Introduction to Hadoop

Dr. Sandeep G. Deshmukh

DataTorrent

1

Introduction to

Contents

Motivation

Scale of Cloud Computing

Hadoop

Hadoop Distributed File System (HDFS)

MapReduce

Sample Code Walkthrough

Hadoop EcoSystem

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Motivation - Traditional Distributed systems

Processor Bound

Using multiple machines

Developer is burdened with managing too many things

Synchronization

Failures

Data moves from shared disk to compute node

Cost of maintaining clusters

Scalability as and when required not present

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What is the scale we are talking about?

100s of CPUs?

Couple of CPUs?

10s of CPUs?

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Hadoop @ Yahoo!

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What we need

Handling failure

One computer = fails once in 1000 days

1000 computers = 1 per day

Petabytes of data to be processed in parallel

1 HDD= 100 MB/sec

1000 HDD= 100 GB/sec

Easy scalability

Relative increase/decrease of performance depending on increase/decrease of nodes

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What we’ve got : Hadoop!

Created by Doug Cutting

Started as a module in nutch and then matured as an apache project

Named it after his son's stuffed

elephant

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http://upload.wikimedia.org/wikipedia/en/a/a7/Doug-Cutting.JPG

What we’ve got : Hadoop!

Fault-tolerant file system Hadoop Distributed File System (HDFS) Modeled on Google File system

Takes computation to data Data Locality

Scalability: Program remains same for 10, 100, 1000,… nodes Corresponding performance improvement

Parallel computation using MapReduce Other components – Pig, Hbase, HIVE, ZooKeeper

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HDFS Hadoop distributed File System

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How HDFS works

NameNode - Master

DataNodes - Slave

Secondary NameNode

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Storing file on HDFS

Motivation Reliability, Availability, Network Bandwidth

The input file (say 1 TB) is split into smaller chunks/blocks of 64 MB (or multiples

of 64MB) The chunks are stored on multiple nodes as independent files on slave nodes To ensure that data is not lost, replicas are stored in the following way:

One on local node One on remote rack (incase local rack fails) One on local rack (incase local node fails) Other randomly placed Default replication factor is 3

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B1

B1

B1

B2

B2

B2

B3 Bn

Hub 1 Hub 2

Data n

od

es File

Master Node

8 gigabit

1 gigabit

Blocks

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NameNode - Master

The master node: NameNode

Functions:

Manages File System- mapping files to blocks and blocks to data nodes

Maintaining status of data nodes

Heartbeat Datanode sends heartbeat at regular intervals

If heartbeat is not received, datanode is declared dead

Blockreport DataNode sends list of blocks on it

Used to check health of HDFS

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NameNode Functions

Replication

On Datanode failure

On Disk failure

On Block corruption

Data integrity

Checksum for each block

Stored in hidden file

Rebalancing- balancer tool

Addition of new nodes

Decommissioning

Deletion of some files

NameNode - Master

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HDFS Robustness

Safemode

At startup: No replication possible

Receives Heartbeats and Blockreports from Datanodes

Only a percentage of blocks are checked for defined replication factor

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All is well Exit Safemode

Replicate blocks wherever necessary

HDFS Summary

Fault tolerant

Scalable

Reliable

File are distributed in large blocks for

Efficient reads

Parallel access

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Questions?

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MapReduce

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What is MapReduce?

It is a powerful paradigm for parallel computation

Hadoop uses MapReduce to execute jobs on files in HDFS

Hadoop will intelligently distribute computation over cluster

Take computation to data

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Origin: Functional Programming

map f [a, b, c] = [f(a), f(b), f(c)]

map sq [1, 2, 3] = [sq(1), sq(2), sq(3)]

= [1,4,9]

Returns a list constructed by applying a function (the first argument) to all items in a list passed as the second argument

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Origin: Functional Programming

reduce f [a, b, c] = f(a, b, c)

reduce sum [1, 4, 9] = sum(1, sum(4,sum(9,sum(NULL))))

= 14

Returns a list constructed by applying a function (the first argument) on the list passed as the second argument

Can be identity (do nothing)

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Sum of squares example

[1,2,3,4]

Sq (1) Sq (2) Sq (3) Sq (4)

16 9 4 1

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Input

Intermediate output

Output

MAPPER

REDUCER

M1 M2 M3 M4

R1

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Sum of squares of even and odd

[1,2,3,4]

Sq (1) Sq (2) Sq (3) Sq (4)

(even, 16) (odd, 9) (even, 4) (odd, 1)

(even, 20) (odd, 10)

Input

Intermediate output

Output

MAPPER

REDUCER

M1 M2 M3 M4

R1 R2

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Programming model- key, value pairs

Format of input- output

(key, value)

Map: (k1, v1) → list (k2, v2)

Reduce: (k2, list v2) → list (k3, v3)

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Sum of squares of even and odd and prime

[1,2,3,4]

Sq (1) Sq (2) Sq (3) Sq (4)

(even, 16) (odd, 9)

(prime, 9)

(even, 4)

(prime, 4)

(odd, 1)

(even, 20) (odd, 10)

(prime, 13)

Input

Intermediate output

Output

R2 R1

R3

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Many keys, many values

Format of input- output

(key, value)

Map: (k1, v1) → list (k2, v2)

Reduce: (k2, list v2) → list (k3, v3)

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Fibonacci sequence

f(n) = f(n-1) + f(n-2)

i.e. f(5) = f(4) + f(3)

0, 1, 1, 2, 3, 5, 8, 13,…

f(5)

f(4) f(3)

f(2) f(3)

f(1) f(2)

f(2) f(1)

f(0) f(1)

•MapReduce will not work on this kind of calculation •No inter-process communication •No data sharing

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Input: 1TB text file containing color names- Blue, Green, Yellow, Purple, Pink, Red, Maroon, Grey,

Desired output:

Occurrence of colors Blue and Green

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N1 f.001

Blue

Purple

Blue

Red

Green

Blue

Maroon

Green

Yellow

N1 f.001

Blue

Blue

Green

Blue

Green

grep Blue|Green

Nn

f.00n

Green

Blue

Blue

Blue

Green

Blue= 3000

Green= 5500

Blue=500 Green=200

Blue=420 Green=200

sort |unique -c

awk ‘{arr[$1]+=$2;} END{print arr[Blue], arr[Green]}’

COMBINER

MAPPER

REDUCER

awk ‘{arr[$1]++;} END{print arr[Blue], arr[Green]}’

Nn f.00n

Blue

Purple

Blue

Red

Green

Blue

Maroon

Green

Yellow

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Input data

Map

Map

Map

Reduce

Reduce

Output

INPUT MAP SHUFFLE REDUCE OUTPUT

Works on a record

Works on output of Map

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MapReduce Overview

Input data

Combine

Combine

Combine

Map

Map

Map

Reduce

Reduce

Output

INPUT MAP REDUCE OUTPUT

Works on output of Map Works on output of Combiner

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MapReduce Overview

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MapReduce Overview

Mapper, reducer and combiner act on <key, value> pairs

Mapper gets one record at a time as an input

Combiner (if present) works on output of map

Reducer works on output of map (or combiner, if present)

Combiner can be thought of local-reducer

Reduces output of maps that are executed on same node

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MapReduce Summary

What Hadoop is not..

It is not a POSIX file system

It is not a SAN file system

It is not for interactive file accessing

It is not meant for a large number of small files- it is for a small number of large files

MapReduce cannot be used for any and all applications

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Hadoop: Take Home

Takes computation to data

Suitable for large data centric operations

Scalable on demand

Fault tolerant and highly transparent

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Questions?

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Coming up next …

First hadoop program

Second hadoop program

Your first program in hadoop

Open up any tutorial on hadoop and first program you see will be of wordcount

Task: Given a text file, generate a list of words with the

number of times each of them appear in the file Input:

Plain text file Expected Output:

<word, frequency> pairs for all words in the file

hadoop is a framework written in java

hadoop supports parallel processing

and is a simple framework

<hadoop, 2>

<is, 2>

<a , 2>

<java , 1>

<framework , 2>

<written , 1>

<in , 1>

<and,1>

<supports , 1>

<parallel , 1>

<processing. , 1> <simple,1>

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Your second program in hadoop

Task: Given a text file containing numbers, one

per line, count sum of squares of odd, even and prime

Input: File containing integers, one per line

Expected Output: <type, sum of squares> for odd, even, prime

1

2

5

3

5

6

3

7

9

4

<odd, 302>

<even, 278>

<prime, 323 >

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File on HDFS

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3

9

6

2

3

7

8

Map: square

3 <odd,9>

7 <odd,49>

2

6 <even,36>

9 <odd,81>

3 <odd,9>

8 <even,64>

<prime,4>

<prime,9>

<prime,9>

<even,4>

Reducer: sum

prime:<9,4,9>

odd:<9,81,9,49>

even:<,36,4,64>

<odd,148>

<even,104>

<prime,22>

Input

Value

Output

(Key,Value)

Input (Key, List of Values)

Output

(Key,Value)


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Map (Invoked on a record)

Reduce (Invoked on a key)

void map (int x){ int sq = x * x; if(x is odd) print(“odd”,sq); if(x is even) print(“even”,sq); if(x is prime) print(“prime”,sq); }

void reduce(List l ){ for(y in List l){ sum += y; } print(Key, sum); }

Library functions boolean odd(int x){ …} boolean even(int x){ …} boolean prime(int x){ …}


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void map (int x){ int sq = x * x; if(x is odd) print(“odd”,sq); if(x is even) print(“even”,sq); if(x is prime) print(“prime”,sq); }

Your second program in hadoop: Reduce

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Reduce (Invoked on a key)

Reduce (Invoked on a

key)

void reduce(List l ){ for(y in List l){ sum += y; } print(Key, sum); }


public static void main(String[] args) throws Exception {

Configuration conf = new Configuration();

Job job = new Job(conf, “OddEvenPrime");

job.setJarByClass(OddEvenPrime.class);

job.setMapperClass(OddEvenPrimeMapper.class);

job.setCombinerClass(OddEvenPrimeReducer.class);

job.setReducerClass(OddEvenPrimeReducer.class);

job.setOutputKeyClass(Text.class);

job.setOutputValueClass(IntWritable.class);

job.setInputFormatClass(TextInputFormat.class);

FileInputFormat.addInputPath(job, new Path(args[0]));

FileOutputFormat.setOutputPath(job, new Path(args[1]));

System.exit(job.waitForCompletion(true) ? 0 : 1);

}

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Questions?

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Coming up next …

More examples

Hadoop Ecosystem

Example: Counting Fans

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Problem: Give Crowd statistics Count fans

supporting India and Pakistan

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45882 67917

Traditional Way

Central Processing

Every fan comes to the centre and presses India/Pak button

Issues

Slow/Bottlenecks

Only one processor

Processing time determined by the speed at which people(data) can move


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Hadoop Way

Appoint processors per block (MAPPER)

Send them to each block and ask them to send a signal for each person

Central processor will aggregate the results (REDUCER)

Can make the processor smart by asking him/her to aggregate locally and only send aggregated value (COMBINER)


Reducer

Combiner

HomeWork : Exit Polls 2014

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Hadoop EcoSystem: Basic

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Hadoop Distributions

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Who all are using Hadoop

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http://wiki.apache.org/hadoop/PoweredBy

References

For understanding Hadoop

Official Hadoop website- http://hadoop.apache.org/

Hadoop presentation wiki- http://wiki.apache.org/hadoop/HadoopPresentations?action=AttachFile

http://developer.yahoo.com/hadoop/

http://wiki.apache.org/hadoop/

http://www.cloudera.com/hadoop-training/

http://developer.yahoo.com/hadoop/tutorial/module2.html#basics

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References

Setup and Installation

Installing on Ubuntu

http://www.michael-noll.com/wiki/Running_Hadoop_On_Ubuntu_Linux_(Single-Node_Cluster)

http://www.michael-noll.com/wiki/Running_Hadoop_On_Ubuntu_Linux_(Multi-Node_Cluster)

Installing on Debian

http://archive.cloudera.com/docs/_apt.html

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http://archive.cloudera.com/docs/_apt.html

Hadoop: The Definitive Guide: Tom White

http://developer.yahoo.com/hadoop/tutorial/

http://www.cloudera.com/content/cloudera-content/cloudera-docs/HadoopTutorial/CDH4/Hadoop-Tutorial.html

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Further Reading

http://www.amazon.com/Hadoop-Definitive-Guide-Tom-White/dp/1449311520





http://developer.yahoo.com/hadoop/tutorial/








Questions?

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Acknowledgements

Surabhi Pendse

Sayali Kulkarni

Parth Shah

Date post:	23-Feb-2017
Category:	Software
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Introduction to Hadoop

Software