Apache Hadoop: DFS and Map Reduce

Apache Hadoop
DFS and Map Reduce
Víctor Sánchez Anguix
Universitat Politècnica de València
MSc. In Artificial Intelligence, Pattern Recognition, and Digital
Image
Course 2014/2015

Apache Hadoop: DFS and Map Reduce. MSc. in Artificial Intelligence, Pattern Recognition and Digital Image
Who has not heard
about Hadoop?

Who knows exactly
what is Hadoop?

➢ Being simplistic:
What is Apache Hadoop?
DFS
Map
Reduce

➢ Google publishes paper about GFS (2003).
http://research.google.com/archive/gfs.html
➢ Distributed data among cluster of computers
➢ Fault tolerant
➢ Highly scalable with commodity hardware
A bit of history: Distributed File
System (DFS)

➢ Google publishes paper about MR (2004).
http://research.google.
com/archive/mapreduce.html
➢ Algorithm for processing distributed data in
parallel
➢ Simple in concept, extremely useful in
practice
A bit of history: Map Reduce (MR)

➢ Doug Cutting and
Mike Caffarella →
Apache Nutch
➢ Doug Cutting goes
to Yahoo
➢ Yahoo implements
Apache Hadoop
A bit of history: Hadoop is born

➢ Framework for distributed computing
➢ Still based on DFS and MR
➢ It is the main actor in Big Data
➢ Last major release: Apache Hadoop 2.6.0
(Nov 2014)
http://hadoop.apache.org/
Apache Hadoop now

DFS architecture

Interacting with Hadoop DFS:
creating dirs
➢ Examples:
hdfs dfs -mkdir data
hdfs dfs -mkdir results

uploading files
➢ Examples:
hdfs dfs -put datasets/students.tsv data/students.tsv
hdfs dfs -put datasets/grades.tsv data/grades.tsv

Interacting with Hadoop DFS: listing
➢ Examples:
hdfs dfs -ls data
Found 2 items
-rw-r--r-- 3 sanguix supergroup 450 2015-02-09 10:50 data/grades.tsv
-rw-r--r-- 3 sanguix supergroup 194 2015-02-09 10:45 data/students.tsv

Interacting with Hadoop DFS: get a
file
➢ Examples:
hdfs dfs -get data/students.tsv
hdfs dfs -get data/grades.tsv

deleting files
➢ Examples:
hdfs dfs -rm data/students.tsv
hdfs dfs -rm data/grades.tsv

Interacting with Hadoop DFS: space
use info
➢ Examples:
hdfs dfs -df -h
Filesystem Size Used Available Use%
hdfs://localhost 1.5 T 12 K 491.6 G 0%

Map Reduce: Overview
Input data
Input data
Input data
Map task
Map task
Map task
Reduce
task
Reduce
task
Reduce
task
Output data
Output data
Output data
chunk of data (key,value) value’

Map: Transform data to (key, value)
Input data
Input data
Input data
Map task
Map task
Map task
chunk of data
chunk of data

Shuffle: Send (key, values)
Reduce
task
Reduce
task
Reduce
task
(key,value)
(key,value)
Map task
Map task
Map task

Reduce: Aggregating (key,values)
Reduce
task
Reduce
task
Reduce
task
Output data
Output data
Output data
value’
value’

Map Reduce
Input data
Input data
Input data
Map task
Map task
Map task
Reduce
task
Reduce
task
Reduce
task
Output data
Output data
Output data

Map Reduce example: word count
CHUNK 1
this class is about big
data and artificial
intelligence
CHUNK 2
there is nothing big
about this example
CHUNK 3
I am a big artificial
intelligence enthusiast
➢ The file is divided in
chunks to be
processed in
parallel
➢ Data is sent
untransformed to
map nodes

Map Reduce example: word count
this class is about big
data and artificial
intelligence
[this, class, is, about, big,
data, and, artificial,
intelligence]
Tokenize
(this,1), (class,1), (is,1),
(about,1), (big,1), (class, 1),
(is, 1), (about 1), (big, 1),
(data, 1), (and, 1), (artificial,1),
(intelligence, 1)
Prepare (key,value)
pairs
MAP TASK
Raw
chunk
Ready to shuffle

Map Reduce example: word countMap Reduce example: word count
(big,1)
(big,1)
(big,1)
(big,3)
Sum
REDUCE TASK
From
shuffle Output

Exercise: Matrix power
row column value
1 1 3.2
2 3 4.3
3 3 5.1
1 3 0.1

Map Reduce variants: No reduce
Input data
Input data
Input data
Map task
Map task
Map task
Output data
Output data
Output data
chunk of data (key,value)
chunk of data (key,value)

Map Reduce variants: chaining
Input
data
Input
data
Input
data
Map
task
Map
task
Map
task
Reduce
task
Reduce
task
Reduce
task
Output
data
Output
data
Output
data
Map
task
Map
task
Map
task
Reduce
task
Reduce
task
Reduce
task
Output
data
Output
data
Output
data

➢ Maps are executed in parallel
➢ Reducers do not start until all maps are
finished
➢ Output is not finished until all reducers are
finished
➢ Bottleneck: Unbalanced map/reduce taks
○ Change key distribution
○ Increase reduces for increasing parallelism
Map Reduce: bottlenecks

➢ Hadoop is implemented in Java
➢ It is possible to program jobs formed by maps
and reduces in Java
➢ We won’t go deep in these matters (bear with
me!)
Map Reduce in Hadoop

http://hadoop.apache.org/
Hadoop architecture

public class WordCount {
public static class TokenizerMapper
extends Mapper<Object, Text, Text,
IntWritable>{
private final static IntWritable one = new
IntWritable(1);
private Text word = new Text();
public void map(Object key, Text value,
Context context) throws IOException,
InterruptedException {
StringTokenizer itr = new StringTokenizer
(value.toString());
while (itr.hasMoreTokens()) {
word.set(itr.nextToken());
context.write(word, one);
}
}
}
Map Reduce job in Hadoop
public static class IntSumReducer
extends Reducer<Text,IntWritable,Text,
IntWritable> {
private IntWritable result = new IntWritable();
public void reduce(Text key,
Iterable<IntWritable> values, Context context )
throws IOException, InterruptedException {
int sum = 0;
for (IntWritable val : values) {
sum += val.get();
}
result.set(sum);
context.write(key, result);
}
}
...

public static void main(String[] args) throws
Exception {
Configuration conf = new Configuration();
Job job = Job.getInstance(conf, "word count");
job.setJarByClass(WordCount.class);
job.setMapperClass(TokenizerMapper.class);
job.setCombinerClass(IntSumReducer.class);
job.setReducerClass(IntSumReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
FileInputFormat.addInputPath(job, new Path
(args[0]));
FileOutputFormat.setOutputPath(job, new Path
(args[1]));
System.exit(job.waitForCompletion(true) ? 0 :
1);
}
}
Map Reduce job in Hadoop

➢ Compiling
javac -cp opt/hadoop/share/hadoop/common/hadoop-common-2.6.0.jar:
opt/hadoop/share/hadoop/mapreduce/hadoop-mapreduce-client-core-2.6.0.jar -d
WordCount source/hadoop/WordCount.java
jar -cvf WordCount.jar -C WordCount/ .
➢ Submitting
hadoop jar WordCount.jar es.upv.dsic.iarfid.haia.WordCount
/user/your_username/data/students.tsv /user/your_username/wc
Compiling and submitting a MR job

Hadoop ecosystem

➢ http://hadoop.apache.org
➢ Hadoop in Practice. Alex Holmes. Ed. Manning
Publications
➢ Hadoop: The Definitive Guide. Tom White. Ed.
O’Reilly.
➢ StackOverflow
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Apache Hadoop: DFS and Map Reduce

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