HOMEWORK 2 Solution



Getting Started

Update your SVN repository.

When needed, you will find additional materials for homework x in the folder hwx. So, for the current assignment the folder is hw2.

Hint: You can check your submission to the SVN repository by viewing https://svn.seas.

wustl.edu/repositories/<yourwustlkey>/cse427s_fl18 in a web browser.



  • all written work needs to be submitted electronically in pdf format1 via GRADESCOPE

  • provide the following information on every page of your pdf file:


student ID

wustlkey (your wustlkey indicates the location (SVN repository) for your code submissions; without this information we will not be able to grade your code!)

  • start every problem on a new page

  • FOR GROUPS: make a group submission on GRADESCOPE and provide the following

information for all group members on every page of your pdf file: names, student IDs, and location of code submissions (one student’s wustlkey)2


    • code needs to be submitted via SVN repository commit (detailed submission instruc-tions are provided whenever code submissions are required)

    • make sure to always use the required file name(s) and submission format(s)

    • comment your code to receive maximum credit

1 Please, type your solutions or use clear hand-writing. If we cannot read your answer, we cannot give you credit nor will we be able to meet any regrade requests concerning your writing.

2It is sufficient to commit code to one SVN repository. If you do not specify the repository for your group’s code submission clearly, we will only check the first repository, sorting wustlkeys alphabetically.


Preparation (this is essentially lab0)

  1. Download and set up the course VM. Find instructions on the course webpage under Resources and HowTos.

  1. Checkout your SVN repository in the VM, then you can commit code submissions directly from there. You will need to install subversion in your VM. To do so run:

  1. If you haven’t done so already, run the course setup script in a terminal window in your VM:

Problem 1: HDFS (50%)

If you haven’t done so, complete Lab 1 as it prepares the data used in this and subsequent homework problems. Note, that you will loose credit if you are not using the correct input data in upcoming problems.

  1. If you haven’t done so, remove the file named glossary. Then, list all the files in the shakespeare folder in HDFS . Provide this command and the result in the hw2.pdf file.

Display the first 16 lines (not more) of the poems data stored in HDFS. Provide the command and result in the hw2.pdf file.

  1. The default replication factor in HDFS is 3. The optimal number of redundant block copies depends on the following factors: cost of replication, cost of data loss, and the probability of failure.

    • What is the downside of a large replication factor with respect to the storage capacity of the data nodes in the cluster?

    • What is the downside of a large replication factor with respect to the amount of memory of the cluster’s NameNode3 in your answer.

  1. The default block size in HDFS is 128MB, which is fairly large. Discuss one benefit and one disadvantage of large block sizes. (HINT: read HTDG pp. 43-45.)

  1. Assume you have a data file of size 642MB, the replication factor in the distributed file system is set to 2, the block size is 128MB, and the cluster consists of 6 nodes (indicate them by N1, N2, . . . , N6) on 3 racks (indicate them by RA, RB, RC). Nodes are evenly distributed among racks and you can assume that the cluster is empty. Now, consider the cluster after storing this file in HDFS. Sketch the cluster state (file/block locations) and provide the meta-data stored on the master node for this example file.

  1. Explain two disadvantages of a distributed files system such as HDFS (HINT: read HTDG pp. 43-45).

  1. Assume you want to download data from HDFS to your local client. Provide the com-mand for this process and briefly describe how the data is located and transferred.

  • NOTE: the NameNode keeps the filesystem metadata in memory for fast access.


Problem 2: MapReduce I (25%)

Suppose the input data to a MapReduce operation consists of integer values (the input keys are not important). The map function takes an integer i and produces the list of pairs (p; i) such that p is a prime divisor of i. For example, map(12) = [(2; 12); (3; 12)]. The reduce function is addition. That is, reduce(p; [i1; i2; :::; ik]) is (p; i1 + i2 + ::: + ik).

Provide the intermediate data and final result of a MAPREDUCE execution for the following integer input i = f15; 21; 24; 30; 49g. Include all Mapper inputs, Mapper outputs, Reducer inputs, and Reducer outputs in your answer.

Problem 3: MapReduce II (25%)

Given the following input data:

2013-03-15 12:39 – /common/logo.gif 1200ms – 2326

2013-03-15 12:39 – /catalog/cat1.html 900ms – 1211

2013-03-15 12:40 – /common/logo.gif 1900ms – 1198

2013-03-15 12:41 – /common/promoex.jpg 4000ms – 2326

2013-03-15 12:44 – /catalog/cat2.html 1100ms – 1451

Consider a MAPREDUCE program that analyzes the log data provided above to retrieve the average processing time for each file type.

  1. Compute the Mapper output, Reducer input, and Reducer output for this particular example input data.

  1. Which data type do you use to represent the keys? Which data type do you use to represent the values?

Bonus Problem (5% up to a max. of 100%) – no group work!

We will be doing a little experiment in the course. We will collect your emotional descrip-tion for each homework assignment and hopefully, we will be able to use this data for sentiment analysis at the end of the semester!

Sentiment Analysis tries to assess the emotion or mood in a text document. Essentially it can be computed by comparing the set of words in each document to an existing dictionary of positive words, negative words, and neutral words. In our experiment, we give you the chance to voice your opinions on each homework by writing a couple of sentences as a review for the homework. You will not be graded on what your review says, but rather solely the completion of it. At the end of the year, given that we have enough data for each homework, you will perform sentiment analysis on this data to see which homework you and your peers regarded as “positive” or “negative”.

So, please write a review for this homework and store it in the file hw2_review.txt provided in your SVN repository. This file should only include the review, no other information such


as name, wustlkey, etc. Remember that you are not graded for the content of your review, solely it’s completion.

To be able to evaluate your sentiment analysis approach we need the ground truth. So, in addition to your textual review, take 1 minute to provide a star rating for hw2 via this link: https://wustl.az1.qualtrics.com/jfe/form/SV_aaAc0ILmBleyKzj.

You can only earn bonus points if you write a meaningful review of at least 50 words and provide the corresponding star rating. Bonus points are given to the owner of the reposi-tory only (no group work!).

Submit your review in the file hw2_review.txt provided in the hw2 folder in your SVN repos-

itory. To commit the file run:

$ svn commit -m ’hw2 review submission’ .


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