3.2.1 Resilient Distributed Dataset (RDD): it works as a collection of elements which are operated in parallel.In the distributed file system  Spark runs on Hadoop cluster, and RDD is created from files in the format of text or sequence files.RDD is used for reading the objects in the collection and when some partition  is lost,  it can  be rebuilt  because RDDs  are distributed  across  a set  of machines.