Using Scala

In order to use Spark in Scala, environment has to bee set up according to Using Spark in UFAL Environment (including the sbt).

Interactive shell can be started using:

spark-shell

As described in Running Spark on Single Machine or on Cluster, environmental variable MASTER specifies which Spark master to use (or whether to start a local one).

Consider the following simple script computing 10 most frequent words of Czech Wikipedia:

(sc.textFile("/net/projects/spark-example-data/wiki-cs", 3*sc.defaultParallelism)
   .flatMap(_.split("\\s"))
   .map((_,1)).reduceByKey(_+_)
   .sortBy(_._2, ascending=false)
   .take(10))

• run interactive shell using existing Spark cluster (i.e., inside spark-srun), or start local Spark cluster using as many threads as there are cores if there is none:

spark-shell

• run interactive shell with local Spark cluster using one thread:

MASTER=local spark-shell

• start Spark cluster (10 machines, 2GB RAM each) via Slurm and run interactive shell:

spark-srun 10 2G spark-shell

Compiled Scala Spark program (JAR) can be started using:

spark-submit

As described in Running Spark on Single Machine or on Cluster, environmental variable MASTER specifies which Spark master to use (or whether to start a local one).

If you do not know how to compile Scala programs, you can use the following directions:

1. create a directory for your project
2. copy /net/projects/spark/sbt/spark-template.sbt to your project directory and rename it to your project name (i.e., my-best-project.sbt)
3. replace the spark-template by your project name in the first line (i.e., name := “my-best-project”)
4. run sbt package to create JAR (note that first run of sbt will take several minutes)

The resulting JAR can be found in target/scala-2.11 subdirectory, named after your project.

Consider the following simple word-count application word_count.scala:

import org.apache.spark.SparkContext
import org.apache.spark.SparkContext._
 
object Main {
  def main(args: Array[String]) {
    if (args.length < 2) sys.error("Usage: input output")
    val (input, output) = (args(0), args(1))
 
    val sc = new SparkContext()
    sc.textFile(input, 3*sc.defaultParallelism)
      .flatMap(_.split("\\s"))
      .map((_,1)).reduceByKey(_+_)
      .sortBy(_._2, ascending=false)
      .saveAsTextFile(output)
    sc.stop()
  }
}

The sbt project file word_count.sbt:

name := "word_count"

version := "1.0"

scalaVersion := "2.12.20"
 
libraryDependencies += "org.apache.spark" %% "spark-core" % "3.5.3"

• compile the application

sbt package

• run word_count application inside existing Spark cluster (i.e., inside spark-sbatch or spark-srun), or start local Spark cluster using as many threads as there are cores if there is none:

spark-submit target/scala-2.12/word_count_2.12-1.0.jar /net/projects/spark-example-data/wiki-cs outdir

• run word_count application with local Spark cluster using one thread:

MASTER=local spark-submit target/scala-2.12/word_count_2.12-1.0.jar /net/projects/spark-example-data/wiki-cs outdir

• start Spark cluster (10 machines, 2GB RAM each) on Slurm and run word_count application:

spark-sbatch 10 2G spark-submit target/scala-2.12/word_count_2.12-1.0.jar /net/projects/spark-example-data/wiki-cs outdir