Ruby-Spark

Apache Spark™ is a fast and general engine for large-scale data processing.

This Gem allows the use Spark functionality on Ruby.

Word count in Spark's Ruby API

file = spark.text_file("hdfs://...")

file.flat_map(:split)
    .map(lambda{|word| [word, 1]})
    .reduce_by_key(lambda{|a, b| a+b})

• Apache Spark
• Wiki
• Rubydoc

Installation

Requirments

• Java 7+
• Ruby 2+
• wget or curl
• MRI or JRuby

Add this line to your application's Gemfile:

gem 'ruby-spark'

And then execute:

$ bundle

Or install it yourself as:

$ gem install ruby-spark

Run rake compile if you are using gem from local filesystem.

Build Apache Spark

This command will download Spark and build extensions for this gem (SBT is used for compiling). For more informations check wiki. Jars will be stored at you HOME directory.

$ ruby-spark build

Usage

You can use Ruby Spark via interactive shell (Pry is used)

$ ruby-spark shell

Or on existing project.

If you want configure Spark first. See configurations for more details.

require 'ruby-spark'

# Configuration
Spark.config do
   set_app_name "RubySpark"
   set 'spark.ruby.serializer', 'oj'
   set 'spark.ruby.serializer.batch_size', 100
end

# Start Apache Spark
Spark.start

# Context reference
Spark.sc

Finally, to stop the cluster. On the shell is Spark stopped automatically when environment exit.

Spark.stop

After first use, global configuration is created at ~/.ruby-spark.conf. There can be specified properties for Spark and RubySpark.

Creating RDD (a new collection)

Single text file:

rdd = sc.text_file(FILE, workers_num, serializer=nil)

All files on directory:

rdd = sc.whole_text_files(DIRECTORY, workers_num, serializer=nil)

Direct uploading structures from ruby:

rdd = sc.parallelize([1,2,3,4,5], workers_num, serializer=nil)
rdd = sc.parallelize(1..5, workers_num, serializer=nil)

There is 2 conditions:

1. choosen serializer must be able to serialize it
2. data must be iterable

If you do not specified serializer -> default is used (defined from spark.ruby.serializer.* options). Check this if you want create custom serializer.

Operations

All operations can be divided into 2 groups:

• Transformations: append new operation to current RDD and return new
• Actions: add operation and start calculations

More informations:

• Wiki page
• Rubydoc
• rdd.rb

You can also check official Spark documentation. First make sure that method is implemented here.

• Transformations
• Actions

Transformations

rdd.map(function)

Return a new RDD by applying a function to all elements of this RDD.

rdd.flat_map(function)

Return a new RDD by first applying a function to all elements of this RDD, and then flattening the results.

rdd.map_partitions(function)

Return a new RDD by applying a function to each partition of this RDD.

rdd.filter(function)

Return a new RDD containing only the elements that satisfy a predicate.

rdd.cartesian(other)

Return the Cartesian product of this RDD and another one, that is, the RDD of all pairs of elements `(a, b)` where `a` is in `self` and `b` is in `other`.

rdd.intersection(other)

Return the intersection of this RDD and another one. The output will not contain any duplicate elements, even if the input RDDs did.

rdd.sample(with_replacement, fraction, seed)

Return a sampled subset of this RDD. Operations are base on Poisson and Uniform distributions.

rdd.group_by_key(num_partitions)

Group the values for each key in the RDD into a single sequence.

...many more...

Actions

rdd.take(count)

Take the first num elements of the RDD.

rdd.reduce(function)

Reduces the elements of this RDD using the specified lambda or method.

rdd.aggregate(zero_value, seq_op, comb_op)

Aggregate the elements of each partition, and then the results for all the partitions, using given combine functions and a neutral “zero value”.

rdd.histogram(buckets)

Compute a histogram using the provided buckets.

rdd.collect

Return an array that contains all of the elements in this RDD.

...many more...

Examples

Basic methods

# Every batch will be serialized by Marshal and will have size 10
ser = Spark::Serializer.build('batched(marshal, 10)')

# Range 0..100, 2 workers, custom serializer
rdd = Spark.sc.parallelize(0..100, 2, ser)

# Take first 5 items
rdd.take(5)
# => [0, 1, 2, 3, 4]

# Numbers reducing
rdd.reduce(lambda{|sum, x| sum+x})
rdd.reduce(:+)
rdd.sum
# => 5050

# Reducing with zero items
seq = lambda{|x,y| x+y}
com = lambda{|x,y| x*y}
rdd.aggregate(1, seq, com)
# 1. Every workers adds numbers
#    => [1226, 3826]
# 2. Results are multiplied
#    => 4690676

# Statistic method
rdd.stats
# => StatCounter: (count, mean, max, min, variance,
#                  sample_variance, stdev, sample_stdev)

# Compute a histogram using the provided buckets.
rdd.histogram(2)
# => [[0.0, 50.0, 100], [50, 51]]

# Mapping
rdd.map(lambda {|x| x*2}).collect
# => [0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, ...]
rdd.map(:to_f).collect
# => [0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, ...]

# Mapping the whole collection
rdd.map_partitions(lambda{|part| part.reduce(:+)}).collect
# => [1225, 3825]

# Selecting
rdd.filter(lambda{|x| x.even?}).collect
# => [0, 2, 4, 6, 8, 10, 12, 14, 16, ...]

# Sampling
rdd.sample(true, 10).collect
# => [3, 36, 40, 54, 58, 82, 86, 95, 98]

# Sampling X items
rdd.take_sample(true, 10)
# => [53, 87, 71, 74, 18, 75, 55, 94, 46, 32]

# Using external process
rdd.pipe('cat', "awk '{print $1*10}'")
# => ["0", "10", "20", "30", "40", "50", ...]

Words count using methods

# Content:
# "first line"
# "second line"
rdd = sc.text_file(PATH)

# ["first", "line", "second", "line"]
rdd = rdd.flat_map(lambda{|line| line.split})

# [["first", 1], ["line", 1], ["second", 1], ["line", 1]]
rdd = rdd.map(lambda{|word| [word, 1]})

# [["first", 1], ["line", 2], ["second", 1]]
rdd = rdd.reduce_by_key(lambda{|a, b| a+b})

# {"first"=>1, "line"=>2, "second"=>1}
rdd.collect_as_hash

Estimating PI with a custom serializer

slices = 3
n = 100000 * slices

def map(_)
  x = rand * 2 - 1
  y = rand * 2 - 1

  if x**2 + y**2 < 1
    return 1
  else
    return 0
  end
end

rdd = Spark.context.parallelize(1..n, slices, serializer: 'oj')
rdd = rdd.map(method(:map))

puts 'Pi is roughly %f' % (4.0 * rdd.sum / n)

Estimating PI

rdd = sc.parallelize([10_000], 1)
rdd = rdd.add_library('bigdecimal/math')
rdd = rdd.map(lambda{|x| BigMath.PI(x)})
rdd.collect # => #<BigDecimal, '0.31415926...'>

Mllib (Machine Learning Library)

Mllib functions are using Spark's Machine Learning Library. Ruby objects are serialized and deserialized in Java so you cannot use custom classes. Supported are primitive types such as string or integers.

All supported methods/models:

• Rubydoc / Mllib
• Github / Mllib

Linear regression

# Import Mllib classes into Object
# Otherwise are accessible via Spark::Mllib::LinearRegressionWithSGD
Spark::Mllib.import(Object)

# Training data
data = [
  LabeledPoint.new(0.0, [0.0]),
  LabeledPoint.new(1.0, [1.0]),
  LabeledPoint.new(3.0, [2.0]),
  LabeledPoint.new(2.0, [3.0])
]

# Train a model
lrm = LinearRegressionWithSGD.train(sc.parallelize(data), initial_weights: [1.0])

lrm.predict([0.0])

K-Mean

Spark::Mllib.import

# Dense vectors
data = [
  DenseVector.new([0.0,0.0]),
  DenseVector.new([1.0,1.0]),
  DenseVector.new([9.0,8.0]),
  DenseVector.new([8.0,9.0])
]

model = KMeans.train(sc.parallelize(data), 2)

model.predict([0.0, 0.0]) == model.predict([1.0, 1.0])
# => true
model.predict([8.0, 9.0]) == model.predict([9.0, 8.0])
# => true

Benchmarks