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<h1>Assignments <small>CS 489/698 Big Data Infrastructure (Winter 2017)</small></h1>
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<section style="padding-top:0px">
<div>
<h3>Assignment 2: Counting in Spark <small>due 1:00pm January 26</small></h3>
<p>In this assignment you will "port" the MapReduce implementations of
the bigram frequency count program
from <a href="http://bespin.io">Bespin</a> over to Spark (in
Scala). Your starting points
are <code>ComputeBigramRelativeFrequencyPairs</code>
and <code>ComputeBigramRelativeFrequencyStripes</code> in
package <code>io.bespin.java.mapreduce.bigram</code> (in Java).
You are welcome to build on the <code>BigramCount</code> (Scala)
implementation <a href="https://github.com/lintool/bespin/blob/master/src/main/scala/io/bespin/scala/spark/bigram/BigramCount.scala">here</a>
for tokenization and "boilerplate" code like command-line argument
parsing. To be consistent in tokenization, you should copy over
the <code>Tokenizer</code> trait
<a href="https://github.com/lintool/bespin/blob/master/src/main/scala/io/bespin/scala/util/Tokenizer.scala">here</a>.</p>
<p>Put your code in the
package <code>ca.uwaterloo.cs.bigdata2017w.assignment2</code>. Since
you'll be writing Scala code, your source files should go
into <code>src/main/scala/ca/uwaterloo/cs/bigdata2017w/assignment2/</code>. Note
that the repository is designed so that Scala/Spark code will also
compile with the same Maven build command:</p>
<pre>
$ mvn clean package
</pre>
<p>Following the Java implementations, you will write both a "pairs"
and a "stripes" implementation in Spark. Not that although Spark has a
different API than MapReduce, the algorithmic concepts are still very
much applicable. Your pairs and stripes implementation should follow
the same logic as in the MapReduce implementations. In particular,
your program should only take one pass through the input data.</p>
<p>Make sure your implementation runs in the Linux student CS
environment on the Shakespeare collection and also on sample Wikipedia
file <code>/shared/cs489/data/enwiki-20161220-sentences-0.1sample.txt</code>
on HDFS in the Altiscale cluster. See
the <a href="software.html">software page</a> for how to set up the
Spark environment on Altiscale.</p>
<p>You can verify the correctness of your algoritm by comparing the
output of the MapReduce implementation with your Spark
implementation. The output should be the same.</p>
<p>Clarification on terminology: informally, we often refer to
"mappers" and "reducers" in the context of Spark. That's a shorthand
way of saying map-like transformations
(<code>map</code>, <code>flatMap</code>, <code>filter</code>, <code>mapPartitions</code>,
etc.) and reduce-like transformations
(e.g., <code>reduceByKey</code>, <code>groupByKey</code>, <code>aggregateByKey</code>,
etc.). Hopefully it's clear from lecture that while Spark represents a
generalization of MapReduce, the notions of per-record processing
(i.e., map-like transformation) and grouping/shuffling (i.e.,
reduce-like transformations) are shared across both frameworks.</p>
<h4 style="padding-top: 10px">Turning in the Assignment</h4>
<p>Please follow these instructions carefully!</p>
<p>The pairs and stripes implementation should be in
package <code>ca.uwaterloo.cs.bigdata2017w.assignment2</code>;
your Scala code should be
in <code>src/main/scala/ca/uwaterloo/cs/bigdata2017w/assignment2/</code>.
There are no questions to answer in this assignment unless there is
something you would like to communicate with us, and if so, put it
in <code>assignment2.md</code>.</p>
<p>When grading, we will pull your repo and build your code:<p>
<pre>
$ mvn clean package
</pre>
<p>Your code should build successfully. We are then going to check
your code (both the pairs and stripes implementations).</p>
<p>We are going to run your code on the Linux student CS environment
as follows (we will make sure the collection is there):</p>
<pre>
$ spark-submit --class ca.uwaterloo.cs.bigdata2017w.assignment2.ComputeBigramRelativeFrequencyPairs \
target/bigdata2017w-0.1.0-SNAPSHOT.jar --input data/Shakespeare.txt \
--output cs489-2017w-lintool-a2-shakespeare-pairs --reducers 5
$ spark-submit --class ca.uwaterloo.cs.bigdata2017w.assignment2.ComputeBigramRelativeFrequencyStripes \
target/bigdata2017w-0.1.0-SNAPSHOT.jar --input data/Shakespeare.txt \
--output cs489-2017w-lintool-a2-shakespeare-stripes --reducers 5
</pre>
<p>Make sure that your code runs in the Linux Student CS environment
(even if you do development on your own machine), which is where we
will be doing the grading. "But it runs on my laptop!" will not be
accepted as an excuse if we can't get your code to run.</p>
<p>We are going to run your code on the Altiscale cluster as follows
(note the addition of the <code>--num-executors</code>
and <code>--executor-cores</code> options):</p>
<pre>
$ spark-submit --class ca.uwaterloo.cs.bigdata2017w.assignment2.ComputeBigramRelativeFrequencyPairs \
--num-executors 7 --executor-cores 2 target/bigdata2017w-0.1.0-SNAPSHOT.jar \
--input /shared/cs489/data/enwiki-20161220-sentences-0.1sample.txt \
--output cs489-2017w-lintool-a2-wiki-pairs --reducers 14
$ spark-submit --class ca.uwaterloo.cs.bigdata2017w.assignment2.ComputeBigramRelativeFrequencyStripes \
--num-executors 7 --executor-cores 2 --executor-memory 4G target/bigdata2017w-0.1.0-SNAPSHOT.jar \
--input /shared/cs489/data/enwiki-20161220-sentences-0.1sample.txt \
--output cs489-2017w-lintool-a2-wiki-stripes --reducers 14
</pre>
<p><b>Important:</b> Make sure that your code accepts the command-line
parameters above!<p>
<p>When you run a Spark job, you need to specify how much cluster
resource to request. The option <code>--num-executors</code> specifies
the number of executors, each with a certain number of cores specified
by <code>--executor-cores</code>. So, in the above commands, we
request a total of 14 workers (7 executors, 2 cores each).</p>
<p>The <code>--reducers</code> flag is the amount of parallelism that
you set in your program in the reduce stage. If the total number of
workers is larger than <code>--reducers</code>, some of the workers
will be sitting idle, since you've allocated more workers for the job
than the parallelism you've specified in your
program. If <code>--reducers</code> is larger than the number of
workers, on the other hand, then your reduce tasks will queue up at
the workers, i.e., a worker will be assigned more than one reduce
task. In the above example we set the two equal.</p>
<p>Note that the setting of these two parameters should not affect the
correctness of your program. The setting above is a reasonable middle
ground between having your jobs finish in a reasonable amount of time
and not monopolizing cluster resources.</p>
<p>A related but still orthogonal concept is partitions. Partitions
describes the physical division of records across workers during
execution. When reading from HDFS, the number of HDFS blocks
determines the number of partitions in your RDD. When you apply a
reduce-like transformation, you can optionally specify the number of
partitions (or Spark applies a default) — in this case, the
number of partitions is equal to the number of reducers.</p>
<p>When you've done everything, commit to your repo and remember to
push back to origin. You should be able to see your edits in the web
interface. Before you consider the assignment "complete", we would
recommend that you verify everything above works by performing a clean
clone of your repo and going through the steps above.</p>
<p>That's it!</p>
<h4 style="padding-top: 10px">Grading</h4>
<p>This assignment is worth a total of 24 points, broken down as
follows:</p>
<ul>
<li>The pairs implementation running locally is worth 6 points; the stripes implementation running locally is worth another 6 points.</li>
<li>The pairs implementation running on Altiscale is worth 6 points; the stripes implementation running on Altiscale is worth another 6 points.</li>
</ul>
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