Sometimes, a variable needs to be shared across tasks, or between tasks and the driver program. By default, when Spark runs a function in parallel as a set of tasks on different nodes, it ships a copy of each variable used in the function to each task. Finally, RDDs automatically recover from node failures.Ī second abstraction in Spark is shared variables that can be used in parallel operations. Users may also ask Spark to persist an RDD in memory, allowing it to be reused efficiently across parallel operations. RDDs are created by starting with a file in the Hadoop file system (or any other Hadoop-supported file system), or an existing Scala collection in the driver program, and transforming it.

The main abstraction Spark provides is a resilient distributed dataset (RDD), which is a collection of elements partitioned across the nodes of the cluster that can be operated on in parallel.

At a high level, every Spark application consists of a driver program that runs the user’s main function and executes various parallel operations on a cluster.

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