Hadoop - Introduction to mapreduce

Terminology
Google calls it: Hadoop equivalent:
MapReduce Hadoop
GFS HDFS
Bigtable HBase
Chubby Zookeeper

Some MapReduce Terminology
 Job – A “full program” - an execution of a Mapper and
Reducer across a data set
 Task – An execution of a Mapper or a Reducer on a slice
of data
 a.k.a. Task-In-Progress (TIP)
 Task Attempt – A particular instance of an attempt to
execute a task on a machine

Task Attempts
 A particular task will be attempted at least once,
possibly more times if it crashes
 If the same input causes crashes over and over, that input
will eventually be abandoned
 Multiple attempts at one task may occur in parallel
with speculative execution turned on
 Task ID from TaskInProgress is not a unique identifier;
don’t use it that way

MapReduce: High Level
In our case: circe.rc.usf.edu

Nodes, Trackers, Tasks
 Master node runs JobTracker instance, which accepts
Job requests from clients
 TaskTracker instances run on slave nodes
 TaskTracker forks separate Java process for task
instances

Job Distribution
 MapReduce programs are contained in a Java “jar”
file + an XML file containing serialized program
configuration options
 Running a MapReduce job places these files into
the HDFS and notifies TaskTrackers where to
retrieve the relevant program code
 … Where’s the data distribution?

Data Distribution
 Implicit in design of MapReduce!
 All mappers are equivalent; so map whatever data
is local to a particular node in HDFS
 If lots of data does happen to pile up on the
same node, nearby nodes will map instead
 Data transfer is handled implicitly by HDFS

What Happens In Hadoop?
Depth First

Job Launch Process: Client
 Client program creates a JobConf
 Identify classes implementing Mapper and
Reducer interfaces
 JobConf.setMapperClass(), setReducerClass()
 Specify inputs, outputs
 FileInputFormat.setInputPath(),
 FileOutputFormat.setOutputPath()
 Optionally, other options too:
 JobConf.setNumReduceTasks(),
JobConf.setOutputFormat()…

Job Launch Process: JobClient
 Pass JobConf to JobClient.runJob() or
submitJob()
 runJob() blocks, submitJob() does not
 JobClient:
 Determines proper division of input into InputSplits
 Sends job data to master JobTracker server

Job Launch Process: JobTracker
 JobTracker:
 Inserts jar and JobConf (serialized to XML) in
shared location
 Posts a JobInProgress to its run queue

Job Launch Process: TaskTracker
 TaskTrackers running on slave nodes
periodically query JobTracker for work
 Retrieve job-specific jar and config
 Launch task in separate instance of Java
 main() is provided by Hadoop

Job Launch Process: Task
 TaskTracker.Child.main():
 Sets up the child TaskInProgress attempt
 Reads XML configuration
 Connects back to necessary MapReduce
components via RPC
 Uses TaskRunner to launch user process

Job Launch Process: TaskRunner
 TaskRunner, MapTaskRunner, MapRunner
work in a daisy-chain to launch your Mapper
 Task knows ahead of time which InputSplits it
should be mapping
 Calls Mapper once for each record retrieved from
the InputSplit
 Running the Reducer is much the same

Creating the Mapper
 You provide the instance of Mapper
 Should extend MapReduceBase
 One instance of your Mapper is initialized by
the MapTaskRunner for a TaskInProgress
 Exists in separate process from all other instances
of Mapper – no data sharing!

Mapper
 void map(K1 key,
V1 value,
OutputCollector<K2, V2> output,
Reporter reporter)
 K types implement WritableComparable
 V types implement Writable

What is Writable?
 Hadoop defines its own “box” classes for
strings (Text), integers (IntWritable), etc.
 All values are instances of Writable
 All keys are instances of WritableComparable

Reading Data
 Data sets are specified by InputFormats
 Defines input data (e.g., a directory)
 Identifies partitions of the data that form an
InputSplit
 Factory for RecordReader objects to extract (k, v)
records from the input source

FileInputFormat and Friends
 TextInputFormat – Treats each ‘n’-terminated line of a
file as a value
 KeyValueTextInputFormat – Maps ‘n’- terminated text
lines of “k SEP v”
 SequenceFileInputFormat – Binary file of (k, v) pairs with
some add’l metadata
 SequenceFileAsTextInputFormat – Same, but maps
(k.toString(), v.toString())

Filtering File Inputs
 FileInputFormat will read all files out of a
specified directory and send them to the
mapper
 Delegates filtering this file list to a method
subclasses may override
 e.g., Create your own “xyzFileInputFormat” to
read *.xyz from directory list

Record Readers
 Each InputFormat provides its own
RecordReader implementation
 Provides (unused?) capability multiplexing
 LineRecordReader – Reads a line from a text
file
 KeyValueRecordReader – Used by
KeyValueTextInputFormat

Input Split Size
 FileInputFormat will divide large files into
chunks
 Exact size controlled by mapred.min.split.size
 RecordReaders receive file, offset, and
length of chunk
 Custom InputFormat implementations may
override split size – e.g., “NeverChunkFile”

Sending Data To Reducers
 Map function receives OutputCollector object
 OutputCollector.collect() takes (k, v) elements
 Any (WritableComparable, Writable) can be
used
 By default, mapper output type assumed to
be same as reducer output type

WritableComparator
 Compares WritableComparable data
 Will call WritableComparable.compare()
 Can provide fast path for serialized data
 JobConf.setOutputValueGroupingComparator()

Sending Data To The Client
 Reporter object sent to Mapper allows simple
asynchronous feedback
 incrCounter(Enum key, long amount)
 setStatus(String msg)
 Allows self-identification of input
 InputSplit getInputSplit()

Partitioner
 int getPartition(key, val, numPartitions)
 Outputs the partition number for a given key
 One partition == values sent to one Reduce task
 HashPartitioner used by default
 Uses key.hashCode() to return partition num
 JobConf sets Partitioner implementation

Reduction
 reduce( K2 key,
Iterator<V2> values,
OutputCollector<K3, V3> output,
Reporter reporter )
 Keys & values sent to one partition all go to
the same reduce task
 Calls are sorted by key – “earlier” keys are
reduced and output before “later” keys

OutputFormat
 Analogous to InputFormat
 TextOutputFormat – Writes “key valn” strings
to output file
 SequenceFileOutputFormat – Uses a binary
format to pack (k, v) pairs
 NullOutputFormat – Discards output
 Only useful if defining own output methods within
reduce()

Example Program - Wordcount
 map()
 Receives a chunk of text
 Outputs a set of word/count pairs
 reduce()
 Receives a key and all its associated values
 Outputs the key and the sum of the values
package org.myorg;
import java.io.IOException;
import java.util.*;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.conf.*;
import org.apache.hadoop.io.*;
import org.apache.hadoop.mapred.*;
import org.apache.hadoop.util.*;
public class WordCount {

Wordcount – main( )
public static void main(String[] args) throws Exception {
JobConf conf = new JobConf(WordCount.class);
conf.setJobName("wordcount");
conf.setOutputKeyClass(Text.class);
conf.setOutputValueClass(IntWritable.class);
conf.setMapperClass(Map.class);
conf.setReducerClass(Reduce.class);
conf.setInputFormat(TextInputFormat.class);
conf.setOutputFormat(TextOutputFormat.class);
FileInputFormat.setInputPaths(conf, new Path(args[0]));
FileOutputFormat.setOutputPath(conf, new Path(args[1]));
JobClient.runJob(conf);
}

Wordcount – map( )
public static class Map extends MapReduceBase … {
private final static IntWritable one = new IntWritable(1);
private Text word = new Text();
public void map(LongWritable key, Text value,
OutputCollector<Text, IntWritable> output, …) … {
String line = value.toString();
StringTokenizer tokenizer = new StringTokenizer(line);
while (tokenizer.hasMoreTokens()) {
word.set(tokenizer.nextToken());
output.collect(word, one);
}
}
}

Wordcount – reduce( )
public static class Reduce extends MapReduceBase … {
public void reduce(Text key, Iterator<IntWritable> values,
OutputCollector<Text, IntWritable> output, …) … {
int sum = 0;
while (values.hasNext()) {
sum += values.next().get();
}
output.collect(key, new IntWritable(sum));
}
}
}

Hadoop Streaming
 Allows you to create and run map/reduce
jobs with any executable
 Similar to unix pipes, e.g.:
 format is: Input | Mapper | Reducer
 echo “this sentence has five lines” | cat | wc

Hadoop Streaming
 Mapper and Reducer receive data from stdin and output to
stdout
 Hadoop takes care of the transmission of data between the
map/reduce tasks
 It is still the programmer’s responsibility to set the correct
key/value
 Default format: “key t valuen”
 Let’s look at a Python example of a MapReduce word count
program…

Streaming_Mapper.py
# read in one line of input at a time from stdin
for line in sys.stdin:
line = line.strip() # string
words = line.split() # list of strings
# write data on stdout
for word in words:
print ‘%st%i’ % (word, 1)

Hadoop Streaming
 What are we outputting?
 Example output: “the 1”
 By default, “the” is the key, and “1” is the value
 Hadoop Streaming handles delivering this key/value pair to a
Reducer
 Able to send similar keys to the same Reducer or to an
intermediary Combiner

Streaming_Reducer.py
wordcount = { } # empty dictionary
# read in one line of input at a time from stdin
for line in sys.stdin:
line = line.strip() # string
key,value = line.split()
wordcount[key] = wordcount.get(key, 0) + value
# write data on stdout
for word, count in sorted(wordcount.items()):
print ‘%st%i’ % (word, count)

Hadoop Streaming Gotcha
 Streaming Reducer receives single lines
(which are key/value pairs) from stdin
 Regular Reducer receives a collection of all the
values for a particular key
 It is still the case that all the values for a particular
key will go to a single Reducer

Using Hadoop Distributed File System
(HDFS)
 Can access HDFS through various shell
commands (see Further Resources slide for
link to documentation)
 hadoop –put <localsrc> … <dst>
 hadoop –get <src> <localdst>
 hadoop –ls
 hadoop –rm file

Configuring Number of Tasks
 Normal method
 jobConf.setNumMapTasks(400)
 jobConf.setNumReduceTasks(4)
 Hadoop Streaming method
 -jobconf mapred.map.tasks=400
 -jobconf mapred.reduce.tasks=4
 Note: # of map tasks is only a hint to the framework. Actual
number depends on the number of InputSplits generated

Running a Hadoop Job
 Place input file into HDFS:
 hadoop fs –put ./input-file input-file
 Run either normal or streaming version:
 hadoop jar Wordcount.jar org.myorg.Wordcount input-file
output-file
 hadoop jar hadoop-streaming.jar
-input input-file
-output output-file
-file Streaming_Mapper.py
-mapper python Streaming_Mapper.py
-file Streaming_Reducer.py
-reducer python Streaming_Reducer.py

Submitting to RC’s GridEngine
 Add appropriate modules
 module add apps/jdk/1.6.0_22.x86_64 apps/hadoop/0.20.2
 Use the submit script posted in the Further Resources slide
 Script calls internal functions hadoop_start and hadoop_end
 Adjust the lines for transferring the input file to HDFS and starting the
hadoop job using the commands on the previous slide
 Adjust the expected runtime (generally good practice to overshoot your
estimate)
 #$ -l h_rt=02:00:00
 NOTICE: “All jobs are required to have a hard run-time specification. Jobs
that do not have this specification will have a default run-time of 10 minutes
and will be stopped at that point.”

Output Parsing
 Output of the reduce tasks must be retrieved:
 hadoop fs –get output-file hadoop-output
 This creates a directory of output files, 1 per reduce task
 Output files numbered part-00000, part-00001, etc.
 Sample output of Wordcount
 head –n5 part-00000
“’tis 1
“come 2
“coming 1
“edwin 1
“found 1

Extra Output
 The stdout/stderr streams of Hadoop itself will be stored in an output file
(whichever one is named in the startup script)
 #$ -o output.$job_id
STARTUP_MSG: Starting NameNode
STARTUP_MSG: host = svc-3024-8-10.rc.usf.edu/10.250.4.205
…
11/03/02 18:28:47 INFO mapred.FileInputFormat: Total input paths to process : 1
11/03/02 18:28:47 INFO mapred.JobClient: Running job: job_local_0001
…
11/03/02 18:28:48 INFO mapred.MapTask: numReduceTasks: 1
…
11/03/02 18:28:48 INFO mapred.TaskRunner: Task 'attempt_local_0001_m_000000_0' done.
11/03/02 18:28:48 INFO mapred.Merger: Merging 1 sorted segments
11/03/02 18:28:48 INFO mapred.Merger: Down to the last merge-pass, with 1 segments left of total
size: 43927 bytes
11/03/02 18:28:48 INFO mapred.JobClient: map 100% reduce 0%
…

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Hadoop - Introduction to mapreduce

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