Scalding

Scalding is a Scala library that makes it easy to specify Hadoop MapReduce jobs. Scalding is built on top of Cascading, a Java library that abstracts away low-level Hadoop details. Scalding is comparable to Pig, but offers tight integration with Scala, bringing advantages of Scala to your MapReduce jobs.

Current version: 0.8.5

Word Count

Hadoop is a distributed system for counting words. Here is how it's done in Scalding.

package com.twitter.scalding.examples

import com.twitter.scalding._

class WordCountJob(args : Args) extends Job(args) {
  TextLine( args("input") )
    .flatMap('line -> 'word) { line : String => tokenize(line) }
    .groupBy('word) { _.size }
    .write( Tsv( args("output") ) )

  // Split a piece of text into individual words.
  def tokenize(text : String) : Array[String] = {
    // Lowercase each word and remove punctuation.
    text.toLowerCase.replaceAll("[^a-zA-Z0-9\\s]", "").split("\\s+")
  }
}

Notice that the tokenize function, which is standard Scala, integrates naturally with the rest of the MapReduce job. This is a very powerful feature of Scalding. (Compare it to the use of UDFs in Pig.)

You can find more example code under examples/. If you're interested in comparing Scalding to other languages, see our Rosetta Code page, which has several MapReduce tasks in Scalding and other frameworks (e.g., Pig and Hadoop Streaming).

Documentation and Getting Started

• Getting Started page on the Scalding Wiki
• Runnable tutorials in the source.
• The API Reference, including many example Scalding snippets:
  • Fields-based API Reference
  • Type-safe API Reference
• Scalding Scaladocs provide details beyond the API References
• The Matrix Library provides a way of working with key-attribute-value scalding pipes:
  • The Introduction to Matrix Library contains an overview and a "getting started" example
  • The Matrix API Reference contains the Matrix Library API reference with examples

Building

0. Install sbt 0.12.2 (sorry, but the assembly plugin is sbt version dependent).
1. sbt update (takes 2 minutes or more)
2. sbt test
3. sbt assembly (needed to make the jar used by the scald.rb script)

The test suite takes a while to run. When you're in sbt, here's a shortcut to run just one test:

> test-only com.twitter.scalding.FileSourceTest

Please refer to FAQ page if you encounter problems when using sbt.

We use Travis CI to verify the build:

Scalding modules are available from maven central.

The current groupid and version for all modules is, respectively, "com.twitter" and 0.8.5.

Current published artifacts are

• scalding-core_2.9.2
• scalding-core_2.10
• scalding-args_2.9.2
• scalding-args_2.10
• scalding-date_2.9.2
• scalding-date_2.10

The suffix denotes the scala version.

Contact

Currently we are using the cascading-user mailing list for discussions: http://groups.google.com/group/cascading-user

In the remote possibility that there exist bugs in this code, please report them to: https://github.com/twitter/scalding/issues

Follow @Scalding on Twitter for updates.

Authors:

• Avi Bryant http://twitter.com/avibryant
• Oscar Boykin http://twitter.com/posco
• Argyris Zymnis http://twitter.com/argyris

Thanks for assistance and contributions:

• Sam Ritchie http://twitter.com/sritchie
• Aaron Siegel: http://twitter.com/asiegel
• Brad Greenlee: http://twitter.com/bgreenlee
• Edwin Chen http://twitter.com/edchedch
• Arkajit Dey: http://twitter.com/arkajit
• Krishnan Raman: http://twitter.com/dxbydt_jasq
• Flavian Vasile http://twitter.com/flavianv
• Chris Wensel http://twitter.com/cwensel
• Ning Liang http://twitter.com/ningliang
• Dmitriy Ryaboy http://twitter.com/squarecog
• Dong Wang http://twitter.com/dongwang218
• Josh Attenberg http://twitter.com/jattenberg
• Kevin Lin http://twitter.com/reconditesea

License

Copyright 2013 Twitter, Inc.

Licensed under the Apache License, Version 2.0: http://www.apache.org/licenses/LICENSE-2.0