Updated: Oct 25, 2019
Apache Spark transformations like Spark reduceByKey, groupByKey, mapPartitions, map etc are very widely used. Apart from these transformations there are several others, I will explain each of them with examples.
But before I proceed with Spark transformation examples, if you are new to Spark and Scala I would highly encourage you to go through this post - Spark RDD, Transformation and Actions example.
Main menu: Spark Scala Tutorial
We will be using lambda functions to pass through most of these Spark transformations. So those who are new to Scala should have basic understanding of lambda functions.
In brief, lambda functions are like normal functions except the fact that they are more convenient when we have to use functions just in one place so that you don't need to worry about defining functions separately.
For example, if you want to double the number you can simply write; x => x + x like you do in Python and other languages. Syntax in Scala would be like this,
scala> val lfunc = (x:Int) => x + x
lfunc: Int => Int = <function1>
res0: Int = 6
We will be using "Where is the Mount Everest?" text data which we created in earlier post (SparkContext and text files). I just picked some random data to go through these examples.
Where is Mount Everest? (MountEverest.txt)
Mount Everest (Nepali: Sagarmatha सगरमाथा; Tibetan: Chomolungma ཇོ་མོ་གླང་མ; Chinese Zhumulangma 珠穆朗玛) is Earth's highest mountain above sea level, located in the Mahalangur Himal sub-range of the Himalayas. The international border between Nepal (Province No. 1) and China (Tibet Autonomous Region) runs across its summit point. - Reference Wikipedia
scala> val mountEverest = sc.textFile("/Users/Rajput/Documents/testdata/MountEverest.txt")
mountEverestRDD: org.apache.spark.rdd.RDD[String] = /Users/Rajput/Documents/testdata/MountEverest.txt MapPartitionsRDD at textFile at <console>:24
I encourage you all to run these examples on Spark-shell side-by-side.
This transformation redistributes the data after passing each element through func.
For example, if you want to split the Mount Everest text into individual words, you just need to pass this lambda func x => x.split(" ") and it will create a new RDD as shown below.
What is this func doing? It's just reading each element and splitting on the basis of space character.
scala> val words = mountEverest.map(x => x.split(" "))
words: org.apache.spark.rdd.RDD[Array[String]] = MapPartitionsRDD at map at <console>:25
res1: Array[Array[String]] = Array(Array(Mount, Everest, (Nepali:, Sagarmatha, सगरमाथा;, Tibetan:, Chomolungma, ཇོ་མོ་གླང་མ;, Chinese, Zhumulangma, 珠穆朗玛), is, Earth's, highest, mountain, above, sea, level,, located, in, the, Mahalangur, Himal, sub-range, of, the, Himalayas., The, international, border, between, Nepal, (Province, No., 1), a