Getting Started on Hadoop

Getting Started on Hadoop

Silicon Valley Cloud Computing Meetup
Mountain View, 2010-07-19
http://www.meetup.com/cloudcomputing/calendar/13911740/

Paco Nathan
@pacoid
http://ceteri.blogspot.com/

Examples of Hadoop Streaming, based on Python scripts
running on the AWS Elastic MapReduce service.

• first, a brief history…
• AWS Elastic MapReduce
• “WordCount” example as “Hello World” for MapReduce
• text mining Enron Email Dataset from Infochimps.com
• inverted index, semantic lexicon, social graph
• data visualization using R and Gephi

All source code for this talk is available at:

http://github.com/ceteri/ceteri-mapred

How Does MapReduce Work?

map(k1, v1) → list(k2, v2)
reduce(k2, list(v2)) → list(v3)

Several phases, which partition a problem into many tasks:
• load data into DFS…
• map phase: input split → (key, value) pairs, with optional combiner
• shuffle phase: sort on keys to group pairs… load-test your network!
• reduce phase: each task receives the values for one key
• pull data from DFS…
NB: “map” phase is required, the rest are optional.
Think of set operations on tuples (and check out Cascading.org).
Meanwhile, given all those (key, value) pairs listed above, it’s no
wonder that key/value stores have become such a popular topic of
conversation…

How Does MapReduce Work?

map(k1, v1) → list(k2, v2)
reduce(k2, list(v2)) → list(v3)

The property of data independence among tasks allows for highly
parallel processing… maybe, if the stars are all aligned :)

Primarily, a MapReduce framework is largely about fault tolerance, and
how to leverage “commodity hardware” to replace “big iron” solutions…

That phrase “big iron” might apply to Oracle + NetApp. Or perhaps an
IBM zSeries mainframe… Or something – expensive, undoubtably.

Bonus questions for self-admitted math geeks: Foresee any concerns
about O(n) complexity, given the functional definitions listed above?
Keep in mind that each phase cannot conclude and progress to the
next phase until after each of its tasks has successfully completed.

A Brief History…

circa 1979 – Stanford, MIT, CMU, etc.
set/list operations in LISP, Prolog, etc., for parallel processing
http://www-formal.stanford.edu/jmc/history/lisp/lisp.htm

circa 2004 – Google
MapReduce: Simplified Data Processing on Large Clusters
Jeffrey Dean and Sanjay Ghemawat
http://labs.google.com/papers/mapreduce.html

circa 2006 – Apache
Hadoop, originating from the Nutch Project
Doug Cutting
http://research.yahoo.com/files/cutting.pdf

circa 2008 – Yahoo
web scale search indexing
Hadoop Summit, HUG, etc.
http://developer.yahoo.com/hadoop/

circa 2009 – Amazon AWS
Elastic MapReduce
Hadoop modified for EC2/S3, plus support for Hive, Pig, etc.
http://aws.amazon.com/elasticmapreduce/

Why run Hadoop in AWS?

• elastic: batch jobs on clusters can consume many nodes,
scalable demand, not 24/7 – great case for using EC2
• commodity hardware: MR is built for fault tolerance, great
case for leveraging AMIs
• right-sizing: difficult to know a priori how large of a cluster
is needed – without running significant jobs (test k/v skew,
data quality, etc.)
• when your input data is already in S3, SDB, EBS, RDS…
• when your output needs to be consumed in AWS …
You really don't want to buy rack space in a datacenter before
assessing these issues – besides, a private datacenter probably
won’t even be cost-effective afterward.

But why run Hadoop on Elastic MapReduce?

• virtualization: Hadoop needs some mods to run well in
that kind of environment
• pay-per-drink: absorbs cost of launching nodes
• secret sauce: Cluster Compute Instances (CCI) and
Spot Instances (SI)
• DevOps: EMR job flow mgmt optimizes where your staff
spends their (limited) time+capital
• logging to S3, works wonders for troubleshooting

A Tale of Two Ventures…

Adknowledge: in 2008, our team became one of the larger
use cases running Hadoop on AWS
• prior to the launch of EMR
• launching clusters of up to 100 m1.xlarge
• initially 12 hrs/day, optimized down to 4 hrs/day
• displaced $3MM capex for Netezza

ShareThis: in 2009, our team used even more Hadoop
on AWS than that previous team
• this time with EMR
• larger/more frequent jobs
• lower batch failure rate
• faster turnaround on results
• excellent support
• smaller team required
• much less budget

“WordCount”, a “Hello World” for MapReduce

Definition: count how often each word appears within a collection
of text documents.

A simple program which illustrates a pretty good test case for what
MapReduce can perform, since it incorporates:

• minimal amount code
• document feature extraction (where words are “terms”)
• symbolic and numeric values
• potential use of a combiner
• bipartite graph of (doc, term) tuples
• not so many steps away from useful indexing…
When a framework can run “WordCount” in parallel at scale, then it
can handle much larger, more interesting compute problems as well.

Bipartite Graph

Wikipedia: “…a bipartite graph is a graph whose vertices can be divided
into two disjoint sets U and V such that every edge connects a vertex in U
to one in V… ”

http://en.wikipedia.org/wiki/Bipartite_graph

Consider the case where:

U ≡ { documents }

V ≡ { terms }

Many kinds of text analytics products
can be constructed based on this
data structure as a foundation.

“WordCount”, in other words…

map(doc_id, text)
→ list(word, count)

reduce(word, list(count))
→ list(sum_count)


void map (String doc_id, String text):
for each word w in segment(text):
emitPartial(w, "1");

void reduce (String word, Iterator partial_counts):
int count = 0;

for each pc in partial_counts:
count += Int(pc);

emitResult(String(count));

Hadoop Streaming

One way to approach MapReduce jobs in Hadoop is to use streaming.
In other words, use any kind of script which can be run from a command
line and read/write data via stdin and stdout:
http://hadoop.apache.org/common/docs/current/streaming.html#Hadoop+Streaming

The following examples use Python scripts for Hadoop Streaming. One
really great benefit is that then you can dev/test/debug your MapReduce
code on small data sets from a command line simply by using pipes:

cat input.txt | mapper.py | sort | reducer.py

BTW, there are much better ways to handle Hadoop Streaming in Python
on Elastic MapReduce – for example, using the “boto” library. However,
these examples are kept simple so they’ll fit into a tech talk!


# this Linux command line...
cat foo.txt | map_wc.py | sort | red_wc.py

# produces output like this...
tuple

9
term

6
tfidf

6
sort

5
analysis
2
wordcount
1
user

1

# depending on input -
# which could be HTML content, tweets, email, etc.

Speaking of Email…

Enron pioneered innovative corporate accounting methods and energy market
manipulations, involving a baffling array of fraud techniques. The firm soared to
a valuation of over $60B (growing 56% in 1999, 87% in 2000) while inducing a
state of emergency in California – which cost the state over $40B. Subsequent
prosecution of top execs plus the meteoric decline in the firm’s 2001 share value
made for a spectacular #EPIC #FAIL
http://en.wikipedia.org/wiki/Enron_scandal
http://en.wikipedia.org/wiki/California_electricity_crisis

Thanks to CALO and Infochimps, we have a half million email messages
collected from Enron managers during their, um, “heyday” period:
http://infochimps.org/datasets/enron-email-dataset--2
http://www.cs.cmu.edu/~enron/

Let’s use Hadoop to help find out: what were some of
the things those managers were talking about?

Simple Text Analytics

Extending from how “WordCount” works, we’ll add multiple kinds of output
tuples, plus two stages of mappers and reducers, to generate different kinds
of text analytics products:

• inverted index
• co-occurrence analysis
• TF-IDF filter
• social graph

While doing that, we'll also perform other statistical
analysis and data visualization using R and Gephi

Mapper 1: RFC822 Parser

map_parse.py takes a list of URI for where to read email messages, parses
each message, then emits multiple kinds of output tuples:

(doc_id, msg_uri, date)

(sender, receiver, doc_id)

(term, term_freq, doc_id)

(term, co_term, doc_id)

Note that our dataset includes approximately 500,000 email messages, with an
average of about 100 words in each message.
Also, there are 10E+5 unique terms. That will tend to be a constant in English
texts, which is great to know when configuring capacity.

Reducer 1: TF-IDF and Co-Occurrence

red_idf.py takes the shuffled output from map_parse.py, collects metadata
for each term, calculates TF-IDF to use in a later stage for filtering, calculates
co-occurrence probability, then emits all these results:

(doc_id, msg_uri, date)

(sender, receiver, doc_id)

(term, idf, count)

(term, co_term, prob_cooc)

(term, tfidf, doc_id)

(term, max_tfidf)

Mapper 2 + Reducer 2: Threshold Filter

map_filter.py and red_filter.py apply a threshold (based on statistical
analysis of TF-IDF) to filter results of co-occurrence analysis so that we begin to
produce a semantic lexicon for exploring the data set.

How do we determine a reasonable value for the TF-IDF threshold, for filtering
terms? Sampling from the (term, max_tfidf) tuple, we run summary stats and
visualization in R:

cat dat.idf | util_extract.py m > thresh.tsv

We also convert the sender/receiver social graph into CSV format for Gephi
visualization:

cat dat.parsed | util_extract.py s | util_gephi.py | sort -u > graph.csv

Elastic MapReduce Job Flows…

Elastic MapReduce Output Part Files…

Exporting Data into Other Tools…

Using R to Determine a Threshold…

data <- read.csv("thresh.tsv", sep='t', header=F)
t_data <- data[,3]
print(summary(t_data))

# pass through values for 80+ percentile
qntile <- .8
t_thresh <- quantile(t_data, qntile)

# CDF plot
title <- "CDF threshold max(tfidf)"
xtitle <- paste("thresh:", t_thresh)
par(mfrow=c(2, 1))
plot(ecdf(t_data), xlab=xtitle, main=title)
abline(v=t_thresh, col="red")
abline(h=qtile, col="yellow")

# box-and-whisker plot
boxplot(t_data, horizontal=TRUE)
rug(t_data, side=1)

Using R to Determine a Threshold…
CDF threshold max(tfidf)
0.8
Fn(x)

0.4
0.0

0 2 4 6 8

thresh: 0.063252

0 1 2 3 4 5 6 7

Using Gephi to Explore the Social Graph…

Best Practices

• Again, there are much more efficient ways to handle Hadoop Streaming
and Text Analytics…
• Unit Tests, Continuous Integration, etc., – all great stuff, but “Big Data”
software engineering requires additional steps
• Sample data, measure data ratios and cluster behaviors, analyze in R,
visualize everything you can, calibrate any necessary “magic numbers”
• Develop and test code on a personal computer in IDE, cmd line, etc., using
a minimal data sets
• Deploy to staging cluster with larger data sets for integration tests and QA
• Run in production with A/B testing were feasible to evaluate changes
quantitatively
• Learn from others at meetups, unconfs, forums, etc.

Great Resources for Diving into Hadoop

Google: Cluster Computing and MapReduce Lectures
http://code.google.com/edu/submissions/mapreduce-minilecture/listing.html

Amazon AWS Elastic MapReduce
http://aws.amazon.com/elasticmapreduce/

Hadoop: The Definitive Guide, by Tom White
http://oreilly.com/catalog/9780596521981

Apache Hadoop
http://hadoop.apache.org/

Python “boto” interface to EMR
http://boto.cloudhackers.com/emr_tut.html

Excellent Products for Hadoop in Production

Datameer
http://www.datameer.com/
“Democratizing Big Data”

Designed for business users, Datameer Analytics Solution (DAS) builds
on the power and scalability of Apache Hadoop to deliver an easy-to-use
and cost-effective solution for big data analytics. The solution integrates
rapidly with existing and new data sources to deliver sophisticated
analytics.

Cascading
http://www.cascading.org/

Cascading is a feature-rich API for defining and executing complex,
scale-free, and fault tolerant data processing workflows on a Hadoop
cluster, which provides a thin Java library that sits on top of Hadoop's
MapReduce layer. Open source in Java.

Scale Unlimited – Hadoop Boot Camp

Santa Clara, 22-23 July 2010
http://www.scaleunlimited.com/courses/hadoop-bootcamp-santaclara

• An extensive overview of the Hadoop architecture
• Theory and practice of solving large scale data processing problems
• Hands-on labs covering Hadoop installation, development, debugging
• Common and advanced “Big Data” tasks and solutions
Special $500 discount for SVCC Meetup members:
http://hadoopbootcamp.eventbrite.com/?discount=DBDatameer

Sample material – list of questions about intro Hadoop from
the recent BigDataCamp:
http://www.scaleunlimited.com/blog/intro-to-hadoop-at-bigdatacamp

Getting Started on Hadoop

Silicon Valley Cloud Computing Meetup
Mountain View, 2010-07-19
http://www.meetup.com/cloudcomputing/calendar/13911740/

Paco Nathan
@pacoid
http://ceteri.blogspot.com/

Examples of Hadoop Streaming, based on Python scripts
running on the AWS Elastic MapReduce service.

All source code for this talk is available at:

http://github.com/ceteri/ceteri-mapred

Getting Started on Hadoop

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