Experimenting with the Spark shell

The best way to learn Spark is through the Spark shell. There are two different shells for Scala and Python. But since the GraphX library is the most complete in Scala at the time this book was written, we are going to use the spark-shell, that is, the Scala shell. Let's launch the Spark shell inside the $SPARKHOME/bin from the command line:

$SPARKHOME/bin/spark-shell

If you set the current directory (cd) to $SPARKHOME, you can simply launch the shell with:

cd $SPARKHOME
./bin/spark-shell

If you were successful in launching the Spark shell, you should see the welcome message like this:

 Welcome to
 ____ __
 / __/__ ___ _____/ /__
 _\ \/ _ \/ _ '/ __/ '_/
 /___/ .__/\_,_/_/ /_/\_\ version 1.4.1
 /_/
Using Scala version 2.10.4 (Java HotSpot(TM) 64-Bit Server VM, Java)

For a sanity check, you can type in some Scala expressions or declarations and have them evaluated. Let's type some commands into the shell now:

scala> sc
res1: org.apache.spark.SparkContext = org.apache.spark.SparkContext@52e52233
scala> val myRDD = sc.parallelize(List(1,2,3,4,5))
myRDD: org.apache.spark.rdd.RDD[Int] = ParallelCollectionRDD[0] at parallelize at <console>:12
scala> sc.textFile("README.md").filter(line => line contains "Spark").count()
res2: Long = 21

Here is what you can tell about the preceding code. First, we displayed the Spark context defined by the variable sc, which is automatically created when you launch the Spark shell. The Spark context is the point of entry to the Spark API. Second, we created an RDD named myRDD that was obtained by calling the parallelize function for a list of five numbers. Finally, we loaded the README.md file into an RDD, filtered the lines that contain the word "Spark", and finally invoked an action on the filtered RDD to count the number of those lines.

Don't panic if you did not fully grasp the mechanisms behind RDDs. The following refresher, however, helps you to remember the important points. RDD is the core data abstraction in Spark to represent a distributed collection of large datasets that can be partitioned and processed in parallel across a cluster of machines. The Spark API provides a uniform set of operations to transform and reduce the data within an RDD. On top of these abstractions and operations, the GraphX library also offers a flexible API that enables us to create graphs and operate on them easily.

Perhaps, when you ran the preceding commands in the Spark shell, you were overwhelmed by the long list of logging statements that start with INFO. There is a way to reduce the amount of information that Spark outputs in the shell.