
			}
		} finally {
			if (lr != null) {
				lr.close();
			}
		}
		return splits;
	}

	/**
	 * Set the number of lines per split
	 * 
	 * @param job
	 *            the job to modify
	 * @param numLines
	 *            the number of lines per split
	 */
	public static void setNumLinesPerSplit(Job job, int numLines) {
		job.getConfiguration().setInt(LINES_PER_MAP, numLines);
	}

	/**
	 * Get the number of lines per split
	 * 
	 * @param job
	 *            the job
	 * @return the number of lines per split
	 */
	public static int getNumLinesPerSplit(JobContext job) {
		return job.getConfiguration().getInt(LINES_PER_MAP, 1);
	}
}

My notes - please disregard

scp /Users/akhanolkar/Documents/hadoop-jars/formatNLineInputFormat.jar akhanolk@cdh-dev01:~/Blog/formatProject/formatNLineInputFormat/jar/

hadoop fs -rm -R formatProject/data/output-formatNLineInputFormat

hadoop jar ~/Blog/formatProject/formatNLineInputFormat/jar/formatNLineInputFormat.jar DriverNLineInputFormat /user/akhanolk/sortProject/data/employees/employees_tsv /user/akhanolk/formatProject/data/output-formatNLineInputFormat