A Simple Example R MapReduce Job

The following example in R performs MapReduce on a large input corpus and counts the number of times each word occurs in the input.

Create the bootstrap script

The following script will download and install the latest version of R on each of your Elastic MapReduce hosts. (The default version of R is very old.)

Name this script bootstrapLatestR.sh and it should contain the following code:

#!/bin/bash

###########################################
# How to get the latest version of R onto Elastic MapReduce
# http://www.r-bloggers.com/bootstrapping-the-latest-r-into-amazon-elastic-map-reduce/
###########################################

# Perform the debian R upgrade
echo "deb http://cran.fhcrc.org/bin/linux/debian lenny-cran/" | sudo tee -a /etc/apt/sources.list
sudo apt-get update
sudo apt-get -t lenny-cran install --yes --force-yes r-base r-base-dev

Create the mapper script

The following script will output each word found in the input passed line by line to STDIN with its count of 1.

Name this script mapper.R and it should contain the following code:

#!/usr/bin/env Rscript

trimWhiteSpace <- function(line) gsub("(^ +)|( +$)", "", line)
splitIntoWords <- function(line) unlist(strsplit(line, "[[:space:]]+"))
    
con <- file("stdin", open = "r")
while (length(line <- readLines(con, n = 1, warn = FALSE)) > 0) {
    line <- trimWhiteSpace(line)
    words <- splitIntoWords(line)
    ## can also be done as cat(paste(words, "\t1\n", sep=""), sep="")
    for (w in words)
        cat(w, "\t1\n", sep="")
}

close(con)

Create the reducer script

The following script will aggregate the count for each word found and output the final results.

Name this script reducer.R and it should contain the following code:

#!/usr/bin/env Rscript

trimWhiteSpace <- function(line) gsub("(^ +)|( +$)", "", line)

splitLine <- function(line) {
    val <- unlist(strsplit(line, "\t"))
    list(word = val[1], count = as.integer(val[2]))
}
    
env <- new.env(hash = TRUE)

con <- file("stdin", open = "r")
while (length(line <- readLines(con, n = 1, warn = FALSE)) > 0) {
    line <- trimWhiteSpace(line)
    split <- splitLine(line)
    word <- split$word
    count <- split$count
    if (exists(word, envir = env, inherits = FALSE)) {
        oldcount <- get(word, envir = env)
        assign(word, oldcount + count, envir = env)
    }
    else assign(word, count, envir = env)
}
close(con)

for (w in ls(env, all = TRUE))
    cat(w, "\t", get(w, envir = env), "\n", sep = "")

Create a small input file for testing

Name this file AnInputFile.txt and it should contain the following text:

Jack and Jill went up the hill
To fetch a pail of water.
Jack fell down and broke his crown,
And Jill came tumbling after.
Up Jack got, and home did trot,
As fast as he could caper,
To old Dame Dob, who patched his nob
With vinegar and brown paper.

Sanity check -> run it locally

The command line to run it

~>cat AnInputFile.txt | ./mapper.R | sort | ./reducer.R 
a       1
after.  1
and     4
And     1
as      1
As      1
broke   1
brown   1
...

How to run it on Elastic MapReduce

./elastic-mapreduce --create --stream \
--bootstrap-action s3://YOUR_BUCKET/scripts/bootstrapLatestR.sh \
--mapper s3://YOUR_BUCKET/scripts/mapper.R \
--reducer s3://YOUR_BUCKET/scripts/reducer.R \
--input s3://YOUR_BUCKET/input/AnInputFile.txt \
--output s3://YOUR_BUCKET/output/try1 \
--name try1

Word Count In R