Dependency class is the base (abstract) class to model a dependency relationship between two or more RDDs.
Dependency has a single method
rdd to access the RDD that is behind a dependency.
def rdd: RDD[T]
NarrowDependency and ShuffleDependency are the two top-level subclasses of
The dependencies of a RDD are available using dependencies method.
You use toDebugString method to print out the RDD lineage in a user-friendly way.