MapReduce is a programming model for processing large datasets in a distributed environment. It consists of a map function that processes input key-value pairs to generate intermediate key-value pairs, and a reduce function that merges all intermediate values associated with the same key. It allows for parallelization of computations across large clusters. Example applications include word count, sorting, and indexing web links. Hadoop is an open source implementation of MapReduce that runs on commodity hardware.

1. MapReduce
Ahmed Elmorsy

2. What is MapReduce?
● MapReduce is a programming model for
processing and generating large data sets.
● Inspired by the map and reduce primitives
present in Lisp and many other functional
languages
● Use of a functional model with user-specified
map and reduce operations allows us to
parallelize large computations easily.

3. Map function
Map, written by the user, takes an input pair
and produces a set of intermediate key/value
pairs. The MapReduce library groups together
all intermediate values associated with the
same intermediate key I and passes them to the
Reduce function.
map (k1,v1) → list(k2,v2)

4. Reduce function
The Reduce function, also written by the user,
accepts an intermediate key I and a set of
values for that key. It merges together these
values to form a possibly smaller set of values.
Typically just zero or one output value is
produced per Reduce invocation.
reduce (k2,list(v2)) → list(v2)

5. Example (Word Count)
Problem
Counting the number of
occurrences of each word in a
large collection of documents

6. Example (Word Count)
Map function:
map(String key, String value):
// key: document name
// value: document contents
for each word w in value:
EmitIntermediate(w, "1");

7. Example (Word Count)
Reduce function:
reduce(String key, Iterator values):
// key: a word
// values: a list of counts
int result = 0;
for each v in values:
result += ParseInt(v);
Emit(AsString(result));

9. Execution Overview
● The Map invocations are distributed across
multiple machines by automatically
partitioning the input data into a set of M splits.
● Reduce invocations are distributed by
partitioning the intermediate key space into R
pieces using a partitioning function.
● The number of partitions (R) and the
partitioning function are specified by the user.

10. How Master works?
● The master picks idle workers and assigns
each one a map task or a reduce task.
● For each map task and reduce task, it stores
the state (idle, in-progress, or
completed), and the identity of the worker
machine (for non-idle tasks).

11. Fault Tolerance
● Worker Failure
● Master Failure

12. Worker Failure
● The master pings every worker periodically.
● If no response is received from a worker,
the master marks the worker as failed.
● Any map task or reduce task in progress on a
failed worker is also reset to idle and
becomes eligible for rescheduling.

13. Worker Failure
● Any map tasks completed by the worker are
reset back to their initial idle state, and
therefore become eligible for scheduling on
other workers (WHY?!).
● Completed reduce tasks do not need to be re-
executed (WHY?!).

14. Master Failure
There are two options:
1. Make the master write periodic checkpoints
of the master data structures described
above. If the master task dies, a new copy
can be started from the last checkpointed
state.
2. Abort the MapReduce computation if the
master fails.

15. Backup Tasks
● One of the common causes that lengthens
the total time taken for a MapReduce
operation is a “straggler”.
● When a MapReduce operation is close to
completion, the master schedules backup
executions of the remaining in-progress
tasks.
● The task is marked as completed whenever
either the primary or the backup execution
completes

16. Refinements
1. Partitioning Function
2. Combiner Function
3. Input and Output Types
4. Skipping Bad Records
5. Status Information
6. Counters

17. More Examples
● Distributed Grep
● Count of URL Access Frequency
● Reverse Web-Link Graph
● Inverted Index
● Distributed Sort

18. Apache Hadoop
Open Source Implementation of MapReduce

19. Hadoop Modules
● Hadoop Common
● Hadoop Distributed File System (HDFS™)
● Hadoop YARN
● Hadoop MapReduce

20. Projects based on Hadoop
● Apache Hive
Developed by Facebook and used by Netflix.
● Apache Pig
Developed at Yahoo! and used by Twitter.
● Apache Cassandra
Developed by Facebook

21. Template Hadoop Program
public class MyJob extends Configured implements Tool {
public static class MapClass extends MapReduceBase
implements Mapper<Text, Text, Text, Text> {
public void map(Text key, Text value,
OutputCollector<Text, Text> output, Reporter reporter)
throws IOException {
//Map Function
}
}
public static class Reduce extends MapReduceBase implements
Reducer<Text, Text, Text, Text> {
public void reduce(Text key, Iterator<Text> values,
OutputCollector<Text, Text> output, Reporter reporter)
throws IOException {
}
}

22. public int run(String[] args) throws Exception {
Configuration conf = getConf();
JobConf job = new JobConf(conf, MyJob.class);
Path in = new Path(args[0]);
Path out = new Path(args[1]);
FileInputFormat.setInputPaths(job, in);
FileOutputFormat.setOutputPath(job, out);
job.setJobName("MyJob");
job.setMapperClass(MapClass.class);
job.setReducerClass(Reduce.class);
job.setInputFormat(KeyValueTextInputFormat.class);
job.setOutputFormat(TextOutputFormat.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(Text.class);
job.set("key.value.separator.in.input.line", ",");
JobClient.runJob(job);
return 0;
}

23. public static void main(String[] args) throws Exception {
int res = ToolRunner.run(new Configuration(), new
MyJob(), args);
System.exit(res);
}
}
To Run it, You have to generate the JAR file, then
you can use the command:
bin/hadoop jar playground/MyJob.jar MyJob
input/cite75_99.txt output

24. Readings
Chapter 4 in
(Lam, Chuck. Hadoop in action.
Manning Publications Co., 2010.)

25. References
[1] Jeffrey Dean and Sanjay Ghemawat. MapReduce:
Simplified data processing
on large clusters. In OSDI, pages 137–150, 2004.
[2] Lam, Chuck. Hadoop in action. Manning
Publications Co., 2010.
[3] http://hadoop.apache.org/