In this tutorial we’ll learn about RDD (Re-silent Distributed Data sets) which is the core concept of spark.
RDD is an immutable (read-only) collection of objects, distributed in the cluster.
RDD can be created from storage data or from other RDD by performing any operation on it.
- In Older Map Reduce paradigm, the map and reduce operation was not effective in terms of memory and speed. So RDD has taken the place to make MapReduce more efficient.
- As data sharing was very slow as it requires map reduce program to write the output on disk. So to reuse data between computations also require o/p to disk.
- Due to replication, serialization and disk IO hadoop spend 90% time on read and write operation.
- In short Iterative and Interactive both processes need faster data sharing.
- Apache spark supports in-memory operations and so the job becomes 10 to 100 times faster than hadoop job.
RDD can be created in two ways,
- By paralleling the existing one
- Loading external dataset from HDFS
Operations on RDD:
Two types of operations can be performed on RDD.
RDD can be transformed from one form to another form. Map, filter, combineByKey etc. are transformation operation which create other RDD.
If you have multiple operations to be performed on the same data, you can store that data explicitly in the memory by calling cache() or persist() functions.
Actions returns final result. Like first, collect, reduce, count etc. are actions.
Until the action operation is called, no transformation operations are performed.
RDD having key/value pairs called Pair RDDs.They are very useful performing or counting aggregations by keys in parallel on various nodes of the cluster.
Pair RDD can be created by calling a map() operation which will emit key/value pairs.
Transformations on Pair RDDs:
ReduceByKey(),groupByKey(),cobineByKey(),mapValues(),flatMapValues(),keys() etc. are functions can be performed on one Pair RDDs where as subtractByKet(),join, cogroup() are functions can be performed on two pair RDDs.
Run the spark-shell command on command line.
Then create the rdd from any text file.
Here media.txt is a list of instagram URLs in it.
scala> val mediaRDD =sc.textFile("D:/instagram-scraper-master/media.txt") rdd: org.apache.spark.rdd.RDD[String] = D:/instagram-scraper-master/media.txt Ma pPartitionsRDD at textFile at <console>:21 scala> mediaRDD.count res0: Long = 1013 scala> mediaRDD.take(2).foreach(println) https://instagram.fbom1-1.fna.fbcdn.net/t50.2886-16/14790206_177359509381923_796 7834812834643968_n.mp4 https://instagram.fbom1-1.fna.fbcdn.net/t50.2886-16/14833228_1020652531380366_85 48718479509815296_n.mp4
Node.txt: It is a network file having node id and it’s neighbors.
scala> val nodeRDD =sc.textFile(“D:/Node.txt”)
nodeRDD: org.apache.spark.rdd.RDD[String] = D:/Node.txt MapPartitionsRDD at textFile at <console>:21
scala> val mapRDD= nodeRDD.map(_.split(” “)).map(v => (v(0).toInt, v(1).toInt))
mapRDD: org.apache.spark.rdd.RDD[(String, String)] = MapPartitionsRDD at ma
fold foreach foreachPartition foreachWith
scala> val result=mapRDD.countByKey()
result: scala.collection.Map[Int,Long] = Map(4 -> 1, 2 -> 1, 1 -> 3, 3 -> 2)
So like this we can perform several paired RDD functions on Paired RDD and it makes easy to perform several aggregation functions.
In the next tutorial we’ll see all the RDD functions in details.