-
Olivier Blanvillain authoreddc3a7d52
StackOverflow
Use the following commands to make a fresh clone of your repository:
git clone -b stackoverflow git@gitlab.epfl.ch:lamp/student-repositories-s21/cs206-GASPAR.git cs206-stackoverflow
Useful links
- The API documentation of Spark
- The API documentation of the Scala standard library
- The API documentation of the Java standard library
If you have issues with the IDE, try reimporting the
build,
if you still have problems, use compile
in sbt instead.
Introduction
For this assignment, you will need to download the stackoverflow dataset (84 MB):
http://alaska.epfl.ch/~dockermoocs/bigdata/stackoverflow-grading.csv
and place it in the folder: src/main/resources/stackoverflow
in your
project directory.
The overall goal of this assignment is to implement a distributed k-means algorithm which clusters posts on the popular question-answer platform StackOverflow according to their score. Moreover, this clustering should be executed in parallel for different programming languages, and the results should be compared.
The motivation is as follows: StackOverflow is an important source of documentation. However, different user-provided answers may have very different ratings (based on user votes) based on their perceived value. Therefore, we would like to look at the distribution of questions and their answers. For example, how many highly-rated answers do StackOverflow users post, and how high are their scores? Are there big differences between higher-rated answers and lower-rated ones?
Finally, we are interested in comparing these distributions for different programming language communities. Differences in distributions could reflect differences in the availability of documentation. For example, StackOverflow could have better documentation for a certain library than that library's API documentation. However, to avoid invalid conclusions we will focus on the well-defined problem of clustering answers according to their scores.
The Data
You are given a CSV (comma-separated values) file with information about StackOverflow posts. Each line in the provided text file has the following format:
<postTypeId>,<id>,[<acceptedAnswer>],[<parentId>],<score>,[<tag>]
A short explanation of the comma-separated fields follows.
<postTypeId>: Type of the post. Type 1 = question,
type 2 = answer.
<id>: Unique id of the post (regardless of type).
<acceptedAnswer>: Id of the accepted answer post. This
information is optional, so maybe be missing
indicated by an empty string.
<parentId>: For an answer: id of the corresponding
question. For a question:missing, indicated
by an empty string.
<score>: The StackOverflow score (based on user
votes).
<tag>: The tag indicates the programming language
that the post is about, in case it's a
question, or missing in case it's an answer.
You will see the following code in the main class:
val lines = sc.textFile("src/main/resources/stackoverflow/stackoverflow-grading.csv")
val raw = rawPostings(lines)
val grouped = groupedPostings(raw)
val scored = scoredPostings(grouped)
val vectors = vectorPostings(scored)
It corresponds to the following steps:
-
lines
: the lines from the csv file as strings -
raw
: the raw Posting entries for each line -
grouped
: questions and answers grouped together -
scored
: questions and scores -
vectors
: pairs of (language, score) for each question
The first two methods are given to you. You will have to implement the rest.
Data processing
We will now look at how you process the data before applying the kmeans algorithm.
Grouping questions and answers
The first method you will have to implement is groupedPostings
:
val grouped = groupedPostings(raw)
In the raw
variable we have simple postings, either questions or
answers, but in order to use the data we need to assemble them together. Questions
are identified using a postTypeId == 1
. Answers to a question with
id == QID
have (a) postTypeId == 2
and (b) parentId == QID
.
Ideally, we want to obtain an RDD with the pairs of (Question, Iterable[Answer])
.
However, grouping on the question directly is expensive (can you imagine why?),
so a better alternative is to match on the QID, thus producing an
RDD[(QID, Iterable[(Question, Answer))]
.
To obtain this, in the groupedPostings
method, first filter the
questions and answers separately and then prepare them for a join
operation by
extracting the QID value in the first element of a tuple. Then, use one of the
join
operations (which one?) to obtain an RDD[(QID, (Question, Answer))]
.
Then, the last step is to obtain an RDD[(QID, Iterable[(Question, Answer)])]
.
How can you do that, what method do you use to group by the key of a pair RDD?
Finally, in the description we used QID, Question and Answer types, which we've
defined as type aliases for Posting
s and Int
s. The full list of type aliases is
available in StackOverflow.scala
:
type Question = Posting
type Answer = Posting
type QID = Int
type HighScore = Int
type LangIndex = Int
The above information should allow you to implement the groupedPostings
method. Its signature is:
def groupedPostings(postings: RDD[Posting]):
RDD[(QID, Iterable[(Question, Answer)])]
Computing Scores
Second, implement the scoredPostings
method, which should return an
RDD containing pairs of (a) questions and (b) the score of the answer with the
highest score (note: this does not have to be the answer marked as
acceptedAnswer
!). The type of this scored RDD is:
val scored: RDD[(Question, HighScore)] = ???
For example, the scored
RDD should contain the following tuples:
((1, 6, None, None, 140, Some(CSS)), 67)
((1, 42, None, None, 155, Some(PHP)), 89)
((1, 72, None, None, 16, Some(Ruby)), 3)
((1, 126, None, None, 33, Some(Java)), 30)
((1, 174, None, None, 38, Some(C#)), 20)
Hint: use the provided answerHighScore
given in scoredPostings
.
Creating vectors for clustering
Next, we prepare the input for the clustering algorithm. For this, we transform
the scored
RDD into a vectors
RDD containing the vectors
to be clustered. In our case, the vectors should be pairs with two components
(in the listed order!):
- Index of the language (in the
langs
list) multiplied by thelangSpread
factor. - The highest answer score (computed above).
The langSpread
factor is provided (set to 50000). Basically, it makes
sure posts about different programming languages have at least distance 50000
using the distance measure provided by the euclideanDist
function.
You will learn later what this distance means and why it is set to this value.
The type of the vectors
RDD is as follows:
val vectors: RDD[(LangIndex, HighScore)] = ???
For example, the vectors
RDD should contain the following tuples:
(350000, 67)
(100000, 89)
(300000, 3)
(50000, 30)
(200000, 20)
Implement this functionality in method vectorPostings
and by using
the given firstLangInTag
helper method.
(Idea for test: scored
RDD should have 1042132 entries)
Kmeans Clustering
val means = kmeans(sampleVectors(vectors), vectors)
Based on these initial means, and the provided variables converged
method, implement the K-means algorithm by iteratively:
- pairing each vector with the index of the closest mean (its cluster);
- computing the new means by averaging the values of each cluster.
To implement these iterative steps, use the provided functions
findClosest
, averageVectors
, and euclideanDistance
.
Note 1:
In our tests, convergence is reached after 44 iterations (for langSpread = 50000
)
and in 104 iterations (for langSpread = 1
), and for the first iterations the
distance kept growing. Although it may look like something is wrong, this is the
expected behavior. Having many remote points forces the kernels to shift quite a
bit and with each shift the effects ripple to other kernels, which also move around,
and so on. Be patient, in 44 iterations the distance will drop from over 100000 to
13, satisfying the convergence condition.
If you want to get the results faster, feel free to downsample the data (each iteration is faster, but it still takes around 40 steps to converge):
val scored = scoredPostings(grouped).sample(true, 0.1, 0)
However, keep in mind that we will test your assignment on the full data set. So that means you can downsample for experimentation, but make sure your algorithm works on the full data set when you submit for grading.
Note 2:
The variable langSpread
corresponds to how far away are languages from
the clustering algorithm's point of view. For a value of 50000, the languages are
too far away to be clustered together at all, resulting in a clustering that only
takes scores into account for each language (similarly to partitioning the data
across languages and then clustering based on the score). A more interesting (but
less scientific) clustering occurs when langSpread
is set to 1 (we can't
set it to 0, as it loses language information completely), where we cluster according
to the score. See which language dominates the top questions now?
Computing Cluster Details
After the call to kmeans, we have the following code in method main
:
val results = clusterResults(means, vectors)
printResults(results)
Implement the clusterResults
method, which, for each cluster, computes:
- (a) the dominant programming language in the cluster;
- (b) the percent of answers that belong to the dominant language;
- (c) the size of the cluster (the number of questions it contains);
- (d) the median of the highest answer scores.
Once this value is returned, it is printed on the screen by the printResults
method.
Questions
- Do you think that partitioning your data would help?
- Have you thought about persisting some of your data? Can you think of why persisting your data in memory may be helpful for this algorithm?
- Of the non-empty clusters, how many clusters have "Java" as their label (based on the majority of questions, see above)? Why?
- Only considering the "Java clusters", which clusters stand out and why?
- How are the "C# clusters" different compared to the "Java clusters"?
Hint: if you break the grader's time or memory constraints, think of how partitioning or persisting could, if at all, help you gain some performance. Please note that our grader only runs unit tests against your methods. It won't run the main method, so make sure to place any caching or partitioning code outside the main.