Write Hadoop jobs in JRuby
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♪ Rubydoop ♫

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If you're reading this on GitHub, please note that this is the readme for the development version and that some features described here might not yet have been released. You can find the readme for a specific version either through rubydoc.info or via the release tags (here is an example).

Rubydoop makes it possible to write Hadoop jobs in Ruby without using the streaming APIs. It configures the Hadoop runtime to run your Ruby code in an embedded JRuby runtime, and it provides a configuration DSL that's way nicer to use than Hadoop's ToolRunner.

Looking for Rubydoop, Brenden Grace's "Simple Ruby Sugar for Hadoop Streaming"? It can still be found at https://github.com/bcg/rubydoop and if you install v0.0.5 from Rubygems, you'll get that gem.

Rubydoop assumes you have some basic experience of Hadoop. The goal of Rubydoop isn't to do someting new on top of Hadoop, it's a way to use Hadoop from JRuby. Feel free to write something awesome that makes Hadoop easier to use on top of it if you like.

Rubydoop is not complete. The configuration DSL only provides the bare basics, but it should make it much easier to set up a Hadoop job compared to a vanilla Java Hadoop project. If you're looking for something where you don't have to handle the Hadoop APIs and not care about how data is encoded in writables, check out Humboldt, which builds on Rubydoop.


Rubydoop uses Bundler to determine how to package your jobs and dependencies into a JAR. The JAR will also contain a complete JRuby runtime, which requires the jruby-jars gem. Add this to your Gemfile:

gem 'rubydoop'
gem 'jruby-jars', "= #{JRUBY_VERSION}"

You also need either Hadoop installed locally or access to a Hadoop cluster to run your jobs.


TL; DR: Just look at the word count example in the examples directory.

Here's how you would implement word count with Rubydoop. First, let's sketch an outline:

module WordCount
  class Mapper
    def map(key, value, context)
      # ...

  class Reducer
    def reduce(key, value, context)
      # ...

Mappers and reducers don't have to inherit from any classes or mix in any modules, they only need to implement methods called map or reduce that takes three arguments: a key, a value (or values iterator in the case of reducers) and a context. You probably recognize these from Hadoop, and in fact Rubydoop mappers and reducers will be run exactly as a Hadoop mappers and reducers, Rubydoop only mediates between the Java side and the Ruby side. This also means that the key and value arguments are Hadoop writables and the context argument is the (mapper or reducer) context passed in by Hadoop. Hadoop has two map/reduce APIs, the old org.apache.hadoop.mapred package and the new org.apache.hadoop.mapreduce, Rubydoop uses the latter.

The mapper

Let's fill in the mapper implementation, as with all word count examples we ignore the input key since it's just the byte offset in the input file. This is a simplistic implementation that just splits on whitespace, removes all non-word characters and downcases. It outputs a one as the value. Rubydoop also aliases the most often used Hadoop classes, like the writables, and makes them easily accessible in Ruby.

module WordCount
  class Mapper
    def map(key, value, context)
      value.to_s.split.each do |word|
        word.gsub!(/\W/, '')
        unless word.empty?
          context.write(Hadoop::Io::Text.new(word), Hadoop::Io::IntWritable.new(1))

The reducer

The reducer implementation is equaly straight forward: we iterate over the values, adding up all the numbers, and output the input key and the sum.

module WordCount
  class Reducer
    def reduce(key, values, context)
      sum = 0
      values.each { |value| sum += value.get }
      context.write(key, Hadoop::Io::IntWritable.new(sum))

The job config

Ok, so let's wire this together. To do that we need to tell Rubydoop about our job. If you saved the mapper and reducer implementation in a file called lib/word_count.rb open another file and call it bin/word-count. In the new file add the following Rubydoop job config:

$LOAD_PATH << File.expand_path('../../lib', __FILE__)

require 'rubydoop'
require 'word_count'

Rubydoop.run do |input_path, output_path|
  job 'word_count' do
    input input_path
    output output_path

    mapper WordCount::Mapper
    reducer WordCount::Reducer

    output_key Hadoop::Io::Text
    output_value Hadoop::Io::IntWritable

That was a lot in one go. The first thing that happens is that we make sure that our lib directory is on the load path, then we require Rubydoop itself along with the file containing the mapper and reducer implementations.

Because of how Rubydoop packages your code to be run in Hadoop it's important that you do not require 'bundler' or require 'bundler/setup' or anything that references Bundler. Bundler will not be available when your code runs.

The next thing is a call to Rubydoop.run. This method takes a block which will be used to define one or more jobs to run. Each job has input, output, a mapper and a reducer, and often some more configuration to control other aspects of how Hadoop should run it.

You can include code before Rubydoop.run, but you should be aware that it will run on both the master and worker nodes. You can't include code after Rubydoop.run, Rubydoop will call exit when all jobs have run.

The arguments to the block are the command line arguments given when telling Hadoop to run our jobs – we'll get to these arguments later, but at this point you should know that there's nothing magic about input_path and output_path, Rubydoop just yields all the arguments given on the command line to the block so it's up to you how to interpret them.

Now finally to the job configuration. You can specify as many jobs as you want, but word count is simple enough to only need one. The things you can specify using the job DSL are the things you would configure in your main method (or run when using Hadoop's ToolRunner):

  • The input and output are aliases for TextInputFormat.setInputPaths (the argument should be a comma-separated list of paths) and TextOutputFormat.setOutputPath (or if you want to use another input/output format just pass :format => XyzFormat as an option to input or output).
  • The mapper and reducer are self-explanatory, and there's also a combiner to set the combiner, just like in Hadoop.
  • The output_key and output_value tells Hadoop what output to expect from the mapper and reducer. This needs to be set correctly otherwise Hadoop will complain. If the mapper's output doesn't match the reducer's you can specify the mapper's separately with map_output_key and map_output_value.
  • You can also use set 'property.name', 'value' to set properties, or raw { |job| ... } to access the raw Job instance.
  • You can control the partitioning and sorting with partitioner, group_comparator and sort_comparator.

Job dependencies and parallel jobs

By default all jobs are run sequentially. This makes it easy to define pipelines of map/reduce jobs where the output of one job is the input of another.

For applications with many jobs you might want to run some of them in parallel, and some in sequence. For this you can group your jobs together with parallel and sequential, like this (the contents of the job blocks are left out to make it easier to follow the example):

Rubydoop.run do |…|
  job 'first' do

  parallel do
    job 'second' do

    sequential do
      job 'third' do

      job 'fourth' do

  job 'fifth' do

Because the run block acts as an implicit sequential, with this config the job first will run, then second will run in parallel with third and fourth. fourth will wait for third to complete before it starts. Finally fifth will run when third and fifth have completed.

You can nest sequential and parallel to any depth, which should make it possible to describe any directed acyclic graph of jobs.

The benefit of running independent jobs in parallel is that many times it leads to better use of the cluster's resources, but it all depends on the cluster and the workload.

Packing it up

The final step before running the job is packing it up for Hadoop. Rubydoop provides the Rubydoop::Package class to do this, and a suitable place to put the necessary code is in a Rakefile:

require 'rubydoop/package'

task :package do

Unless you hate using defaults that's all you need to do (most of the defaults can be changed, so don't worry). When you run rake package it will create a JAR file in a directory called build. The JAR will be named after the directory that the Rakefile is in (it assumes this is your project directory).

The packaging is done with Puck, which creates JAR files that are standalone JRuby applications that include a full JRuby runtime, all gems and the application code. Rubydoop applications aren't completely standalone and can't be run with java -jar … as they require Hadoop, but they contain everything needed to run them with hadoop jar … – Hadoop won't even know that it's running Ruby code.

As mentioned above, it's important that your code does not reference Bundler at runtime (for example with require 'bundler/setup'), because Bundler is not included in the JAR file.

Running it, finally

Now when we have a JAR we can run it with Hadoop. Assuming you have Hadoop set up already you only need to submit the job using the hadoop command, like this:

$ hadoop jar build/word_count.jar word-count -config hadoop-config.xml path/to/input path/to/output

The only surprise there is the word-count argument. That's the name of the file with the Rudoop.run block. Think of this argument as the equivalent of the main-class argument you have to give Hadoop if your JAR's manifest doesn't specify it. If that last sentence doesn't make sense, just ignore it, the important thing is that you need to tell Rubydoop which job configuration you want to run. Why couldn't this be set when you packaged the JAR? Doing it this way makes it possible to pack multiple configurations into the same JAR. In the future there might be a way to choose, just like you can with a normal Hadoop job: either you specifiy the main-class when you create the JAR, or you have to pass it on the command line.

You can pass any other ToolRunner arguments like -config if you want (as long as you pass them after the job file name, e.g. word-count), but the rest of the command line arguments will end up as arguments to the Rubydoop.run block, as mentioned before.

Running the tests and building from source

Rubydoop requires Hadoop, RVM, and Bundler to be installed to compile from source.

Run the tests like this:

$ bundle install
$ rake setup
$ rake spec

The two first commands are one-time setup steps. The third will build the Java extensions before running the tests, but if you want to just build run rake build.

Should these instructions not work check the .travis.yml file and see how it runs the tests.


Copyright 2012-2015 Theo Hultberg and contributors

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License You may obtain a copy of the License at


Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.