hadoop.ppt

Lecture 3 – Hadoop
Technical Introduction
CSE 490H

Announcements
 My office hours: M 2:30—3:30 in CSE 212
 Cluster is operational; instructions in
assignment 1 heavily rewritten
 Eclipse plugin is “deprecated”
 Students who already created accounts:
let me know if you have trouble

Breaking news!
 Hadoop tested on 4,000 node cluster
32K cores (8 / node)
16 PB raw storage (4 x 1 TB disk / node)
(about 5 PB usable storage)
 http://developer.yahoo.com/blogs/hadoop/2008/09/
scaling_hadoop_to_4000_nodes_a.html

You Say, “tomato…”
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

Terminology Example
 Running “Word Count” across 20 files is
one job
 20 files to be mapped imply 20 map tasks
+ some number of reduce tasks
 At least 20 map task attempts will be
performed… more if a machine crashes,
etc.

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
JobTracker
MapReduce job
submitted by
client computer
Master node
TaskTracker
Slave node
Task instance
TaskTracker
Slave node
Task instance
TaskTracker
Slave node
Task instance

Node-to-Node Communication
 Hadoop uses its own RPC protocol
 All communication begins in slave nodes
Prevents circular-wait deadlock
Slaves periodically poll for “status” message
 Classes must provide explicit serialization

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

Configuring With JobConf
 MR Programs have many configurable options
 JobConf objects hold (key, value) components
mapping String  ’a
 e.g., “mapred.map.tasks”  20
 JobConf is serialized and distributed before running
the job
 Objects implementing JobConfigurable can
retrieve elements from a JobConf

What Happens In MapReduce?
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.addInputPath(),
 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

Getting Data To The Mapper
Input file
InputSplit InputSplit InputSplit InputSplit
Input file
RecordReader RecordReader RecordReader RecordReader
Mapper
(intermediates)
Mapper
(intermediates)
Mapper
(intermediates)
Mapper
(intermediates)
InputFormat

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()

Partition And Shuffle
Mapper
(intermediates)
Mapper
(intermediates)
Mapper
(intermediates)
Mapper
(intermediates)
Reducer Reducer Reducer
(intermediates) (intermediates) (intermediates)
Partitioner Partitioner Partitioner Partitioner
shuffling

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

Finally: Writing The Output
Reducer Reducer Reducer
RecordWriter RecordWriter RecordWriter
output file output file output file
OutputFormat

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

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